<?xml version="1.0" encoding="utf-8"?>
<!--XBRL document created with Toppan Merrill Bridge  11.3.0.1 -->
<!--Based on XBRL 2.1-->
<!--Created on: 5/20/2026 3:31:39 PM-->
<!--Modified on: 5/20/2026 3:31:39 PM-->
<link:linkbase xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt">
  <roleRef xlink:type="simple" xlink:href="arqq-20260331.xsd#StatementCondensedConsolidatedStatementOfChangesInEquity" roleURI="http://www.arqitquantum.com/role/StatementCondensedConsolidatedStatementOfChangesInEquity" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="arqq-20260331.xsd#DisclosureRevenueDetails" roleURI="http://www.arqitquantum.com/role/DisclosureRevenueDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="arqq-20260331.xsd#DisclosureDiscontinuedOperationsImpactOnStatementOfComprehensiveIncomeDetails" roleURI="http://www.arqitquantum.com/role/DisclosureDiscontinuedOperationsImpactOnStatementOfComprehensiveIncomeDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="arqq-20260331.xsd#DisclosureIntangibleFixedAssetsDetails" roleURI="http://www.arqitquantum.com/role/DisclosureIntangibleFixedAssetsDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="arqq-20260331.xsd#DisclosureShareCapitalDetails" roleURI="http://www.arqitquantum.com/role/DisclosureShareCapitalDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="arqq-20260331.xsd#DisclosureLeasesLeasesAsLesseeDetails" roleURI="http://www.arqitquantum.com/role/DisclosureLeasesLeasesAsLesseeDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="arqq-20260331.xsd#DisclosureLeasesRightOfUseAssetsDetails" roleURI="http://www.arqitquantum.com/role/DisclosureLeasesRightOfUseAssetsDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <roleRef xlink:type="simple" xlink:href="arqq-20260331.xsd#DisclosurePriorPeriodErrorDetails" roleURI="http://www.arqitquantum.com/role/DisclosurePriorPeriodErrorDetails" xmlns="http://www.xbrl.org/2003/linkbase" />
  <link:arcroleRef arcroleURI="http://www.esma.europa.eu/xbrl/esef/arcrole/wider-narrower" xlink:type="simple" xlink:href="http://www.xbrl.org/lrr/arcrole/esma-arcrole-2018-11-21.xsd#wider-narrower" />
  <link:roleRef roleURI="http://www.arqitquantum.com/role/DisclosureStandardAndCustomAxisDomainDefaults" xlink:href="arqq-20260331.xsd#DisclosureStandardAndCustomAxisDomainDefaults" xlink:type="simple" />
  <link:roleRef roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-roles-2025.xsd#eedm" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" />
  <link:definitionLink xlink:type="extended" xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:title="Extensible Enumeration Domain Members" />
  <link:definitionLink xlink:role="http://www.arqitquantum.com/role/DisclosureStandardAndCustomAxisDomainDefaults" xlink:type="extended" xlink:title="99900 - Disclosure - Standard And Custom Axis Domain Defaults">
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalAxis" xlink:label="ifrs-full_ClassesOfShareCapitalAxis_1" xlink:title="ifrs-full_ClassesOfShareCapitalAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfShareCapitalDomain" xlink:label="ifrs-full_ClassesOfShareCapitalDomain_1" xlink:title="ifrs-full_ClassesOfShareCapitalMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_ClassesOfShareCapitalAxis_1" xlink:to="ifrs-full_ClassesOfShareCapitalDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_ClassesOfShareCapitalAxis_1 To ifrs-full_ClassesOfShareCapitalMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityAddressesAddressTypeAxis" xlink:label="dei_EntityAddressesAddressTypeAxis_1" xlink:title="dei_EntityAddressesAddressTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_AddressTypeDomain" xlink:label="dei_AddressTypeDomain_1" xlink:title="dei_AddressTypeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="dei_EntityAddressesAddressTypeAxis_1" xlink:to="dei_AddressTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:dei_EntityAddressesAddressTypeAxis_1 To dei_AddressTypeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComponentsOfEquityAxis" xlink:label="ifrs-full_ComponentsOfEquityAxis_1" xlink:title="ifrs-full_ComponentsOfEquityAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComponentsOfEquityDomain" xlink:label="ifrs-full_ComponentsOfEquityDomain_1" xlink:title="ifrs-full_EquityMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_ComponentsOfEquityAxis_1" xlink:to="ifrs-full_ComponentsOfEquityDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_ComponentsOfEquityAxis_1 To ifrs-full_EquityMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:label="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis_1" xlink:title="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountMember" xlink:label="ifrs-full_CarryingAmountMember_1" xlink:title="ifrs-full_CarryingAmountMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis_1" xlink:to="ifrs-full_CarryingAmountMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis_1 To ifrs-full_CarryingAmountMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis" xlink:label="ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis_1" xlink:title="ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfPropertyPlantAndEquipmentDomain" xlink:label="ifrs-full_ClassesOfPropertyPlantAndEquipmentDomain_1" xlink:title="ifrs-full_PropertyPlantAndEquipmentMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis_1" xlink:to="ifrs-full_ClassesOfPropertyPlantAndEquipmentDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_ClassesOfPropertyPlantAndEquipmentAxis_1 To ifrs-full_PropertyPlantAndEquipmentMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis" xlink:label="ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis_1" xlink:title="ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillDomain" xlink:label="ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillDomain_1" xlink:title="ifrs-full_IntangibleAssetsOtherThanGoodwillMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis_1" xlink:to="ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_ClassesOfIntangibleAssetsOtherThanGoodwillAxis_1 To ifrs-full_IntangibleAssetsOtherThanGoodwillMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RangeAxis" xlink:label="ifrs-full_RangeAxis_1" xlink:title="ifrs-full_RangeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RangeDomain" xlink:label="ifrs-full_RangeDomain_1" xlink:title="ifrs-full_RangesMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_RangeAxis_1" xlink:to="ifrs-full_RangeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_RangeAxis_1 To ifrs-full_RangesMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_FinancialEffectOfTransitionFromPreviousGAAPToIFRSsAxis" xlink:label="ifrs-full_FinancialEffectOfTransitionFromPreviousGAAPToIFRSsAxis_1" xlink:title="ifrs-full_FinancialEffectOfTransitionFromPreviousGAAPToIFRSsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IFRSsMember" xlink:label="ifrs-full_IFRSsMember_1" xlink:title="ifrs-full_IFRSsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_FinancialEffectOfTransitionFromPreviousGAAPToIFRSsAxis_1" xlink:to="ifrs-full_IFRSsMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_FinancialEffectOfTransitionFromPreviousGAAPToIFRSsAxis_1 