<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:77313e87-e3e6-4d07-b1d9-3107d11e88e8,g:dc8b0e8f-06fe-4734-aeed-98ea8a31bbc2-->
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:qsi="http://quantum-si.com/20251231" xmlns:xbrli="http://www.xbrl.org/2003/instance" xmlns:dtr-types="http://www.xbrl.org/dtr/type/2024-01-31" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" attributeFormDefault="unqualified" elementFormDefault="qualified" targetNamespace="http://quantum-si.com/20251231">
  <xs:import namespace="http://fasb.org/srt/2025" schemaLocation="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd"/>
  <xs:import namespace="http://fasb.org/us-gaap/2025" schemaLocation="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/instance" schemaLocation="http://www.xbrl.org/2003/xbrl-instance-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/2003/linkbase" schemaLocation="http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd"/>
  <xs:import namespace="http://www.xbrl.org/dtr/type/2024-01-31" schemaLocation="https://www.xbrl.org/dtr/type/2024-01-31/types.xsd"/>
  <xs:import namespace="http://xbrl.org/2005/xbrldt" schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/country/2025" schemaLocation="https://xbrl.sec.gov/country/2025/country-2025.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/cyd-af-sub/2025" schemaLocation="https://xbrl.sec.gov/cyd/2025/cyd-af-sub-2025.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/dei/2025" schemaLocation="https://xbrl.sec.gov/dei/2025/dei-2025.xsd"/>
  <xs:import namespace="http://xbrl.sec.gov/ecd/2025" schemaLocation="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd"/>
  <xs:annotation>
    <xs:appinfo>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="qsi-20251231_pre.xml" xlink:role="http://www.xbrl.org/2003/role/presentationLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="qsi-20251231_def.xml" xlink:role="http://www.xbrl.org/2003/role/definitionLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="qsi-20251231_lab.xml" xlink:role="http://www.xbrl.org/2003/role/labelLinkbaseRef" xlink:type="simple"/>
      <link:linkbaseRef xmlns:xlink="http://www.w3.org/1999/xlink" xlink:arcrole="http://www.w3.org/1999/xlink/properties/linkbase" xlink:href="qsi-20251231_cal.xml" xlink:role="http://www.xbrl.org/2003/role/calculationLinkbaseRef" xlink:type="simple"/>
      <link:roleType id="Cover" roleURI="http://quantum-si.com/role/Cover">
        <link:definition>0000001 - Document - Cover</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AuditInformation" roleURI="http://quantum-si.com/role/AuditInformation">
        <link:definition>0000002 - Document - Audit Information</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CONSOLIDATEDBALANCESHEETS" roleURI="http://quantum-si.com/role/CONSOLIDATEDBALANCESHEETS">
        <link:definition>9952151 - Statement - CONSOLIDATED BALANCE SHEETS</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CONSOLIDATEDBALANCESHEETSParenthetical" roleURI="http://quantum-si.com/role/CONSOLIDATEDBALANCESHEETSParenthetical">
        <link:definition>9952152 - Statement - CONSOLIDATED BALANCE SHEETS (Parenthetical)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CONSOLIDATEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSS" roleURI="http://quantum-si.com/role/CONSOLIDATEDSTATEMENTSOFOPERATIONSANDCOMPREHENSIVELOSS">
        <link:definition>9952153 - Statement - CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CONSOLIDATEDSTATEMENTSOFCHANGESINSTOCKHOLDERSEQUITYDEFICIT" roleURI="http://quantum-si.com/role/CONSOLIDATEDSTATEMENTSOFCHANGESINSTOCKHOLDERSEQUITYDEFICIT">
        <link:definition>9952154 - Statement - CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS&#8217; EQUITY (DEFICIT)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CONSOLIDATEDSTATEMENTSOFCASHFLOWS" roleURI="http://quantum-si.com/role/CONSOLIDATEDSTATEMENTSOFCASHFLOWS">
        <link:definition>9952155 - Statement - CONSOLIDATED STATEMENTS OF CASH FLOWS</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DescriptionofBusinessandBasisofPresentation" roleURI="http://quantum-si.com/role/DescriptionofBusinessandBasisofPresentation">
        <link:definition>9952156 - Disclosure - Description of Business and Basis of Presentation</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPolicies" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPolicies">
        <link:definition>9952157 - Disclosure - Summary of Significant Accounting Policies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InvestmentsinMarketableSecurities" roleURI="http://quantum-si.com/role/InvestmentsinMarketableSecurities">
        <link:definition>9952158 - Disclosure - Investments in Marketable Securities</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueofFinancialInstruments" roleURI="http://quantum-si.com/role/FairValueofFinancialInstruments">
