Cited by 502Quantum Computing Algorithms and ArchitectureQuantum Information and CryptographyQuantum Mechanics and Applications

Scalable noise estimation with random unitary operators

Open access
Emerson, Joseph, Alicki, Robert, Życzkowski, Karol·2005·Journal of Optics B: Quantum and Semiclassical Optics

Abstract

We describe a scalable stochastic method for the experimental measurement of generalized fidelities characterizing the accuracy of the implementation of a coherent quantum transformation. The method is based on the motion reversal of random unitary operators. In the simplest case our method enables direct estimation of the average gate fidelity. The more general fidelities are characterized by a universal exponential rate of fidelity loss. In all cases the measurable fidelity decrease is directly related to the strength of the noise affecting the implementation, quantified by the trace of the superoperator describing the non-unitary dynamics. While the scalability of our stochastic protocol makes it most relevant in large Hilbert spaces (when quantum process tomography is infeasible), our method should be immediately useful for evaluating the degree of control that is achievable in any prototype quantum processing device. By varying over different experimental arrangements and error-correction strategies, additional information about the noise can be determined.

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Written from the paper’s full text.

TL;DR

The paper proposes a scalable stochastic protocol for estimating noise in quantum implementations by applying random unitary operators and their inverses (motion reversal), enabling direct estimation of average gate fidelity and generalized fidelities. It shows the measurable fidelity loss depends on the trace of the cumulative noise superoperator and can be estimated with accuracy \(O(1/\sqrt{DN})\) for \(N\) independent experiments, avoiding the exponential scaling of quantum process tomography.

Key findings

  • Under Haar-random motion reversal, the average gate fidelity depends only on a single noise-strength parameter \(p \in [0,1]\), equivalently \(\mathrm{Tr}(\hat{\Lambda})\), and can be read out as the residual population of the initial computational basis state.
  • The method estimates noise strength with accuracy \(O(1/\sqrt{DN})\) for \(N\) independent experiments, where \(D\) is the Hilbert-space dimension. In contrast, quantum process tomography requires \(O(D^4)\) experiments and manipulation of \(D^2 \times D^2\) matrices; Nielsen’s direct \(F_g\) method requires \(D^4\) experiments but only \(D \times D\) matrices.
  • Generalized iterated motion-reversal and Loschmidt-echo sequences yield a universal exponential decay of averaged fidelity depending only on noise strength; as the number of iterations \(n\) grows, \(F_n \to D^{-1}\).
  • For continuous-time weak Markovian noise, under an ergodic hypothesis and small noise parameter \(\epsilon\), the paper derives a universal exponential fidelity decay. A generalized Loschmidt echo with noise in both forward and backward sequences is conjectured to satisfy relation (31), with analytic and numerical evidence for unitary noise.

Methods

  • Theoretical/analytical protocol: apply a Haar-random unitary \(U\) and its inverse \(U^{-1}\) to an initial state, average over the Haar measure, and use superoperator representation. The Haar-averaged superoperator is \(U(D)\)-invariant and equivalent to a depolarizing channel characterized by \(\mathrm{Tr}(\hat{\Lambda})\).
  • Generalized discrete-time analysis: iterated motion reversal and Loschmidt-echo sequences with independent random unitaries, arbitrary mixed initial states, and averaging over Haar measure.
  • Continuous-time analysis: Markovian master equation with controlled reversible dynamics and a small noise parameter \(\epsilon\), using a cumulant expansion and an ergodic hypothesis for the time-dependent generator.

Limitations

  • The paper states the only inefficiency is the requirement to experimentally implement a Haar-random unitary: its decomposition into elementary one- and two-qubit gates requires an exponentially long gate sequence. It leaves open whether efficient random circuits can provide adequate randomization.
  • The paper states it assumes the noise superoperator is independent of the specific target transformation, but notes that actual noise generally depends sensitively on the target \(U\) and the gate sequence; it argues this assumption may become statistically well motivated but does not prove it.
  • The generalized Loschmidt-echo relation (31) is not directly evaluated analytically in the general case; the paper reports only analytic and numerical evidence for unitary noise and conjectures it holds under general noise. Continuous-time results rely on the stated ergodic hypothesis and small-\(\epsilon\), long-time conditions.

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Integrity signals

Registry

Crossref · 10.1088/1464-4266/7/10/021

Cites

24 of 24 references verified0 citation errors · 0 not checkable

Statistics

Nothing to recompute.

Claims the paper makes

0 of 12 cited sources checked · 0 supported · 0 flagged

  • Findingnone given
    § Scalable Noise Estimation with Random Unitary Operators · p. 1

    The more general fidelities are characterized by a universal exponential rate of fidelity loss.

    “The more general fidelities are characterized by a universal exponential rate of fidelity loss.”
    Quote in the paperNot a substantial claim
  • Findingnone given
    § Scalable Noise Estimation with Random Unitary Operators · p. 1

    In all cases the measurable fidelity decrease is directly related to the strength of the noise affecting the implementation, quantified by the trace of the superoperator describing the non-unitary dynamics.