To ifrs-full_IFRSsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_BusinessCombinationsAxis" xlink:label="ifrs-full_BusinessCombinationsAxis_1" xlink:title="ifrs-full_BusinessCombinationsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_BusinessCombinationsDomain" xlink:label="ifrs-full_BusinessCombinationsDomain_1" xlink:title="ifrs-full_EntitysTotalForBusinessCombinationsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_BusinessCombinationsAxis_1" xlink:to="ifrs-full_BusinessCombinationsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_BusinessCombinationsAxis_1 To ifrs-full_EntitysTotalForBusinessCombinationsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeAxis" xlink:label="arqq_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeAxis_1" xlink:title="arqq_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeDomain" xlink:label="arqq_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeDomain_1" xlink:title="arqq_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="arqq_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeAxis_1" xlink:to="arqq_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:arqq_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeAxis_1 To arqq_IfrsCollaborativeArrangementAndArrangementOtherThanCollaborativeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SignificantInvestmentsInSubsidiariesAxis" xlink:label="ifrs-full_SignificantInvestmentsInSubsidiariesAxis_1" xlink:title="ifrs-full_SignificantInvestmentsInSubsidiariesAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SignificantInvestmentsInSubsidiariesDomain" xlink:label="ifrs-full_SignificantInvestmentsInSubsidiariesDomain_1" xlink:title="ifrs-full_EntitysTotalForSubsidiariesMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_SignificantInvestmentsInSubsidiariesAxis_1" xlink:to="ifrs-full_SignificantInvestmentsInSubsidiariesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_SignificantInvestmentsInSubsidiariesAxis_1 To ifrs-full_EntitysTotalForSubsidiariesMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IfrsMeasurementInputTypeAxis" xlink:label="arqq_IfrsMeasurementInputTypeAxis_1" xlink:title="arqq_IfrsMeasurementInputTypeAxis_1" />
    <link:loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IfrsMeasurementInputTypeDomain" xlink:label="arqq_IfrsMeasurementInputTypeDomain_1" xlink:title="arqq_IfrsMeasurementInputTypeDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="arqq_IfrsMeasurementInputTypeAxis_1" xlink:to="arqq_IfrsMeasurementInputTypeDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:arqq_IfrsMeasurementInputTypeAxis_1 To arqq_IfrsMeasurementInputTypeDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_JointVenturesAxis" xlink:label="ifrs-full_JointVenturesAxis_1" xlink:title="ifrs-full_JointVenturesAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_JointVenturesDomain" xlink:label="ifrs-full_JointVenturesDomain_1" xlink:title="ifrs-full_EntitysTotalForJointVenturesMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_JointVenturesAxis_1" xlink:to="ifrs-full_JointVenturesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_JointVenturesAxis_1 To ifrs-full_EntitysTotalForJointVenturesMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_BorrowingsByNameAxis" xlink:label="ifrs-full_BorrowingsByNameAxis_1" xlink:title="ifrs-full_BorrowingsByNameAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_BorrowingsByNameDomain" xlink:label="ifrs-full_BorrowingsByNameDomain_1" xlink:title="ifrs-full_BorrowingsByNameMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_BorrowingsByNameAxis_1" xlink:to="ifrs-full_BorrowingsByNameDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_BorrowingsByNameAxis_1 To ifrs-full_BorrowingsByNameMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IfrsClassOfWarrantOrRightAxis" xlink:label="arqq_IfrsClassOfWarrantOrRightAxis_1" xlink:title="arqq_IfrsClassOfWarrantOrRightAxis_1" />
    <link:loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IfrsClassOfWarrantOrRightDomain" xlink:label="arqq_IfrsClassOfWarrantOrRightDomain_1" xlink:title="arqq_IfrsClassOfWarrantOrRightDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="arqq_IfrsClassOfWarrantOrRightAxis_1" xlink:to="arqq_IfrsClassOfWarrantOrRightDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:arqq_IfrsClassOfWarrantOrRightAxis_1 To arqq_IfrsClassOfWarrantOrRightDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementScenarioAxis" xlink:label="srt_StatementScenarioAxis_1" xlink:title="srt_StatementScenarioAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="srt_ScenarioUnspecifiedDomain_1" xlink:title="srt_ScenarioUnspecifiedDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="srt_StatementScenarioAxis_1" xlink:to="srt_ScenarioUnspecifiedDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:srt_StatementScenarioAxis_1 To srt_ScenarioUnspecifiedDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis" xlink:label="ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis_1" xlink:title="ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsDomain" xlink:label="ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsDomain_1" xlink:title="ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis_1" xlink:to="ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsAxis_1 To ifrs-full_TemporaryDifferenceUnusedTaxLossesAndUnusedTaxCreditsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_Ifrs_subsidiarySaleOfStockAxis" xlink:label="arqq_Ifrs_subsidiarySaleOfStockAxis_1" xlink:title="arqq_Ifrs_subsidiarySaleOfStockAxis_1" />
    <link:loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IfrsSaleOfStockNameOfTransactionDomain" xlink:label="arqq_IfrsSaleOfStockNameOfTransactionDomain_1" xlink:title="arqq_IfrsSaleOfStockNameOfTransactionDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="arqq_Ifrs_subsidiarySaleOfStockAxis_1" xlink:to="arqq_IfrsSaleOfStockNameOfTransactionDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:arqq_Ifrs_subsidiarySaleOfStockAxis_1 To arqq_IfrsSaleOfStockNameOfTransactionDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfFinancialAssetsAxis" xlink:label="ifrs-full_ClassesOfFinancialAssetsAxis_1" xlink:title="ifrs-full_ClassesOfFinancialAssetsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfFinancialAssetsDomain" xlink:label="ifrs-full_ClassesOfFinancialAssetsDomain_1" xlink:title="ifrs-full_FinancialAssetsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_ClassesOfFinancialAssetsAxis_1" xlink:to="ifrs-full_ClassesOfFinancialAssetsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_ClassesOfFinancialAssetsAxis_1 To ifrs-full_FinancialAssetsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfFinancialAssetsAxis" xlink:label="ifrs-full_CategoriesOfFinancialAssetsAxis_1" xlink:title="ifrs-full_CategoriesOfFinancialAssetsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfFinancialAssetsDomain" xlink:label="ifrs-full_CategoriesOfFinancialAssetsDomain_1" xlink:title="ifrs-full_FinancialAssetsCategoryMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_CategoriesOfFinancialAssetsAxis_1" xlink:to="ifrs-full_CategoriesOfFinancialAssetsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_CategoriesOfFinancialAssetsAxis_1 To ifrs-full_FinancialAssetsCategoryMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfFinancialLiabilitiesAxis" xlink:label="ifrs-full_ClassesOfFinancialLiabilitiesAxis_1" xlink:title="ifrs-full_ClassesOfFinancialLiabilitiesAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfFinancialLiabilitiesDomain" xlink:label="ifrs-full_ClassesOfFinancialLiabilitiesDomain_1" xlink:title="ifrs-full_FinancialLiabilitiesMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_ClassesOfFinancialLiabilitiesAxis_1" xlink:to="ifrs-full_ClassesOfFinancialLiabilitiesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_ClassesOfFinancialLiabilitiesAxis_1 To ifrs-full_FinancialLiabilitiesMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfFinancialLiabilitiesAxis" xlink:label="ifrs-full_CategoriesOfFinancialLiabilitiesAxis_1" xlink:title="ifrs-full_CategoriesOfFinancialLiabilitiesAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfFinancialLiabilitiesDomain" xlink:label="ifrs-full_CategoriesOfFinancialLiabilitiesDomain_1" xlink:title="ifrs-full_FinancialLiabilitiesCategoryMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_CategoriesOfFinancialLiabilitiesAxis_1" xlink:to="ifrs-full_CategoriesOfFinancialLiabilitiesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_CategoriesOfFinancialLiabilitiesAxis_1 To ifrs-full_FinancialLiabilitiesCategoryMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LevelsOfFairValueHierarchyAxis" xlink:label="ifrs-full_LevelsOfFairValueHierarchyAxis_1" xlink:title="ifrs-full_LevelsOfFairValueHierarchyAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LevelsOfFairValueHierarchyDomain" xlink:label="ifrs-full_LevelsOfFairValueHierarchyDomain_1" xlink:title="ifrs-full_AllLevelsOfFairValueHierarchyMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_LevelsOfFairValueHierarchyAxis_1" xlink:to="ifrs-full_LevelsOfFairValueHierarchyDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_LevelsOfFairValueHierarchyAxis_1 To ifrs-full_AllLevelsOfFairValueHierarchyMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesAxis" xlink:label="ifrs-full_CategoriesOfRelatedPartiesAxis_1" xlink:title="ifrs-full_CategoriesOfRelatedPartiesAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CategoriesOfRelatedPartiesDomain" xlink:label="ifrs-full_CategoriesOfRelatedPartiesDomain_1" xlink:title="ifrs-full_EntitysTotalForRelatedPartiesMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_CategoriesOfRelatedPartiesAxis_1" xlink:to="ifrs-full_CategoriesOfRelatedPartiesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_CategoriesOfRelatedPartiesAxis_1 To ifrs-full_EntitysTotalForRelatedPartiesMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_PercentageOfReasonablyPossibleChangeInRiskAssumptionAxis" xlink:label="arqq_PercentageOfReasonablyPossibleChangeInRiskAssumptionAxis_1" xlink:title="arqq_PercentageOfReasonablyPossibleChangeInRiskAssumptionAxis_1" />
    <link:loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_PercentageOfReasonablyPossibleChangeInRiskAssumptionDomain" xlink:label="arqq_PercentageOfReasonablyPossibleChangeInRiskAssumptionDomain_1" xlink:title="arqq_PercentageOfReasonablyPossibleChangeInRiskAssumptionDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="arqq_PercentageOfReasonablyPossibleChangeInRiskAssumptionAxis_1" xlink:to="arqq_PercentageOfReasonablyPossibleChangeInRiskAssumptionDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:arqq_PercentageOfReasonablyPossibleChangeInRiskAssumptionAxis_1 To arqq_PercentageOfReasonablyPossibleChangeInRiskAssumptionDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MaturityAxis" xlink:label="ifrs-full_MaturityAxis_1" xlink:title="ifrs-full_MaturityAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MaturityDomain" xlink:label="ifrs-full_MaturityDomain_1" xlink:title="ifrs-full_AggregatedTimeBandsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_MaturityAxis_1" xlink:to="ifrs-full_MaturityDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_MaturityAxis_1 To ifrs-full_AggregatedTimeBandsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NonadjustingEventsAfterReportingPeriodAxis" xlink:label="ifrs-full_NonadjustingEventsAfterReportingPeriodAxis_1" xlink:title="ifrs-full_NonadjustingEventsAfterReportingPeriodAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NonadjustingEventsAfterReportingPeriodDomain" xlink:label="ifrs-full_NonadjustingEventsAfterReportingPeriodDomain_1" xlink:title="ifrs-full_NonadjustingEventsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_NonadjustingEventsAfterReportingPeriodAxis_1" xlink:to="ifrs-full_NonadjustingEventsAfterReportingPeriodDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_NonadjustingEventsAfterReportingPeriodAxis_1 To ifrs-full_NonadjustingEventsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesAxis" xlink:label="ifrs-full_CounterpartiesAxis_1" xlink:title="ifrs-full_CounterpartiesAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CounterpartiesDomain" xlink:label="ifrs-full_CounterpartiesDomain_1" xlink:title="ifrs-full_CounterpartiesMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_CounterpartiesAxis_1" xlink:to="ifrs-full_CounterpartiesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_CounterpartiesAxis_1 To ifrs-full_CounterpartiesMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis" xlink:label="ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis_1" xlink:title="ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RestatedMember" xlink:label="ifrs-full_RestatedMember_1" xlink:title="ifrs-full_RestatedMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis_1" xlink:to="ifrs-full_RestatedMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis_1 To ifrs-full_RestatedMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesAxis" xlink:label="ifrs-full_ProductsAndServicesAxis_1" xlink:title="ifrs-full_ProductsAndServicesAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesDomain" xlink:label="ifrs-full_ProductsAndServicesDomain_1" xlink:title="ifrs-full_ProductsAndServicesMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_ProductsAndServicesAxis_1" xlink:to="ifrs-full_ProductsAndServicesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_ProductsAndServicesAxis_1 To ifrs-full_ProductsAndServicesMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GeographicalAreasAxis" xlink:label="ifrs-full_GeographicalAreasAxis_1" xlink:title="ifrs-full_GeographicalAreasAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GeographicalAreasDomain" xlink:label="ifrs-full_GeographicalAreasDomain_1" xlink:title="ifrs-full_GeographicalAreasMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_GeographicalAreasAxis_1" xlink:to="ifrs-full_GeographicalAreasDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_GeographicalAreasAxis_1 To