        <link:definition>9952159 - Disclosure - Fair Value of Financial Instruments</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Inventory" roleURI="http://quantum-si.com/role/Inventory">
        <link:definition>9952160 - Disclosure - Inventory</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyandEquipmentNet" roleURI="http://quantum-si.com/role/PropertyandEquipmentNet">
        <link:definition>9952161 - Disclosure - Property and Equipment, Net</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Leases" roleURI="http://quantum-si.com/role/Leases">
        <link:definition>9952162 - Disclosure - Leases</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpensesandOtherCurrentLiabilities" roleURI="http://quantum-si.com/role/AccruedExpensesandOtherCurrentLiabilities">
        <link:definition>9952163 - Disclosure - Accrued Expenses and Other Current Liabilities</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquityTransactions" roleURI="http://quantum-si.com/role/EquityTransactions">
        <link:definition>9952164 - Disclosure - Equity Transactions</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockbasedCompensation" roleURI="http://quantum-si.com/role/StockbasedCompensation">
        <link:definition>9952165 - Disclosure - Stock-based Compensation</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="WarrantLiabilities" roleURI="http://quantum-si.com/role/WarrantLiabilities">
        <link:definition>9952166 - Disclosure - Warrant Liabilities</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetLossPerShare" roleURI="http://quantum-si.com/role/NetLossPerShare">
        <link:definition>9952167 - Disclosure - Net Loss Per Share</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="Restructuring" roleURI="http://quantum-si.com/role/Restructuring">
        <link:definition>9952168 - Disclosure - Restructuring</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxes" roleURI="http://quantum-si.com/role/IncomeTaxes">
        <link:definition>9952169 - Disclosure - Income Taxes</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SegmentReporting" roleURI="http://quantum-si.com/role/SegmentReporting">
        <link:definition>9952170 - Disclosure - Segment Reporting</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RelatedPartyTransactions" roleURI="http://quantum-si.com/role/RelatedPartyTransactions">
        <link:definition>9952171 - Disclosure - Related Party Transactions</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsandContingencies" roleURI="http://quantum-si.com/role/CommitmentsandContingencies">
        <link:definition>9952172 - Disclosure - Commitments and Contingencies</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesPolicies" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesPolicies">
        <link:definition>9955511 - Disclosure - Summary of Significant Accounting Policies (Policies)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesTables" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesTables">
        <link:definition>9955512 - Disclosure - Summary of Significant Accounting Policies (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InvestmentsinMarketableSecuritiesTables" roleURI="http://quantum-si.com/role/InvestmentsinMarketableSecuritiesTables">
        <link:definition>9955513 - Disclosure - Investments in Marketable Securities (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueofFinancialInstrumentsTables" roleURI="http://quantum-si.com/role/FairValueofFinancialInstrumentsTables">
        <link:definition>9955514 - Disclosure - Fair Value of Financial Instruments (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InventoryTables" roleURI="http://quantum-si.com/role/InventoryTables">
        <link:definition>9955515 - Disclosure - Inventory (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyandEquipmentNetTables" roleURI="http://quantum-si.com/role/PropertyandEquipmentNetTables">
        <link:definition>9955516 - Disclosure - Property and Equipment, Net (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesTables" roleURI="http://quantum-si.com/role/LeasesTables">
        <link:definition>9955517 - Disclosure - Leases (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpensesandOtherCurrentLiabilitiesTables" roleURI="http://quantum-si.com/role/AccruedExpensesandOtherCurrentLiabilitiesTables">
        <link:definition>9955518 - Disclosure - Accrued Expenses and Other Current Liabilities (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockbasedCompensationTables" roleURI="http://quantum-si.com/role/StockbasedCompensationTables">
        <link:definition>9955519 - Disclosure - Stock-based Compensation (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetLossPerShareTables" roleURI="http://quantum-si.com/role/NetLossPerShareTables">
        <link:definition>9955520 - Disclosure - Net Loss Per Share (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringTables" roleURI="http://quantum-si.com/role/RestructuringTables">