    “In all cases the measurable fidelity decrease is directly related to the strength of the noise affecting the implementation - quantified by the trace of the superoperator describing the non-unitary dynamics.”
    Quote in the paperNot a substantial claim
  • Findingreasoning
    § GENERALIZED FIDELITIES IN A DISCRETE-TIME SCENARIO · p. 4

    Assuming the cumulative noise for each step has the same strength p_j = p, one obtains for arbitrary noise a universal exponential form for the decay of fidelity in the Loschmidt echo sequence.

    “As before, we can simplify this result by assuming that the cumulative noise for each step has the same strength ( p j = p ), in which case we obtain for arbitrary noise a universal exponential form for the decay of fidelity”
    Quote in the paperSubstantial claim
    • The authors state this as an assumption: The cumulative noise for each U_j, or each step, has the same strength p_j = p.
  • Findingcitation
    § GENERALIZED FIDELITIES IN A DISCRETE-TIME SCENARIO · p. 4

    In the limit of large n, F_n approaches D^{-1}, as may be expected from the average fidelity between random states.

    “In the limit of large n , we see that F n → D -1 , as may be expected from the average fidelity between random states [16].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [16] source

      Not checked in the free run.

  • Findingreasoning
    § GENERALIZED FIDELITIES IN A DISCRETE-TIME SCENARIO · p. 4

    For an iterated motion reversal sequence, the Haar-averaged noise operator is a depolarizing channel with a strength parameter.

    “As noted above and shown in the Appendix, ˆ Λ ave ≡ ∫ dU ˆ U ˆ Λ ˆ U -1 is a depolarizing channel with strength parameter”
    Quote in the paperSubstantial claim
  • May combine two claims. 4
  • Classified as probably not a substantial claim. 6
  • May combine two claims.

    Findingreasoning
    § GENERALIZED FIDELITIES FOR CONTINUOUS-TIME WEAK NOISE · p. 6

    For small enough ε and long enough times t such that εt is fixed, the first term dominates and a specific expression can be written.

    “Therefore for small enough ǫ and long enough times t such that ǫt is fixed the first term dominates and we can write”
    Quote in the paperSubstantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingnone given
    § APPENDIX: HAAR AVERAGED SUPEROPERATORS · p. 7

    Any superoperator Λ possesses exactly two linear U(D) invariants.

    “Any superoperator ˆ Λ possesses exactly two linear U ( D ) invariants, i.e. the linear functionals on superoperator space which are invariant with respect to all transformation of the form ˆ Λ ↦→ ˆ U ˆ Λ ˆ U † :”
    Quote in the paperNot a substantial claim
  • Probably not a substantial claim: may not pass S3 (load-bearing).

    Findingreasoning
    § APPENDIX: HAAR AVERAGED SUPEROPERATORS · p. 8

    From the fact that Tr(Λ_ave) = Tr(Λ), a specific expression is obtained.

    “From the fact that Tr( ˆ Λ ave ) = Tr( ˆ Λ) we obtain”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Findingreasoning
    § EFFICIENT ESTIMATION OF THE AVERAGE GATE FIDELITY · p. 2

    The Haar-averaged superoperator Λ_ave is U(D)-invariant and thus can be expressed as a depolarizing channel.

    “As shown in the Appendix, the Haar-averaged superoperator ˆ Λ ave is U ( D ) -invariant and thus can be expressed as a depolarizing channel”
    Quote in the paperSubstantial claim
  • Classified as probably not a substantial claim.

    Premisecitation
    § INTRODUCTION · p. 1

    Quantum process tomography (QPT) is the exact method for characterizing the noise affecting an implementation.

    “The exact method for characterizing the noise affecting an implementation is quantum process tomography (QPT) [8].”
    Quote in the paperSubstantial claim0 of 1 sources checked
    • cites [8] Nielsen M A, 2000, Quantum Computation and Quantum Information

      Not checked in the free run.

  • Probably not a substantial claim: may not pass S3 (load-bearing), S4 (based).

    Interpretationnone given
    § Scalable Noise Estimation with Random Unitary Operators · p. 1

    By varying over different experimental arrangements and error-correction strategies, additional information about the noise can be determined.

    “By varying over different experimental arrangements and error-correction strategies additional information about the noise can be determined.”
    Quote in the paperNot a substantial claim
  • Probably not a substantial claim: may not pass S2 (checkable).

    Interpretationreasoning
    § Scalable Noise Estimation with Random Unitary Operators · p. 1

    The method should be immediately useful for evaluating the degree of control achievable in any prototype quantum processing device.

    “our method should be immediately useful for evaluating the degree of control that is achievable in any prototype quantum processing device.”
    Quote in the paperNot a substantial claim
  • Probably not a substantial claim: may not pass S4 (based).