ifrs-full_GeographicalAreasMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis_1" xlink:title="dei_LegalEntityAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain_1" xlink:title="dei_EntityDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="dei_LegalEntityAxis_1" xlink:to="dei_EntityDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:dei_LegalEntityAxis_1 To dei_EntityDomain_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MeasurementAxis" xlink:label="ifrs-full_MeasurementAxis_1" xlink:title="ifrs-full_MeasurementAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MeasurementDomain" xlink:label="ifrs-full_MeasurementDomain_1" xlink:title="ifrs-full_AggregatedMeasurementMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_MeasurementAxis_1" xlink:to="ifrs-full_MeasurementDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_MeasurementAxis_1 To ifrs-full_AggregatedMeasurementMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AttributionOfExpensesByNatureToTheirFunctionAxis" xlink:label="ifrs-full_AttributionOfExpensesByNatureToTheirFunctionAxis_1" xlink:title="ifrs-full_AttributionOfExpensesByNatureToTheirFunctionAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AttributionOfExpensesByNatureToTheirFunctionDomain" xlink:label="ifrs-full_AttributionOfExpensesByNatureToTheirFunctionDomain_1" xlink:title="ifrs-full_LineItemsByFunctionMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_AttributionOfExpensesByNatureToTheirFunctionAxis_1" xlink:to="ifrs-full_AttributionOfExpensesByNatureToTheirFunctionDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_AttributionOfExpensesByNatureToTheirFunctionAxis_1 To ifrs-full_LineItemsByFunctionMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsAxis" xlink:label="ifrs-full_ClassesOfAssetsAxis_1" xlink:title="ifrs-full_ClassesOfAssetsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsDomain" xlink:label="ifrs-full_ClassesOfAssetsDomain_1" xlink:title="ifrs-full_ClassesOfAssetsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_ClassesOfAssetsAxis_1" xlink:to="ifrs-full_ClassesOfAssetsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_ClassesOfAssetsAxis_1 To ifrs-full_ClassesOfAssetsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfRisksAxis" xlink:label="ifrs-full_TypesOfRisksAxis_1" xlink:title="ifrs-full_TypesOfRisksAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfRisksDomain" xlink:label="ifrs-full_TypesOfRisksDomain_1" xlink:title="ifrs-full_TypesOfRisksMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_TypesOfRisksAxis_1" xlink:to="ifrs-full_TypesOfRisksDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_TypesOfRisksAxis_1 To ifrs-full_TypesOfRisksMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfSharebasedPaymentArrangementsAxis" xlink:label="ifrs-full_TypesOfSharebasedPaymentArrangementsAxis_1" xlink:title="ifrs-full_TypesOfSharebasedPaymentArrangementsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TypesOfSharebasedPaymentArrangementsDomain" xlink:label="ifrs-full_TypesOfSharebasedPaymentArrangementsDomain_1" xlink:title="ifrs-full_SharebasedPaymentArrangementsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_TypesOfSharebasedPaymentArrangementsAxis_1" xlink:to="ifrs-full_TypesOfSharebasedPaymentArrangementsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_TypesOfSharebasedPaymentArrangementsAxis_1 To ifrs-full_SharebasedPaymentArrangementsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_PerformanceObligationsAxis" xlink:label="ifrs-full_PerformanceObligationsAxis_1" xlink:title="ifrs-full_PerformanceObligationsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_PerformanceObligationsDomain" xlink:label="ifrs-full_PerformanceObligationsDomain_1" xlink:title="ifrs-full_PerformanceObligationsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_PerformanceObligationsAxis_1" xlink:to="ifrs-full_PerformanceObligationsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_PerformanceObligationsAxis_1 To ifrs-full_PerformanceObligationsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AssetsAndLiabilitiesClassifiedAsHeldForSaleAxis" xlink:label="ifrs-full_AssetsAndLiabilitiesClassifiedAsHeldForSaleAxis_1" xlink:title="ifrs-full_AssetsAndLiabilitiesClassifiedAsHeldForSaleAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AssetsAndLiabilitiesNotClassifiedAsHeldForSaleMember" xlink:label="ifrs-full_AssetsAndLiabilitiesNotClassifiedAsHeldForSaleMember_1" xlink:title="ifrs-full_AssetsAndLiabilitiesNotClassifiedAsHeldForSaleMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_AssetsAndLiabilitiesClassifiedAsHeldForSaleAxis_1" xlink:to="ifrs-full_AssetsAndLiabilitiesNotClassifiedAsHeldForSaleMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_AssetsAndLiabilitiesClassifiedAsHeldForSaleAxis_1 To ifrs-full_AssetsAndLiabilitiesNotClassifiedAsHeldForSaleMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ContinuingAndDiscontinuedOperationsAxis" xlink:label="ifrs-full_ContinuingAndDiscontinuedOperationsAxis_1" xlink:title="ifrs-full_ContinuingAndDiscontinuedOperationsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ContinuingOperationsMember" xlink:label="ifrs-full_ContinuingOperationsMember_1" xlink:title="ifrs-full_ContinuingOperationsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_ContinuingAndDiscontinuedOperationsAxis_1" xlink:to="ifrs-full_ContinuingOperationsMember_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_ContinuingAndDiscontinuedOperationsAxis_1 To ifrs-full_ContinuingOperationsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MarketsOfCustomersAxis" xlink:label="ifrs-full_MarketsOfCustomersAxis_1" xlink:title="ifrs-full_MarketsOfCustomersAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MarketsOfCustomersDomain" xlink:label="ifrs-full_MarketsOfCustomersDomain_1" xlink:title="ifrs-full_MarketsOfCustomersMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_MarketsOfCustomersAxis_1" xlink:to="ifrs-full_MarketsOfCustomersDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_MarketsOfCustomersAxis_1 To ifrs-full_MarketsOfCustomersMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_PastDueStatusAxis" xlink:label="ifrs-full_PastDueStatusAxis_1" xlink:title="ifrs-full_PastDueStatusAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_PastDueStatusDomain" xlink:label="ifrs-full_PastDueStatusDomain_1" xlink:title="ifrs-full_PastDueStatusMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_PastDueStatusAxis_1" xlink:to="ifrs-full_PastDueStatusDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_PastDueStatusAxis_1 To ifrs-full_PastDueStatusMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CreditImpairmentOfFinancialInstrumentsAxis" xlink:label="ifrs-full_CreditImpairmentOfFinancialInstrumentsAxis_1" xlink:title="ifrs-full_CreditImpairmentOfFinancialInstrumentsAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CreditImpairmentOfFinancialInstrumentsDomain" xlink:label="ifrs-full_CreditImpairmentOfFinancialInstrumentsDomain_1" xlink:title="ifrs-full_CreditImpairmentOfFinancialInstrumentsMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_CreditImpairmentOfFinancialInstrumentsAxis_1" xlink:to="ifrs-full_CreditImpairmentOfFinancialInstrumentsDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_CreditImpairmentOfFinancialInstrumentsAxis_1 To ifrs-full_CreditImpairmentOfFinancialInstrumentsMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfContingentLiabilitiesAxis" xlink:label="ifrs-full_ClassesOfContingentLiabilitiesAxis_1" xlink:title="ifrs-full_ClassesOfContingentLiabilitiesAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfContingentLiabilitiesDomain" xlink:label="ifrs-full_ClassesOfContingentLiabilitiesDomain_1" xlink:title="ifrs-full_ContingentLiabilitiesMember_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_ClassesOfContingentLiabilitiesAxis_1" xlink:to="ifrs-full_ClassesOfContingentLiabilitiesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_ClassesOfContingentLiabilitiesAxis_1 To ifrs-full_ContingentLiabilitiesMember_1" order="1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TimingOfTransferOfGoodsOrServicesAxis" xlink:label="ifrs-full_TimingOfTransferOfGoodsOrServicesAxis_1" xlink:title="ifrs-full_TimingOfTransferOfGoodsOrServicesAxis_1" />