        <link:definition>9955521 - Disclosure - Restructuring (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesTables" roleURI="http://quantum-si.com/role/IncomeTaxesTables">
        <link:definition>9955522 - Disclosure - Income Taxes (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SegmentInformationTables" roleURI="http://quantum-si.com/role/SegmentInformationTables">
        <link:definition>9955523 - Disclosure - Segment Information (Tables)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="DescriptionofBusinessandBasisofPresentationNarrativeDetails" roleURI="http://quantum-si.com/role/DescriptionofBusinessandBasisofPresentationNarrativeDetails">
        <link:definition>9955524 - Disclosure - Description of Business and Basis of Presentation - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesSegmentReportingDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesSegmentReportingDetails">
        <link:definition>9955525 - Disclosure - Summary of Significant Accounting Policies - Segment Reporting (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesAccountsReceivableNetDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesAccountsReceivableNetDetails">
        <link:definition>9955526 - Disclosure - Summary of Significant Accounting Policies - Accounts Receivable, Net (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesPropertyandEquipmentNetDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesPropertyandEquipmentNetDetails">
        <link:definition>9955527 - Disclosure - Summary of Significant Accounting Policies - Property and Equipment, Net (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesCapitalizedSoftwareDevelopmentCostsDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesCapitalizedSoftwareDevelopmentCostsDetails">
        <link:definition>9955528 - Disclosure - Summary of Significant Accounting Policies - Capitalized Software Development Costs (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesImpairmentofLongLivedAssetsDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesImpairmentofLongLivedAssetsDetails">
        <link:definition>9955529 - Disclosure - Summary of Significant Accounting Policies - Impairment of Long-Lived Assets (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesLeasesDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesLeasesDetails">
        <link:definition>9955530 - Disclosure - Summary of Significant Accounting Policies - Leases (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesWarrantLiabilitiesDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesWarrantLiabilitiesDetails">
        <link:definition>9955531 - Disclosure - Summary of Significant Accounting Policies - Warrant Liabilities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesWarrantyDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesWarrantyDetails">
        <link:definition>9955532 - Disclosure - Summary of Significant Accounting Policies, Warranty (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesRevenueRecognitionDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesRevenueRecognitionDetails">
        <link:definition>9955533 - Disclosure - Summary of Significant Accounting Policies - Revenue Recognition (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesDeferredRevenueDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesDeferredRevenueDetails">
        <link:definition>9955534 - Disclosure - Summary of Significant Accounting Policies - Deferred Revenue (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesDeferredRevenueDetails_1" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesDeferredRevenueDetails_1">
        <link:definition>9955534 - Disclosure - Summary of Significant Accounting Policies - Deferred Revenue (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesStockBasedCompensationDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesStockBasedCompensationDetails">
        <link:definition>9955535 - Disclosure - Summary of Significant Accounting Policies - Stock-Based Compensation (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SummaryofSignificantAccountingPoliciesIncomeTaxesDetails" roleURI="http://quantum-si.com/role/SummaryofSignificantAccountingPoliciesIncomeTaxesDetails">
        <link:definition>9955536 - Disclosure - Summary of Significant Accounting Policies - Income Taxes (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InvestmentsinMarketableSecuritiesUnrealizedandRealizedGainsLossesandDividendIncomeonMarketableSecuritiesDetails" roleURI="http://quantum-si.com/role/InvestmentsinMarketableSecuritiesUnrealizedandRealizedGainsLossesandDividendIncomeonMarketableSecuritiesDetails">
        <link:definition>9955537 - Disclosure - Investments in Marketable Securities -Unrealized and Realized Gains/(Losses) and Dividend Income on Marketable Securities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InvestmentsinMarketableSecuritiesScheduleofAvailableforSaleSecuritiesReconciliationDetails" roleURI="http://quantum-si.com/role/InvestmentsinMarketableSecuritiesScheduleofAvailableforSaleSecuritiesReconciliationDetails">
