• Photonics Research
  • Vol. 11, Issue 1, 81 (2023)
Yihong Zhang, Wenjun Yu, Pei Zeng, Guoding Liu, and Xiongfeng Ma*
Author Affiliations
  • Center for Quantum Information, Institute for Interdisciplinary Information Sciences, Tsinghua University, Beijing 100084, China
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    DOI: 10.1364/PRJ.473970 Cite this Article Set citation alerts
    Yihong Zhang, Wenjun Yu, Pei Zeng, Guoding Liu, Xiongfeng Ma. Scalable fast benchmarking for individual quantum gates with local twirling[J]. Photonics Research, 2023, 11(1): 81 Copy Citation Text show less

    Abstract

    With the development of controllable quantum systems, fast and practical characterization of multi-qubit gates has become essential for building high-fidelity quantum computing devices. The usual way to fulfill this requirement via randomized benchmarking demands complicated implementation of numerous multi-qubit twirling gates. How to efficiently and reliably estimate the fidelity of a quantum process remains an open problem. This work thus proposes a character-cycle benchmarking protocol and a character-average benchmarking protocol using only local twirling gates to estimate the process fidelity of an individual multi-qubit operation. Our protocols were able to characterize a large class of quantum gates including and beyond the Clifford group via the local gauge transformation, which forms a universal gate set for quantum computing. We demonstrated numerically our protocols for a non-Clifford gate—controlled-(TX) and a Clifford gate—five-qubit quantum error-correcting encoding circuit. The numerical results show that our protocols can efficiently and reliably characterize the gate process fidelities. Compared with the cross-entropy benchmarking, the simulation results show that the character-average benchmarking achieves three orders of magnitude improvements in terms of sampling complexity.
    F(Λ)=1d2i=04n1λi,

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    F(Λ)1MΣ{Pj}λj,

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    ΛP=14nΣ{PjPn}Pj1ΛPj,

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    Fccb=F(U1ΛP()UΛP),

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    Fccb(Λ)=F(U1ΛPUΛP).

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    Pr(|F^ccbF¯ccb|ϵM+ϵb)1δ,

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    ϕ:GGL(V),gϕ(g),  gG,(A1)

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    ϕ(g1)ϕ(g2)=ϕ(g1g2).(A2)

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    ϕ(g)wW.(A3)

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    ϕ(g)σRGσ(g)mσ,(A4)

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    χσ(g)=Tr[σ(g)].(A5)

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    Πσ=dσ|G|gGχσ(g)ϕ(g),(A6)

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    ΛG=1|G|gGϕ(g)Λϕ(g).(A7)

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    ΛG=σRGTr(ΛΠσ)Tr(Πσ)Πσ,(A8)

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    E(O)=l=1mKlOKl,(A9)

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    l=1mKlKl=I.(A10)

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    E2E1(O)=l2=1m2l1=1m1Kl2Kl1OKl1Kl2,(A11)

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    Pn=i=1n{Ii,Xi,Yi,Zi},(A12)

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    {σi=12nPi|PiPn}.(A13)

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    Tr(σiσj)=δij.(A14)

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    |O=iPnTr(σiO)σi.(A15)

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    |Λ(O)=Λ|O.(A16)

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    Λij=σi|Λ|σj=Tr(σiΛ(σj)).(A17)

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    |Λ2Λ1(O)=Λ2|Λ1(O)=Λ2Λ1|O.(A18)

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    pi=Fi|ρ=Tr(Fiρ).(A19)

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    Λ(ρ)=di,jχijσiρσj.(A20)

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    F(Λ)=χ00(Λ).(A21)

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    λj=d1Tr(PjΛ(Pj)),(A22)

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    λj=i(1)i,jχii.(A23)

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    F(Λ)=χ00(Λ)=1d2jλj,(A24)

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    Fave=(dF+1)/(d+1).(A25)

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    Fave=dψTr(|ψψ|Λ(|ψψ|)),(A26)

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    Srb=Gm+1GmG1,(A27)

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    f(m)=EG1GmQ0|G˜m+1G˜mG˜2G˜1|ρψ0,(A28)

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    G˜i=ΛLGiΛR,(A29)

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    f(m)=Q|(EgGGΛG)m|ρψ,(A30)

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    f(m)=σRGQ|Πσm|ρψλσm,(A31)

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    λσ=Tr(ΛΠσ)Tr(Πσ).(A32)

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    F(Λ,I)=4nσdσλσ,(A33)

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    Πσ=dσ|G|gGχσ(g)G.(B1)

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    fi(m)=djQj|((EPtPnPt1U1ΛUPt)(EPrPnPr1ΛPr))m(EPPnχj(P)P)|ρψj.(B7)

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    ΛPEPrPnPr1ΛPr=PrPnPr|Λ|PrΠPr,ΛPUEPtPnPt1U1ΛUPr=PtPnPt|U1ΛU|PtΠPt=U1ΛPU,(B8)