    <link:loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TimingOfTransferOfGoodsOrServicesDomain" xlink:label="ifrs-full_TimingOfTransferOfGoodsOrServicesDomain_1" xlink:title="ifrs-full_TimingOfTransferOfGoodsOrServicesDomain_1" />
    <link:definitionArc xlink:type="arc" xlink:from="ifrs-full_TimingOfTransferOfGoodsOrServicesAxis_1" xlink:to="ifrs-full_TimingOfTransferOfGoodsOrServicesDomain_1" use="optional" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:title="definition:ifrs-full_TimingOfTransferOfGoodsOrServicesAxis_1 To ifrs-full_TimingOfTransferOfGoodsOrServicesDomain_1" order="1" />
  </link:definitionLink>
  <definitionLink xlink:role="http://www.arqitquantum.com/role/StatementCondensedConsolidatedStatementOfChangesInEquity" xlink:type="extended" xlink:title="00300 - Statement - Condensed Consolidated Statement of Changes in Equity" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_StatementOfChangesInEquityTable" xlink:label="ifrs-full_StatementOfChangesInEquityTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComponentsOfEquityAxis" xlink:label="ifrs-full_ComponentsOfEquityAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_StatementOfChangesInEquityTable" xlink:to="ifrs-full_ComponentsOfEquityAxis" priority="2" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComponentsOfEquityDomain" xlink:label="ifrs-full_ComponentsOfEquityDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_ComponentsOfEquityAxis" xlink:to="ifrs-full_ComponentsOfEquityDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IssuedCapitalMember" xlink:label="ifrs-full_IssuedCapitalMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_ComponentsOfEquityDomain" xlink:to="ifrs-full_IssuedCapitalMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_SharePremiumMember" xlink:label="ifrs-full_SharePremiumMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_ComponentsOfEquityDomain" xlink:to="ifrs-full_SharePremiumMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OtherReservesMember" xlink:label="ifrs-full_OtherReservesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_ComponentsOfEquityDomain" xlink:to="ifrs-full_OtherReservesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ReserveOfExchangeDifferencesOnTranslationMember" xlink:label="ifrs-full_ReserveOfExchangeDifferencesOnTranslationMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_ComponentsOfEquityDomain" xlink:to="ifrs-full_ReserveOfExchangeDifferencesOnTranslationMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ReserveOfSharebasedPaymentsMember" xlink:label="ifrs-full_ReserveOfSharebasedPaymentsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="ifrs-full_ComponentsOfEquityDomain" xlink:to="ifrs-full_ReserveOfSharebasedPaymentsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RetainedEarningsMember" xlink:label="ifrs-full_RetainedEarningsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="ifrs-full_ComponentsOfEquityDomain" xlink:to="ifrs-full_RetainedEarningsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_StatementOfChangesInEquityLineItems" xlink:label="ifrs-full_StatementOfChangesInEquityLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_StatementOfChangesInEquityTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_Equity" xlink:label="ifrs-full_Equity" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_Equity" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProfitLoss" xlink:label="ifrs-full_ProfitLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_ProfitLoss" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OtherComprehensiveIncome" xlink:label="ifrs-full_OtherComprehensiveIncome" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_OtherComprehensiveIncome" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComprehensiveIncome" xlink:label="ifrs-full_ComprehensiveIncome" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_ComprehensiveIncome" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IssueOfEquity" xlink:label="ifrs-full_IssueOfEquity" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_IssueOfEquity" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ChangesInEquityAbstract" xlink:label="ifrs-full_ChangesInEquityAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="7" xlink:from="ifrs-full_StatementOfChangesInEquityLineItems" xlink:to="ifrs-full_ChangesInEquityAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseThroughSharebasedPaymentTransactions" xlink:label="ifrs-full_IncreaseDecreaseThroughSharebasedPaymentTransactions" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_ChangesInEquityAbstract" xlink:to="ifrs-full_IncreaseDecreaseThroughSharebasedPaymentTransactions" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.arqitquantum.com/role/DisclosureRevenueDetails" xlink:type="extended" xlink:title="40201 - Disclosure - Revenue (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:label="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesAxis" xlink:label="ifrs-full_ProductsAndServicesAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="ifrs-full_ProductsAndServicesAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProductsAndServicesDomain" xlink:label="ifrs-full_ProductsAndServicesDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_ProductsAndServicesAxis" xlink:to="ifrs-full_ProductsAndServicesDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_SkaPlatformMember" xlink:label="arqq_SkaPlatformMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_ProductsAndServicesDomain" xlink:to="arqq_SkaPlatformMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MarketsOfCustomersAxis" xlink:label="ifrs-full_MarketsOfCustomersAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" xlink:to="ifrs-full_MarketsOfCustomersAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_MarketsOfCustomersDomain" xlink:label="ifrs-full_MarketsOfCustomersDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_MarketsOfCustomersAxis" xlink:to="ifrs-full_MarketsOfCustomersDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_GB" xlink:label="country_GB" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_MarketsOfCustomersDomain" xlink:to="country_GB" use="optional" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_OtherCountriesMember" xlink:label="arqq_OtherCountriesMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_MarketsOfCustomersDomain" xlink:to="arqq_OtherCountriesMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:label="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RevenueFromRenderingOfServices" xlink:label="ifrs-full_RevenueFromRenderingOfServices" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="ifrs-full_RevenueFromRenderingOfServices" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RevenueFromGovernmentGrants" xlink:label="ifrs-full_RevenueFromGovernmentGrants" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_DisclosureOfDisaggregationOfRevenueFromContractsWithCustomersLineItems" xlink:to="ifrs-full_RevenueFromGovernmentGrants" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.arqitquantum.com/role/DisclosureDiscontinuedOperationsImpactOnStatementOfComprehensiveIncomeDetails" xlink:type="extended" xlink:title="40402 - Disclosure - Discontinued operations - Impact on the statement of comprehensive income (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsTable" xlink:label="ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ContinuingAndDiscontinuedOperationsAxis" xlink:label="ifrs-full_ContinuingAndDiscontinuedOperationsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsTable" xlink:to="ifrs-full_ContinuingAndDiscontinuedOperationsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ContinuingOperationsMember" xlink:label="ifrs-full_ContinuingOperationsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_ContinuingAndDiscontinuedOperationsAxis" xlink:to="ifrs-full_ContinuingOperationsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DiscontinuedOperationsMember" xlink:label="ifrs-full_DiscontinuedOperationsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_ContinuingOperationsMember" xlink:to="ifrs-full_DiscontinuedOperationsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsLineItems" xlink:label="ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsLineItems" xlink:to="ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RevenueFromGovernmentGrants" xlink:label="ifrs-full_RevenueFromGovernmentGrants" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsLineItems" xlink:to="ifrs-full_RevenueFromGovernmentGrants" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_OtherIncome" xlink:label="ifrs-full_OtherIncome" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsLineItems" xlink:to="ifrs-full_OtherIncome" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AdministrativeExpense" xlink:label="ifrs-full_AdministrativeExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsLineItems" xlink:to="ifrs-full_AdministrativeExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ImpairmentLossReversalOfImpairmentLossRecognisedInProfitOrLoss" xlink:label="ifrs-full_ImpairmentLossReversalOfImpairmentLossRecognisedInProfitOrLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsLineItems" xlink:to="ifrs-full_ImpairmentLossReversalOfImpairmentLossRecognisedInProfitOrLoss" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProfitLossFromDiscontinuedOperations" xlink:label="ifrs-full_ProfitLossFromDiscontinuedOperations" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="ifrs-full_DisclosureOfAnalysisOfSingleAmountOfDiscontinuedOperationsLineItems" xlink:to="ifrs-full_ProfitLossFromDiscontinuedOperations" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.arqitquantum.com/role/DisclosureIntangibleFixedAssetsDetails" xlink:type="extended" xlink:title="40501 - Disclosure - Intangible fixed assets (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfIntangibleAssetsTable" xlink:label="ifrs-full_DisclosureOfIntangibleAssetsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:label="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfIntangibleAssetsTable" xlink:to="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" priority="2" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountMember" xlink:label="ifrs-full_CarryingAmountMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:to="ifrs-full_CarryingAmountMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GrossCarryingAmountMember" xlink:label="ifrs-full_GrossCarryingAmountMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_CarryingAmountMember" xlink:to="ifrs-full_GrossCarryingAmountMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AccumulatedDepreciationAndAmortisationMember" xlink:label="ifrs-full_AccumulatedDepreciationAndAmortisationMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_CarryingAmountMember" xlink:to="ifrs-full_AccumulatedDepreciationAndAmortisationMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RangeAxis" xlink:label="ifrs-full_RangeAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="ifrs-full_DisclosureOfIntangibleAssetsTable" xlink:to="ifrs-full_RangeAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RangeDomain" xlink:label="ifrs-full_RangeDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_RangeAxis" xlink:to="ifrs-full_RangeDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_BottomOfRangeMember" xlink:label="ifrs-full_BottomOfRangeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_RangeDomain" xlink:to="ifrs-full_BottomOfRangeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_TopOfRangeMember" xlink:label="ifrs-full_TopOfRangeMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_RangeDomain" xlink:to="ifrs-full_TopOfRangeMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfIntangibleAssetsLineItems" xlink:label="ifrs-full_DisclosureOfIntangibleAssetsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfIntangibleAssetsLineItems" xlink:to="ifrs-full_DisclosureOfIntangibleAssetsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ReconciliationOfChangesInIntangibleAssetsOtherThanGoodwillAbstract" xlink:label="ifrs-full_ReconciliationOfChangesInIntangibleAssetsOtherThanGoodwillAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfIntangibleAssetsLineItems" xlink:to="ifrs-full_ReconciliationOfChangesInIntangibleAssetsOtherThanGoodwillAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IntangibleAssetsOtherThanGoodwill" xlink:label="ifrs-full_IntangibleAssetsOtherThanGoodwill" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_ReconciliationOfChangesInIntangibleAssetsOtherThanGoodwillAbstract" xlink:to="ifrs-full_IntangibleAssetsOtherThanGoodwill" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AmortisationIntangibleAssetsOtherThanGoodwill" xlink:label="ifrs-full_AmortisationIntangibleAssetsOtherThanGoodwill" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_ReconciliationOfChangesInIntangibleAssetsOtherThanGoodwillAbstract" xlink:to="ifrs-full_AmortisationIntangibleAssetsOtherThanGoodwill" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseThroughNetExchangeDifferencesIntangibleAssetsOtherThanGoodwill" xlink:label="ifrs-full_IncreaseDecreaseThroughNetExchangeDifferencesIntangibleAssetsOtherThanGoodwill" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