        <link:definition>9955538 - Disclosure - Investments in Marketable Securities - Schedule of Available-for-Sale Securities Reconciliation (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InvestmentsinMarketableSecuritiesInvestmentsClassifiedbyContractualMaturityDateDetails" roleURI="http://quantum-si.com/role/InvestmentsinMarketableSecuritiesInvestmentsClassifiedbyContractualMaturityDateDetails">
        <link:definition>9955539 - Disclosure - Investments in Marketable Securities - Investments Classified by Contractual Maturity Date (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueofFinancialInstrumentsNarrativeDetails" roleURI="http://quantum-si.com/role/FairValueofFinancialInstrumentsNarrativeDetails">
        <link:definition>9955540 - Disclosure - Fair Value of Financial Instruments - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="FairValueofFinancialInstrumentsAssetsandLiabilitiesMeasuredatFairValueonRecurringBasisDetails" roleURI="http://quantum-si.com/role/FairValueofFinancialInstrumentsAssetsandLiabilitiesMeasuredatFairValueonRecurringBasisDetails">
        <link:definition>9955541 - Disclosure - Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InventoryScheduleofNetInventoryDetails" roleURI="http://quantum-si.com/role/InventoryScheduleofNetInventoryDetails">
        <link:definition>9955542 - Disclosure - Inventory - Schedule of Net Inventory (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="InventoryNarrativeDetails" roleURI="http://quantum-si.com/role/InventoryNarrativeDetails">
        <link:definition>9955543 - Disclosure - Inventory - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyandEquipmentNetDetails" roleURI="http://quantum-si.com/role/PropertyandEquipmentNetDetails">
        <link:definition>9955544 - Disclosure - Property and Equipment, Net (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="PropertyandEquipmentNetNarrativeDetails" roleURI="http://quantum-si.com/role/PropertyandEquipmentNetNarrativeDetails">
        <link:definition>9955545 - Disclosure - Property and Equipment, Net - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesLeaseRelatedCostsDetails" roleURI="http://quantum-si.com/role/LeasesLeaseRelatedCostsDetails">
        <link:definition>9955546 - Disclosure - Leases - Lease-Related Costs (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesFutureMinimumLeasePaymentsUnderNonCancellableLeasesDetails" roleURI="http://quantum-si.com/role/LeasesFutureMinimumLeasePaymentsUnderNonCancellableLeasesDetails">
        <link:definition>9955547 - Disclosure - Leases - Future Minimum Lease Payments Under Non-Cancellable Leases (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesFutureMinimumLeasePaymentsUnderNonCancellableLeasesDetails_1" roleURI="http://quantum-si.com/role/LeasesFutureMinimumLeasePaymentsUnderNonCancellableLeasesDetails_1">
        <link:definition>9955547 - Disclosure - Leases - Future Minimum Lease Payments Under Non-Cancellable Leases (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesSupplementalCashFlowInformationRelatedtoOperatingLeasesDetails" roleURI="http://quantum-si.com/role/LeasesSupplementalCashFlowInformationRelatedtoOperatingLeasesDetails">
        <link:definition>9955548 - Disclosure - Leases - Supplemental Cash Flow Information Related to Operating Leases (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="LeasesNarrativeDetails" roleURI="http://quantum-si.com/role/LeasesNarrativeDetails">
        <link:definition>9955549 - Disclosure - Leases - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="AccruedExpensesandOtherCurrentLiabilitiesDetails" roleURI="http://quantum-si.com/role/AccruedExpensesandOtherCurrentLiabilitiesDetails">
        <link:definition>9955550 - Disclosure - Accrued Expenses and Other Current Liabilities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="EquityTransactionsDetails" roleURI="http://quantum-si.com/role/EquityTransactionsDetails">
        <link:definition>9955551 - Disclosure - Equity Transactions (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockbasedCompensationNarrativeDetails" roleURI="http://quantum-si.com/role/StockbasedCompensationNarrativeDetails">
        <link:definition>9955552 - Disclosure - Stock-based Compensation - Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockbasedCompensationFairValueofEachStockOptionAwardDetails" roleURI="http://quantum-si.com/role/StockbasedCompensationFairValueofEachStockOptionAwardDetails">
        <link:definition>9955553 - Disclosure - Stock-based Compensation - Fair Value of Each Stock Option Award (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockbasedCompensationStockOptionsDetails" roleURI="http://quantum-si.com/role/StockbasedCompensationStockOptionsDetails">