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    fi(m)=Qj|(ΛPUΛP)m(djEPPnχj(P)P)|ρs.(B9)

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    Πj=dj4nPPnχj(P)P,(B10)

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    fj(m)=Qj|(PtPnPt|U1ΛU|PtΠPt×PrPnPr|Λ|PrΠPr)mΠj|ρs=Qj|Πj|ρs(Pj|U1ΛU|PjPj|Λ|Pj)m.(B11)

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    λj=Pj|U1ΛU|PjPj|Λ|Pj,Aj=Qj|Πj|ρs.(B12)

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    Fccb(Λ)=F((U1ΛPU)ΛP,I).(B13)

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    xny1++x1ynxτ(1)y1++xτ(n)ynx1y1++xnyn,(B14)

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    UPiU1=Pu(i),(B15)

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    ωu(1)ω1++ωu(4n)ω4nω12++ω4n2.(B16)

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    ωu(1)ω1++ωu(4n)ω4nω1++ω4n.(B17)

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    Fccb(Λ)F(Λ,I).(B18)

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    fj(m)=djQj|(EPtPnPt1L1LU1L1ΛLUL1LPt)mL1L(EPrPnPr1L1ΛLPr)L1L(EPPnχj(P)P)|ρs=djQj|(EPtPnPt1U1ΛLUPt)m(EPrPnPr1ΛLPr)(EPPnχj(P)P)|ρs,(B19)

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    F(ΛL,I)=d2Tr(ΛL)=d2Tr(Λ)=F(Λ,I).(B20)

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    Rn={I,ϒ}n.(B21)

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    ΛC=σkRnTr(ΛΠσk)Tr(Πσk)Πσk,(B22)

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    γk=Tr(ΛΠσk)Tr(Πσk)=1|σk|PjσkPj|Λ|Pj,(B23)

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    σ1={II}={II},σ2={Iϒ}={IX,IY,IZ},σ3={ϒI}={XI,YI,ZI},σ4={ϒϒ}={XX,XY,XZ,YX,YY,YZ,ZX,ZY,ZZ},(B24)

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    Tr(Πσ1)=|σ1|=30,Tr(Πσ2)=|σ2|=31,Tr(Πσ3)=|σ3|=31,Tr(Πσ4)=|σ4|=32.(B25)

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    ΛC=Tr(ΛΠσ1)Πσ1+Tr(ΛΠσ2)3Πσ2+Tr(ΛΠσ3)3Πσ3+Tr(ΛΠσ4)9Πσ4,(B26)

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    Λin=U1ΛPUΛP,(B27)

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    ΛC(m)=ECC1nC1ΛinmC=σkRnTr(ΛinmΠσk)Tr(Πσk)Πσk.(B28)

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    fk(m)=Qk|ΛC(m)|ρs,(B29)

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    fk(m)=Qk|ΠQkΠσkTr(ΛinmΠσk)Tr(Πσk)|ρs=Qk|ρsPjσk(Pj|U1ΛU|PjPj|Λ|Pj)mTr(Πσk)=Qk|ρsPjσkλj2mTr(Πσk),(B30)

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    Fcab=4nσkRndσkμk,(B31)

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    fk(m)=Qk|ρs|σk|Pjσkλj2m,(B32)

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    fk(m)=Akμk2m,(B33)

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    y=β0+β1m,(B34)

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    y=lnfk(m),β0=lnAk,β1=2lnμk.(B35)

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    M=(1m11m21mq),Y=(y1y2yq).(B36)

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    β^=(β^0β^1)=(MTM)1MTY.(B37)

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    μk=exp(β^12),(B38)

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    β^1=qi=1qmiyii=1qmii=1qyiqi=1qmi2(i=1qmi)2.(B39)

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    μk1|σk|Pjσkλj,(B40)

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    Fcab(Λ)Fccb(Λ).(B41)

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    β^1mini(yi+1yimi+1mi).(B42)

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    1|σk|Pjσkλj2mi+1=1|σk|Pjσkλj2miλj2(mi+1mi)(1|σk|Pjσkλj2mi)(1|σk|Pjσkλj2(mi+1mi)).(B43)

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    yi+1yi=ln(Pjσkλj2mi+1Pjσkλj2mi)ln(1|σk|Pjσkλj2(mi+1mi))2(mi+1mi)ln(1|σk|Pjσkλj),(B44)

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    β^12ln(1|σk|Pjσkλj).(B45)

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    μk1|σk|Pjσkλj.(B46)

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    σkRn|σk|=σkRnTr(Πσk)=4n.(B47)

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    Fcab=4nσkRn|σk|μk4nσkRnPjσkλj=Fccb(Λ),(B48)

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    Fccb(Λ)=14njλj,(C1)

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    Δλj:=|ES1,S2[λ^j]λ¯j|,(C6)

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    λ¯j=(f¯j(m2)f¯j(m1))1m2m1.(C7)

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    f¯j(m1)=Es1Sm1[f^j(m1,s1)],f¯j(m2)=Es2Sm2[f^j(m2,s2)].(C8)