_ReconciliationOfChangesInIntangibleAssetsOtherThanGoodwillAbstract" xlink:to="ifrs-full_IncreaseDecreaseThroughNetExchangeDifferencesIntangibleAssetsOtherThanGoodwill" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_UsefulLifeMeasuredAsPeriodOfTimeIntangibleAssetsOtherThanGoodwill" xlink:label="ifrs-full_UsefulLifeMeasuredAsPeriodOfTimeIntangibleAssetsOtherThanGoodwill" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_ReconciliationOfChangesInIntangibleAssetsOtherThanGoodwillAbstract" xlink:to="ifrs-full_UsefulLifeMeasuredAsPeriodOfTimeIntangibleAssetsOtherThanGoodwill" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ImpairmentLossRecognisedInProfitOrLossIntangibleAssetsOtherThanGoodwill" xlink:label="ifrs-full_ImpairmentLossRecognisedInProfitOrLossIntangibleAssetsOtherThanGoodwill" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="ifrs-full_ReconciliationOfChangesInIntangibleAssetsOtherThanGoodwillAbstract" xlink:to="ifrs-full_ImpairmentLossRecognisedInProfitOrLossIntangibleAssetsOtherThanGoodwill" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.arqitquantum.com/role/DisclosureShareCapitalDetails" xlink:type="extended" xlink:title="40801 - Disclosure - Share capital (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfClassesOfShareCapitalTable" xlink:label="ifrs-full_DisclosureOfClassesOfShareCapitalTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComponentsOfEquityAxis" xlink:label="ifrs-full_ComponentsOfEquityAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfClassesOfShareCapitalTable" xlink:to="ifrs-full_ComponentsOfEquityAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ComponentsOfEquityDomain" xlink:label="ifrs-full_ComponentsOfEquityDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_ComponentsOfEquityAxis" xlink:to="ifrs-full_ComponentsOfEquityDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IssuedCapitalMember" xlink:label="ifrs-full_IssuedCapitalMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_ComponentsOfEquityDomain" xlink:to="ifrs-full_IssuedCapitalMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" xlink:label="ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" xlink:to="ifrs-full_DisclosureOfClassesOfShareCapitalTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfSharesIssuedAbstract" xlink:label="ifrs-full_NumberOfSharesIssuedAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" xlink:to="ifrs-full_NumberOfSharesIssuedAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_NumberOfSharesOutstanding" xlink:label="ifrs-full_NumberOfSharesOutstanding" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_NumberOfSharesIssuedAbstract" xlink:to="ifrs-full_NumberOfSharesOutstanding" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ParValuePerShare" xlink:label="ifrs-full_ParValuePerShare" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_NumberOfSharesIssuedAbstract" xlink:to="ifrs-full_ParValuePerShare" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_NumberOfSharesIssuedInAtMarketOffering" xlink:label="arqq_NumberOfSharesIssuedInAtMarketOffering" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_NumberOfSharesIssuedAbstract" xlink:to="arqq_NumberOfSharesIssuedInAtMarketOffering" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_NumberOfSharesIssuedOnExerciseOfWarrants" xlink:label="arqq_NumberOfSharesIssuedOnExerciseOfWarrants" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_NumberOfSharesIssuedAbstract" xlink:to="arqq_NumberOfSharesIssuedOnExerciseOfWarrants" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IncreaseInNumberOfSharesOutstandingThroughExerciseOfEmis" xlink:label="arqq_IncreaseInNumberOfSharesOutstandingThroughExerciseOfEmis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="ifrs-full_NumberOfSharesIssuedAbstract" xlink:to="arqq_IncreaseInNumberOfSharesOutstandingThroughExerciseOfEmis" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IncreaseInNumberOfSharesOutstandingThroughGrantingOfRestrictedStockUnits" xlink:label="arqq_IncreaseInNumberOfSharesOutstandingThroughGrantingOfRestrictedStockUnits" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="ifrs-full_NumberOfSharesIssuedAbstract" xlink:to="arqq_IncreaseInNumberOfSharesOutstandingThroughGrantingOfRestrictedStockUnits" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IssuedCapitalAbstract" xlink:label="ifrs-full_IssuedCapitalAbstract" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_DisclosureOfClassesOfShareCapitalLineItems" xlink:to="ifrs-full_IssuedCapitalAbstract" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IssuedCapital" xlink:label="ifrs-full_IssuedCapital" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_IssuedCapitalAbstract" xlink:to="ifrs-full_IssuedCapital" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IssueOfEquityInAtMarketOffering" xlink:label="arqq_IssueOfEquityInAtMarketOffering" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_IssuedCapitalAbstract" xlink:to="arqq_IssueOfEquityInAtMarketOffering" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseThroughExerciseOfWarrantsEquity" xlink:label="ifrs-full_IncreaseDecreaseThroughExerciseOfWarrantsEquity" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_IssuedCapitalAbstract" xlink:to="ifrs-full_IncreaseDecreaseThroughExerciseOfWarrantsEquity" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IncreaseInIssuedCapitalThroughExerciseOfEmis" xlink:label="arqq_IncreaseInIssuedCapitalThroughExerciseOfEmis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_IssuedCapitalAbstract" xlink:to="arqq_IncreaseInIssuedCapitalThroughExerciseOfEmis" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IncreaseInIssuedCapitalThroughGrantingOfRestrictedStockUnits" xlink:label="arqq_IncreaseInIssuedCapitalThroughGrantingOfRestrictedStockUnits" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="ifrs-full_IssuedCapitalAbstract" xlink:to="arqq_IncreaseInIssuedCapitalThroughGrantingOfRestrictedStockUnits" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.arqitquantum.com/role/DisclosureLeasesLeasesAsLesseeDetails" xlink:type="extended" xlink:title="41001 - Disclosure - Leases - Leases as lessee (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" xlink:label="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsAxis" xlink:label="ifrs-full_ClassesOfAssetsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" xlink:to="ifrs-full_ClassesOfAssetsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsDomain" xlink:label="ifrs-full_ClassesOfAssetsDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_ClassesOfAssetsAxis" xlink:to="ifrs-full_ClassesOfAssetsDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_BuildingsMember" xlink:label="ifrs-full_BuildingsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_ClassesOfAssetsDomain" xlink:to="ifrs-full_BuildingsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CommunicationAndNetworkEquipmentMember" xlink:label="ifrs-full_CommunicationAndNetworkEquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_ClassesOfAssetsDomain" xlink:to="ifrs-full_CommunicationAndNetworkEquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:label="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:to="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_AverageLeaseTerm" xlink:label="arqq_AverageLeaseTerm" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:to="arqq_AverageLeaseTerm" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.arqitquantum.com/role/DisclosureLeasesRightOfUseAssetsDetails" xlink:type="extended" xlink:title="41002 - Disclosure - Leases - Right-of-use assets (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" xlink:label="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsAxis" xlink:label="ifrs-full_ClassesOfAssetsAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" xlink:to="ifrs-full_ClassesOfAssetsAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ClassesOfAssetsDomain" xlink:label="ifrs-full_ClassesOfAssetsDomain" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_ClassesOfAssetsAxis" xlink:to="ifrs-full_ClassesOfAssetsDomain" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_LandAndBuildingsMember" xlink:label="ifrs-full_LandAndBuildingsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_ClassesOfAssetsDomain" xlink:to="ifrs-full_LandAndBuildingsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CommunicationAndNetworkEquipmentMember" xlink:label="ifrs-full_CommunicationAndNetworkEquipmentMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_ClassesOfAssetsDomain" xlink:to="ifrs-full_CommunicationAndNetworkEquipmentMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:label="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="2" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" xlink:to="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CarryingAmountMember" xlink:label="ifrs-full_CarryingAmountMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_CarryingAmountAccumulatedDepreciationAmortisationAndImpairmentAndGrossCarryingAmountAxis" xlink:to="ifrs-full_CarryingAmountMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_GrossCarryingAmountMember" xlink:label="ifrs-full_GrossCarryingAmountMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_CarryingAmountMember" xlink:to="ifrs-full_GrossCarryingAmountMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AccumulatedDepreciationAndAmortisationMember" xlink:label="ifrs-full_AccumulatedDepreciationAndAmortisationMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_CarryingAmountMember" xlink:to="ifrs-full_AccumulatedDepreciationAndAmortisationMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:label="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:to="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RightofuseAssets" xlink:label="ifrs-full_RightofuseAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:to="ifrs-full_RightofuseAssets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AdditionsOtherThanThroughBusinessCombinationsRightofuseAssets" xlink:label="ifrs-full_AdditionsOtherThanThroughBusinessCombinationsRightofuseAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:to="ifrs-full_AdditionsOtherThanThroughBusinessCombinationsRightofuseAssets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_IncreaseDecreaseThroughModificationsRightOfUseAssets" xlink:label="arqq_IncreaseDecreaseThroughModificationsRightOfUseAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:to="arqq_IncreaseDecreaseThroughModificationsRightOfUseAssets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_DepreciationRightofuseAssets" xlink:label="ifrs-full_DepreciationRightofuseAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:to="ifrs-full_DepreciationRightofuseAssets" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_IncreaseDecreaseThroughNetExchangeDifferencesRightofuseAssets" xlink:label="ifrs-full_IncreaseDecreaseThroughNetExchangeDifferencesRightofuseAssets" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="6" xlink:from="ifrs-full_DisclosureOfQuantitativeInformationAboutRightofuseAssetsLineItems" xlink:to="ifrs-full_IncreaseDecreaseThroughNetExchangeDifferencesRightofuseAssets" priority="1" use="optional" />
  </definitionLink>
  <definitionLink xlink:role="http://www.arqitquantum.com/role/DisclosurePriorPeriodErrorDetails" xlink:type="extended" xlink:title="41101 - Disclosure - Prior period error (Details)" xmlns="http://www.xbrl.org/2003/linkbase">
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_PriorPeriodErrorTable" xlink:label="arqq_PriorPeriodErrorTable" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis" xlink:label="ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" order="1" xlink:from="arqq_PriorPeriodErrorTable" xlink:to="ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_RestatedMember" xlink:label="ifrs-full_RestatedMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" order="1" xlink:from="ifrs-full_RetrospectiveApplicationAndRetrospectiveRestatementAxis" xlink:to="ifrs-full_RestatedMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_PreviouslyStatedMember" xlink:label="ifrs-full_PreviouslyStatedMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="1" xlink:from="ifrs-full_RestatedMember" xlink:to="ifrs-full_PreviouslyStatedMember" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_FinancialEffectOfCorrectionsOfAccountingErrorsMember" xlink:label="ifrs-full_FinancialEffectOfCorrectionsOfAccountingErrorsMember" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="ifrs-full_RestatedMember" xlink:to="ifrs-full_FinancialEffectOfCorrectionsOfAccountingErrorsMember" use="optional" />
    <loc xlink:type="locator" xlink:href="arqq-20260331.xsd#arqq_PriorPeriodErrorLineItems" xlink:label="arqq_PriorPeriodErrorLineItems" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" order="1" xlink:from="arqq_PriorPeriodErrorLineItems" xlink:to="arqq_PriorPeriodErrorTable" use="optional" xbrldt:contextElement="segment" xbrldt:closed="true" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_CashAndCashEquivalents" xlink:label="ifrs-full_CashAndCashEquivalents" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="2" xlink:from="arqq_PriorPeriodErrorLineItems" xlink:to="ifrs-full_CashAndCashEquivalents" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_AdministrativeExpense" xlink:label="ifrs-full_AdministrativeExpense" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="3" xlink:from="arqq_PriorPeriodErrorLineItems" xlink:to="ifrs-full_AdministrativeExpense" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProfitLossBeforeTax" xlink:label="ifrs-full_ProfitLossBeforeTax" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="4" xlink:from="arqq_PriorPeriodErrorLineItems" xlink:to="ifrs-full_ProfitLossBeforeTax" priority="1" use="optional" />
    <loc xlink:type="locator" xlink:href="https://xbrl.ifrs.org/taxonomy/2025-03-27/full_ifrs/full_ifrs-cor_2025-03-27.xsd#ifrs-full_ProfitLoss" xlink:label="ifrs-full_ProfitLoss" />
    <definitionArc xlink:type="arc" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" order="5" xlink:from="arqq_PriorPeriodErrorLineItems" xlink:to="ifrs-full_ProfitLoss" priority="1" use="optional" />
  </definitionLink>
</link:linkbase>