        <link:definition>9955554 - Disclosure - Stock-based Compensation - Stock Options (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockbasedCompensationRestrictedStockUnitsDetails" roleURI="http://quantum-si.com/role/StockbasedCompensationRestrictedStockUnitsDetails">
        <link:definition>9955555 - Disclosure - Stock-based Compensation - Restricted Stock Units (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="StockbasedCompensationStockBasedCompensationDetails" roleURI="http://quantum-si.com/role/StockbasedCompensationStockBasedCompensationDetails">
        <link:definition>9955556 - Disclosure - Stock-based Compensation - Stock-Based Compensation (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="WarrantLiabilitiesDetails" roleURI="http://quantum-si.com/role/WarrantLiabilitiesDetails">
        <link:definition>9955557 - Disclosure - Warrant Liabilities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetLossPerShareBasicandDilutedNetLossPerShareDetails" roleURI="http://quantum-si.com/role/NetLossPerShareBasicandDilutedNetLossPerShareDetails">
        <link:definition>9955558 - Disclosure - Net Loss Per Share - Basic and Diluted Net Loss Per Share (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="NetLossPerShareAntidilutiveCommonEquivalentSharesDetails" roleURI="http://quantum-si.com/role/NetLossPerShareAntidilutiveCommonEquivalentSharesDetails">
        <link:definition>9955559 - Disclosure - Net Loss Per Share -Anti-dilutive Common Equivalent Shares (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RestructuringDetails" roleURI="http://quantum-si.com/role/RestructuringDetails">
        <link:definition>9955560 - Disclosure - Restructuring (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesComponentsofLossbeforeProvisionforIncomeTaxesDetails" roleURI="http://quantum-si.com/role/IncomeTaxesComponentsofLossbeforeProvisionforIncomeTaxesDetails">
        <link:definition>9955561 - Disclosure - Income Taxes - Components of Loss before Provision for Income Taxes (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesProvisionforIncomeTaxesDetails" roleURI="http://quantum-si.com/role/IncomeTaxesProvisionforIncomeTaxesDetails">
        <link:definition>9955562 - Disclosure - Income Taxes - Provision for Income Taxes (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesStatutorytoEffectiveIncomeTaxRateReconciliationDetails" roleURI="http://quantum-si.com/role/IncomeTaxesStatutorytoEffectiveIncomeTaxRateReconciliationDetails">
        <link:definition>9955563 - Disclosure - Income Taxes - Statutory to Effective Income Tax Rate Reconciliation (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesIncomeTaxesPaidDetails" roleURI="http://quantum-si.com/role/IncomeTaxesIncomeTaxesPaidDetails">
        <link:definition>9955564 - Disclosure - Income Taxes - Income Taxes Paid (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesDeferredTaxAssetsandLiabilitiesDetails" roleURI="http://quantum-si.com/role/IncomeTaxesDeferredTaxAssetsandLiabilitiesDetails">
        <link:definition>9955565 - Disclosure - Income Taxes - Deferred Tax Assets and Liabilities (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesNetOperatingLossCarryforwardsDetails" roleURI="http://quantum-si.com/role/IncomeTaxesNetOperatingLossCarryforwardsDetails">
        <link:definition>9955566 - Disclosure - Income Taxes - Net Operating Loss Carryforwards (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesTaxCreditCarryforwardsDetails" roleURI="http://quantum-si.com/role/IncomeTaxesTaxCreditCarryforwardsDetails">
        <link:definition>9955567 - Disclosure - Income Taxes - Tax Credit Carryforwards (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesUnrecognizedTaxBenefitsDetails" roleURI="http://quantum-si.com/role/IncomeTaxesUnrecognizedTaxBenefitsDetails">
        <link:definition>9955568 - Disclosure - Income Taxes - Unrecognized Tax Benefits (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="IncomeTaxesUnrecognizedTaxBenefitsNarrativeDetails" roleURI="http://quantum-si.com/role/IncomeTaxesUnrecognizedTaxBenefitsNarrativeDetails">
        <link:definition>9955569 - Disclosure - Income Taxes - Unrecognized Tax Benefits Narrative (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="SegmentInformationDetails" roleURI="http://quantum-si.com/role/SegmentInformationDetails">
        <link:definition>9955570 - Disclosure - Segment Information (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="RelatedPartyTransactionsDetails" roleURI="http://quantum-si.com/role/RelatedPartyTransactionsDetails">
        <link:definition>9955571 - Disclosure - Related Party Transactions (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
      <link:roleType id="CommitmentsandContingenciesDetails" roleURI="http://quantum-si.com/role/CommitmentsandContingenciesDetails">
        <link:definition>9955572 - Disclosure - Commitments and Contingencies (Details)</link:definition>
        <link:usedOn>link:presentationLink</link:usedOn>