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    ES1,S2[λ^j]=ES2[f^j(m2)1m2m1]ES1[f^j(m1)1m2m1].(C9)

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    b^:=f^j(m2),b¯:=f¯j(m2),a^:=f^j(m1),a¯:=f¯j(m1),t:=1m2m1,(C10)

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    ES1,S2[λ^j]=ES1[a^t]ES2[b^t].(C11)

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    δa:=a^a¯a¯1,δb:=b^b¯b¯1,(C12)

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    a^t=a¯t(1+δa)t=a¯t(1tδa+t(t+1)δa2+O(δa3)),b^t=b¯t(1+δb)t=b¯t(1+tδb+t(t1)δb2+O(δb3)).(C13)

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    ES1[a^t]=a¯t(1+t(t+1)Var[a^]a¯2+O(δa3)),ES2[b^t]=b¯t(1+t(t1)Var[b^]b¯2+O(δb3)).(C14)

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    ES1,S2[λ^j]=(b¯a¯)t(1+t(t+1)Var[a^]a¯2)(1+t(t1)Var[b^]b¯2)+O(δa3,δb3)=λ¯j(1+t(t+1)Var[a^]a¯2+t(t1)Var[b^]b¯2)+O(δa3,δb3).(C15)

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    Δλj=λ¯j(t(t+1)Var[a^]a¯2+t(t1)Var[b^]b¯2)+O(δa3,δb3).(C16)

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    Pr(|XE[X]|>ϵ)2exp(nϵ2/2V2+bϵ/3).(C17)

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    Pr(|f^j(m)f¯j(m)|>ϵ)2exp(Kϵ2/2Var[f^j(m,s)]+ϵ/3).(C18)

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    Pr(|a^a¯|>ϵ1)2exp(K1ϵ12/2Var[f^j(m1,s1)]+ϵ1/3):=ϒa(K1,ϵ1),Pr(|b^b¯|>ϵ2)2exp(K2ϵ22Var[f^j(m2,s2)]+ϵ2/3):=ϒb(K2,ϵ2).(C19)

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    Δλj1(m2m1)2|1+m2m1K1f¯j2(m1)Var[f^j(m1,s1)]+1m2+m1K2f¯j2(m2)Var[f^j(m2,s2)]|+O(ϵ13,ϵ23):=ϵb(m1,m2;K1,K2;ϵ1,ϵ2),(C20)

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    F¯ccb=1M{Pj}λ¯j,F¯ccb=14njλ¯j.(C21)

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    Pr(|XE[X]|>ϵ)2exp(2n2ϵ2i=1n(biai)2).(C22)

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    Pr(|F¯ccbF¯ccb|>ϵM)2exp(2MϵM2).(C23)

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    Pr(|F^ccbF¯ccb|>ϵb)Mϒj(K1,K2;ϵ1,ϵ2),(C24)

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    Pr(|F^ccbF¯ccb|>ϵM+ϵb)2exp(MϵM2/2)+Mϒj(K1,K2;ϵ1,ϵ2).(C25)

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    0Var[f^j(m1,s1)]1,0Var[f^j(m2,s2)]1,(C26)

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    Pr(|F^ccbF¯ccb|>ϵM+ϵb)2exp(MϵM2/2)+2Mexp(K1ϵ12/21+ϵ1/3)+2Mexp(K2ϵ22/21+ϵ2/3).(C27)

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    12<f¯j(m1)<1,(C28)

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    Pr(|F^ccbF¯ccb|ϵM+ϵb)1δ,(C29)

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    ϵb4K11m2m1(1m2m1+1)+O(ϵ13,ϵ23),(C30)

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    2exp(2MϵM2)+2Mexp(K1ϵ12/21+ϵ1/3)+2Mexp(K2ϵ22/21+ϵ2/3)=δ.(C31)

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    CTX=(IT)CNOT(IT1).(D6)

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    Pctx=(IT)P2(IT1).(D7)

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    CT˜X=CTXΛt(μ,σ),CTX˜1=CTX1Λt(μ,σ).(D8)

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    P˜ctx=ΛrefPctx.(D9)

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    ρψ=Λspam(|ψψ|),Q˜=QΛspam.(D10)

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    U˜=UΛt.(D19)

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    fk(m)=1KScabQ˜k|S˜cab|ρψ=1KScabTr(QkS˜cab(|00|))=1KScabztkzTr(|zz|S˜cab(|00|)),(D24)

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    CZ˜=CZΛ2Λ1Λ0.(D30)

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    the number of circuit depths×sample complexity for one depth×single shots for one sequence.(D31)

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    2n×the number of circuit depths×sample complexity for one depth×single shots for one sequence.(D32)

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    Yihong Zhang, Wenjun Yu, Pei Zeng, Guoding Liu, Xiongfeng Ma. Scalable fast benchmarking for individual quantum gates with local twirling[J]. Photonics Research, 2023, 11(1): 81
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