    Premisenone given
    § GENERALIZED FIDELITIES FOR CONTINUOUS-TIME WEAK NOISE · p. 6

    The ergodic hypothesis conditions are satisfied if the time-dependent dynamics can be modelled by a random walk on U(D) or by a trajectory on U(D) given by a deterministic dynamics with strong enough ergodic properties.

    “These conditions are satisfied , for instance if the time-dependent dynamics t ↦→ U ( t ) can be modelled by a random walk on the group U ( D ) or by a trajectory on U ( D ) given by a certain deterministic dynamics with strong enough ergodic properties.”
    Quote in the paperNot a substantial claim
  • May combine two claims.

    Premisecitation
    § EFFICIENT ESTIMATION OF THE AVERAGE GATE FIDELITY · p. 2

    The average fidelity depends only on the noise operator and can be expressed in a known form.

    “Due to the invariance of the Fubini-Study measure the average fidelity depends only on the noise operator and can been expressed in the form [9, 10, 11]”
    Quote in the paperSubstantial claim0 of 3 sources checked
    • cites [9] source

      Not checked in the free run.

    • cites [10] source

      Not checked in the free run.

    • cites [11] source

      Not checked in the free run.

  • May combine two claims.

    Interpretationcitation
    § GENERALIZED FIDELITIES IN A DISCRETE-TIME SCENARIO · p. 4

    Due to the concentration of measure, for large D the fidelity loss under iterated motion reversal of a single sequence of random unitary operators will be exponentially close to the Haar-average, and hence the noise strength can be estimated with only a few experimental runs.

    “Most importantly, due to the concentration of measure (14), for large D the fidelity loss under iterated motion reversal of a single sequence of random unitary operators will be exponentially close to the Haar-average, and hence the noise strength can be estimated with only a few experimental runs.”
    Quote in the paperSubstantial claim
10 of 10
  • The authors state this as an assumption.

    The cumulative noise for each U_j, or each step, has the same strength p_j = p.

    “we can further simplify this result by assuming that the cumulative noise for each U j has the same strength p j = p”
    “As before, we can simplify this result by assuming that the cumulative noise for each step has the same strength ( p j = p )”
  • The authors state this as an assumption.

    The ergodic mean exists and is equal to the Haar average.

    “We assume now the following ergodic hypothesis: a) the ergodic mean exists and is equal to the Haar average”
  • The authors state this as an assumption.

    The fluctuations δL(t) around the ergodic mean are normal for long t.

    “b) the fluctuations δ ˆ L ( t ) ≡ ˆ L ( t ) -ˆ L ave around ergodic mean are normal, i.e. for long t”
  • The authors state this as an assumption.

    The noise superoperator Λ is independent of the specific target transformation but depends only on the duration of the experiment.

    “We have throughout our analysis made the usual assumption that the noise superoperator Λ is independent of the specific target transformation but depends only on the duration of the experiment.”
4 of 4

Claims other papers make about this one

Each is checked against this paper’s own text: does it say what the citing paper says it does?

  • The uniform probability distribution over Clif_n comprises a unitary 2-design.

    “One extremely useful property of Clif n , especially for noise estimation, is that the uniform probability distribution over Clif n comprises a unitary 2-design [17].”

    From Characterizing quantum gates via randomized benchmarking (2012) · cites this paper as [17]

Cites

Flagged first, then by citations

Research works printed but not linked: 1

  1. Alicki R, 1987, Quantum Dynamical Semigroups and their Applications

Cited by

Sorted by citations

Authors
Emerson, J., Alicki, R., & Życzkowski, K.
Affiliations
  • Perimeter Institute (CA)
  • University of Gdańsk (PL)
  • Jagiellonian University (PL)
Venue
Journal of Optics B: Quantum and Semiclassical Optics
Published
2005-09-21
Volume
7
Issue
10
Pages
S347-S352
ISSN
  • 1464-4266 (ISSN-L)
  • 1741-3575
OpenAlex
W2148118219
Cited by
502
References
24
Citations by year
2026: 33 · 2025: 67 · 2024: 52 · 2023: 47 · 2022: 42 · 2021: 58 · 2020: 41 · 2019: 40 · 2018: 15 · 2017: 17 · 2016: 19 · 2015: 16 · 2014: 12 · 2013: 5 · 2012: 8
FWCI
5.97
Type
journal article
Language
en
Open access
Locations
  • Journal of Optics B Quantum and Semiclassical Optics (publishedVersion)
  • arXiv (Cornell University) (publishedVersion)
  • CERN Document Server (European Organization for Nuclear Research) (submittedVersion)
  • arXiv (Cornell University)
Topics
  • Physical Sciences › Computer Science › Artificial Intelligence › Quantum Computing Algorithms and Architecture
  • Physical Sciences › Computer Science › Artificial Intelligence › Quantum Information and Cryptography
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Quantum Mechanics and Applications
Keywords
noise estimation, average gate fidelity, quantum process tomography, quantum control, non-unitary dynamics, superoperators, fidelity loss, error correction, scalable quantum devices

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