        <link:usedOn>link:calculationLink</link:usedOn>
        <link:usedOn>link:definitionLink</link:usedOn>
      </link:roleType>
    </xs:appinfo>
  </xs:annotation>
  <xs:element id="qsi_RedeemableWarrantsEachWholeWarrantExercisableForOneShareOfClassACommonStockEachAtAnExercisePriceOf1150PerShareMember" abstract="true" name="RedeemableWarrantsEachWholeWarrantExercisableForOneShareOfClassACommonStockEachAtAnExercisePriceOf1150PerShareMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_AuditInformationAbstract" abstract="true" name="AuditInformationAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="qsi_AccruedContractedServicesCurrent" abstract="false" name="AccruedContractedServicesCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_LegalSettlementExpenseNetOfInsuranceProceeds" abstract="false" name="LegalSettlementExpenseNetOfInsuranceProceeds" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_AdjustmentsToAdditionalPaidInCapitalRefundOfIssuanceCosts" abstract="false" name="AdjustmentsToAdditionalPaidInCapitalRefundOfIssuanceCosts" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_IncreaseDecreaseInOperatingLeaseRightOfUseAssets" abstract="false" name="IncreaseDecreaseInOperatingLeaseRightOfUseAssets" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_IncreaseDecreaseInLitigationLiability" abstract="false" name="IncreaseDecreaseInLitigationLiability" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_ProceedsFromVestingOfRestrictedStock" abstract="false" name="ProceedsFromVestingOfRestrictedStock" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_ProceedsFromRefundOfIssuanceCosts" abstract="false" name="ProceedsFromRefundOfIssuanceCosts" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_CashReceivedFromExchangeOfResearchAndDevelopmentTaxCredits" abstract="false" name="CashReceivedFromExchangeOfResearchAndDevelopmentTaxCredits" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_TransfersFromInventoryToPropertyAndEquipment" abstract="false" name="TransfersFromInventoryToPropertyAndEquipment" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_DeferredOfferingCostsPayable" abstract="false" name="DeferredOfferingCostsPayable" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_PrepaidExpensesAndOtherCurrentAssetsPolicyPolicyTextBlock" abstract="false" name="PrepaidExpensesAndOtherCurrentAssetsPolicyPolicyTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="qsi_DeferredRevenuePolicyPolicyTextBlock" abstract="false" name="DeferredRevenuePolicyPolicyTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="qsi_ShippingAndHandlingCostsPolicyTextBlock" abstract="false" name="ShippingAndHandlingCostsPolicyTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="qsi_PropertyPlantAndEquipmentEstimatedUsefulLivesTableTextBlock" abstract="false" name="PropertyPlantAndEquipmentEstimatedUsefulLivesTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="qsi_GainLossOnSecuritiesAndInvestmentIncomeTableTextBlock" abstract="false" name="GainLossOnSecuritiesAndInvestmentIncomeTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="qsi_SupplementalCashFlowInformationRelatedToOperatingLeasesTableTextBlock" abstract="false" name="SupplementalCashFlowInformationRelatedToOperatingLeasesTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="qsi_NumberOfWarrantsIssuedPerUnitIssued" abstract="false" name="NumberOfWarrantsIssuedPerUnitIssued" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="qsi_StandardProductWarrantyWarrantyPeriod" abstract="false" name="StandardProductWarrantyWarrantyPeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="qsi_DebtSecuritiesAvailableForSaleCurrentMember" abstract="true" name="DebtSecuritiesAvailableForSaleCurrentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_DebtSecuritiesAvailableForSaleNoncurrentMember" abstract="true" name="DebtSecuritiesAvailableForSaleNoncurrentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_PrivatePlacementWarrantsMember" abstract="true" name="PrivatePlacementWarrantsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_RedeemableWarrantsMember" abstract="true" name="RedeemableWarrantsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_PropertyAndEquipmentDisposalsMember" abstract="true" name="PropertyAndEquipmentDisposalsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_RightOfUseAssetAndOperatingLeaseLiabilityLeaseTerminationDerecognition" abstract="false" name="RightOfUseAssetAndOperatingLeaseLiabilityLeaseTerminationDerecognition" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_SpaceLocatedInNewHavenConnecticutMember" abstract="true" name="SpaceLocatedInNewHavenConnecticutMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_IncreaseDecreaseInRightOfUseAssets" abstract="false" name="IncreaseDecreaseInRightOfUseAssets" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_SeptemberSalesAgreementMember" abstract="true" name="SeptemberSalesAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_AtTheMarketOfferingMember" abstract="true" name="AtTheMarketOfferingMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_JulyRegisteredDirectOfferingMember" abstract="true" name="JulyRegisteredDirectOfferingMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_RegisteredDirectOfferingMember" abstract="true" name="RegisteredDirectOfferingMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_CommonStockNumberOfVotesPerShare" abstract="false" name="CommonStockNumberOfVotesPerShare" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="qsi_AtTheMarketEquityOfferingProgramAbstract" abstract="true" name="AtTheMarketEquityOfferingProgramAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="qsi_SaleOfStockAuthorizedAmount" abstract="false" name="SaleOfStockAuthorizedAmount" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_SaleOfStockCommonStockThatCanBeSold" abstract="false" name="SaleOfStockCommonStockThatCanBeSold" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_SaleOfStockPlacementAgentFeePercentageOfProceedsFromSaleOfStockOffering" abstract="false" name="SaleOfStockPlacementAgentFeePercentageOfProceedsFromSaleOfStockOffering" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="qsi_QuantumSiIncorporated2021EquityIncentivePlanMember" abstract="true" name="QuantumSiIncorporated2021EquityIncentivePlanMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_InducementEquityIncentivePlan2023Member" abstract="true" name="InducementEquityIncentivePlan2023Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_AwardedNovember92022Member" abstract="true" name="AwardedNovember92022Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_CommonStockAdditionalCapitalSharesReservedForFutureIssuance" abstract="false" name="CommonStockAdditionalCapitalSharesReservedForFutureIssuance" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="qsi_ShareBasedPaymentArrangementPlanModificationToBeRecognized" abstract="false" name="ShareBasedPaymentArrangementPlanModificationToBeRecognized" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_RestrictedStockExpenseCredit" abstract="false" name="RestrictedStockExpenseCredit" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_ClassOfWarrantOrRightRedemptionPrice" abstract="false" name="ClassOfWarrantOrRightRedemptionPrice" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="dtr-types:perShareItemType"/>
  <xs:element id="qsi_NoticePeriodToRedeemWarrants" abstract="false" name="NoticePeriodToRedeemWarrants" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="qsi_ThresholdTradingDays" abstract="false" name="ThresholdTradingDays" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="qsi_ThresholdConsecutiveTradingDays" abstract="false" name="ThresholdConsecutiveTradingDays" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="qsi_PeriodPriorToNoticeOfRedemption" abstract="false" name="PeriodPriorToNoticeOfRedemption" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="qsi_TradingDayPeriodToCalculateFairMarketValueOverExercisePriceOfWarrants" abstract="false" name="TradingDayPeriodToCalculateFairMarketValueOverExercisePriceOfWarrants" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="qsi_BeneficialOwnershipPercentage" abstract="false" name="BeneficialOwnershipPercentage" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="qsi_ClassOfWarrantOrRightLimitationPeriodToTransferAssignOrSellWarrants" abstract="false" name="ClassOfWarrantOrRightLimitationPeriodToTransferAssignOrSellWarrants" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="qsi_ClassOfWarrantOrRightRedeemed" abstract="false" name="ClassOfWarrantOrRightRedeemed" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="qsi_ClassOfWarrantOrRightExercised" abstract="false" name="ClassOfWarrantOrRightExercised" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="qsi_CashPaymentsAndOtherAdjustmentsResearchAndDevelopmentMember" abstract="true" name="CashPaymentsAndOtherAdjustmentsResearchAndDevelopmentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_CashPaymentsAndOtherAdjustmentsSellingGeneralAndAdministrativeMember" abstract="true" name="CashPaymentsAndOtherAdjustmentsSellingGeneralAndAdministrativeMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_RestructuringReservePaymentsForRestructuringAndOtherAdjustments" abstract="false" name="RestructuringReservePaymentsForRestructuringAndOtherAdjustments" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_ForeignTaxEffectAmountAbstract" abstract="true" name="ForeignTaxEffectAmountAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="qsi_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseCompensationAmount" abstract="false" name="EffectiveIncomeTaxRateReconciliationNondeductibleExpenseCompensationAmount" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_ForeignTaxEffectPercentAbstract" abstract="true" name="ForeignTaxEffectPercentAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="qsi_EffectiveIncomeTaxRateReconciliationNondeductibleExpenseCompensationPercent" abstract="false" name="EffectiveIncomeTaxRateReconciliationNondeductibleExpenseCompensationPercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:percentItemType"/>
  <xs:element id="qsi_DeferredTaxAssetsLeasingArrangements" abstract="false" name="DeferredTaxAssetsLeasingArrangements" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_DeferredTaxAssetsUnrealizedGainLossOnDebtSecuritiesTrading" abstract="false" name="DeferredTaxAssetsUnrealizedGainLossOnDebtSecuritiesTrading" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_DeferredTaxAssetsCapitalizedResearchAndDevelopmentExpenses" abstract="false" name="DeferredTaxAssetsCapitalizedResearchAndDevelopmentExpenses" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_Pre2018Member" abstract="true" name="Pre2018Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_OperatingLossCarryforwardsNotSubjectToExpirationButSubjectTo80TaxableIncomeLimitation" abstract="false" name="OperatingLossCarryforwardsNotSubjectToExpirationButSubjectTo80TaxableIncomeLimitation" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_OperatingLossCarryforwardsNotSubjectToExpiration" abstract="false" name="OperatingLossCarryforwardsNotSubjectToExpiration" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_ConnecticutDepartmentOfRevenueServicesMember" abstract="true" name="ConnecticutDepartmentOfRevenueServicesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_IncomeTaxDisclosureTable" abstract="true" name="IncomeTaxDisclosureTable" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrldt:hypercubeItem" type="xbrli:stringItemType"/>
  <xs:element id="qsi_IncomeTaxDisclosureLineItems" abstract="true" name="IncomeTaxDisclosureLineItems" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="qsi_IncomeTaxExaminationIncreaseDecreaseInOperatingLoss" abstract="false" name="IncomeTaxExaminationIncreaseDecreaseInOperatingLoss" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_IncomeTaxExaminationOperatingLossReclassified" abstract="false" name="IncomeTaxExaminationOperatingLossReclassified" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_EffectiveIncomeTaxRateReconciliationTaxCreditResearchExchangedForCashAmount" abstract="false" name="EffectiveIncomeTaxRateReconciliationTaxCreditResearchExchangedForCashAmount" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_Catalyzer4CorporationMember" abstract="true" name="Catalyzer4CorporationMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_OtherCompaniesControlledByRothbergFamilyMember" abstract="true" name="OtherCompaniesControlledByRothbergFamilyMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_MonthlyServicesUnderAmendedAndRestatedTechnologyServicesAgreementMember" abstract="true" name="MonthlyServicesUnderAmendedAndRestatedTechnologyServicesAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_MonthToMonthSubleaseArrangementsForOfficeAndLaboratorySpacesMember" abstract="true" name="MonthToMonthSubleaseArrangementsForOfficeAndLaboratorySpacesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_ServicesProvidedToParticipantCompaniesUnderAmendedAndRestatedTechnologyServicesAgreementMember" abstract="true" name="ServicesProvidedToParticipantCompaniesUnderAmendedAndRestatedTechnologyServicesAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesThatCanBePurchasedWithOptionGranted" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardSharesThatCanBePurchasedWithOptionGranted" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="qsi_LicensesRelatedToCertainIntellectualPropertyMember" abstract="true" name="LicensesRelatedToCertainIntellectualPropertyMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_DelawareStockholderLitigationMember" abstract="true" name="DelawareStockholderLitigationMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_ContractManufacturerMatterMember" abstract="true" name="ContractManufacturerMatterMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="qsi_OtherCommitmentAnnualMinimumFixedPayments" abstract="false" name="OtherCommitmentAnnualMinimumFixedPayments" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_LeaseIncentives" abstract="false" name="LeaseIncentives" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_OperatingLeaseLeaseTerminationCashSettlement" abstract="false" name="OperatingLeaseLeaseTerminationCashSettlement" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_OperatingLeaseLeaseTerminationSecurityDepositSurrendered" abstract="false" name="OperatingLeaseLeaseTerminationSecurityDepositSurrendered" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_OperatingLeaseLeaseTerminationMonthlyCharges" abstract="false" name="OperatingLeaseLeaseTerminationMonthlyCharges" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="qsi_OperatingLeaseLeaseTerminationNet" abstract="false" name="OperatingLeaseLeaseTerminationNet" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
</xs:schema>
