Source-linked AI summary
Wigner's Friend as a Circuit: Inter-Branch Communication Witness Benchmarks on Superconducting Quantum Hardware
Christopher Altman
TL;DR
The paper addresses how inter-branch communication witnesses can be operationally benchmarked without treating the experiment as a test of quantum interpretations. It implements Violaris-inspired circuits on IBM hardware with coherence diagnostics and calibrated noise modeling, obtaining degraded but unitary-consistent witness values and a pipeline for constraining specified non-ideal channels.
Problem
Wigner’s-friend-inspired circuit experiments need operational diagnostics for branch-transfer correlations while remaining distinct from claims about interpreting quantum mechanics.
Method
The paper executes a five-qubit branch-transfer circuit on IBM hardware, measures population and coherence witnesses, and compares results with backend-matched noise models and parameterized channel constraints.
Results
Hardware measurements remain consistent with unitary evolution plus device noise, with visibility V = 0.8771 lower than the noisy simulation prediction V = 0.9381.
Takeaways & Limitations
The experiment establishes reproducible baseline measurements and a methodology for assessing detectability of specified nonunitary perturbations relative to calibrated device noise.
Takeaways & Limitations
The channel constraint applies only to the assumed channel family, insertion point, and noise calibration, and does not uniquely identify physical mechanisms.
Abstract
from arXiv · showhide
We implement and benchmark on IBM Quantum hardware the circuit family proposed by Violaris for estimating operational inter-branch communication witnesses, defined as correlations in classical measurement records produced by compiled Wigner's-friend-style circuits. We realize a five-qubit instance of the protocol as an inter-register message-transfer pattern within a single circuit, rather than physical signaling, and evaluate its behavior under realistic device noise and compilation constraints. The circuit encodes branch-conditioned evolution of an observer subsystem whose dynamics depend on a control qubit, followed by a controlled transfer operation that probes correlations between conditional measurement contexts. Executing on the ibm_fez backend with 20000 shots, we observe population-based visibility of 0.877, coherence witnesses of 0.840 and -0.811 along orthogonal axes, and a phase-sensitive magnitude of approximately 1.17. While the visibility metric is insensitive to some classes of dephasing, the coherence witnesses provide complementary sensitivity to off-diagonal noise. This work does not test or discriminate among interpretations of quantum mechanics. Instead, it provides a reproducible operational constraint pipeline for evaluating detectability of non-ideal channels relative to calibrated device noise.
1 Introduction
This work translates inter-branch communication protocols into reproducible quantum-circuit benchmarks on IBM hardware, emphasizing coherence diagnostics and calibrated noise constraints rather than interpretation-level tests.
- Motivation: Violaris’s inter-branch communication protocols provide circuit templates for probing branch-interference correlations on near-term quantum processors.The protocols use operations on a superposed friend register to enable conditional message transfer between branch-conditioned internal states.
- Contributions: The benchmark provides a publicly reproducible IBM-hardware execution of a minimal branch-transfer circuit.The reproducibility package includes job IDs, calibration snapshots, software versions, and SHA256 hashes for independent verification.
- Contributions: Coherence-witness diagnostics supplement population-based visibility with multi-qubit Pauli-parity correlators sensitive to off-diagonal coherences.These correlators are intended to detect coherence loss that population measurements may miss.
- Contributions: Backend-matched Qiskit Aer simulations compare hardware results against noise models derived from contemporaneous ibm fez calibration data.This comparison targets device-noise behavior under realistic hardware conditions.
- Contributions: Parameterized dephasing channels are constrained relative to observed witness values and device-noise bands.The pipeline operationalizes detectability thresholds for specified non-ideal channels without assigning interpretation-specific meanings.
- Scope: The experiment does not uniquely test or confirm interpretations of quantum mechanics because unitary completions can predict identical statistics for these circuits.Its stated value is methodological: establishing hardware baselines and a framework for constraining specific nonunitary perturbations.
2 Protocol and Metrics
The five-qubit branch-transfer circuit uses branch-conditioned evolution and controlled message transfer, then measures population visibility or four-qubit coherence witnesses. These observables probe complementary aspects of the resulting correlations, with visibility relying on computational-basis populations and coherence witnesses probing off-diagonal structure.
- Registers and Qubit Mapping: The circuit assigns Q as the branch-control qubit, R as the message-encoding reference, F as the observer-state register, P as the readout probe, and a fifth auxiliary qubit to controlled transfer.The auxiliary qubit is not measured in coherence-witness mode.
- Protocol Stages: The protocol prepares branch-conditioned evolution, applies controlled message transfer, and measures either computational-basis visibility or rotated-basis coherence witnesses.Visibility measures R and P, whereas coherence witnesses measure Q, R, F, and P.
- Visibility: Visibility V is extracted from conditional Z-basis populations using P(R = 0 | P = 1) − P(R = 1 | P = 1).The conditional probabilities refer to measuring R given the outcome on P.
- Coherence Witnesses: The four-qubit coherence witnesses are WX = ⟨XQ ⊗ XR ⊗ XF ⊗ XP⟩ and WY = ⟨YQ ⊗ YR ⊗ YF ⊗ YP⟩ on (Q, R, F, P).X and Y are standard Pauli operators; basis rotations precede Z-basis measurement.
- Phase-Sensitive Magnitude: Cmag combines WX and WY into a phase-sensitive magnitude that is not a probability or coherence fraction and is bounded above by √2.The supplied equation text is incomplete, but the passage states the interpretation and bound.
3 Implementation on IBM Quantum Hardware
The experiment executes the five-qubit circuits on ibm fez and compares hardware with backend-matched Aer simulations based on contemporaneous calibration data. The modeling provides a calibrated noise-floor proxy, while hardware results remain subject to effects not captured by calibration snapshots.
- Hardware Execution: The circuits were executed on ibm fez with 20,000 shots per circuit using Qiskit IBM Runtime.Hardware runs used transpiler optimization level 2 and included coherence-witness and rp z visibility modes.
- Reproducibility: Hardware provenance includes job IDs, software versions, calibration settings, and SHA256 hashes for the reproducibility bundle.The listed software includes Qiskit 2.3.0, Qiskit Aer 0.17.2, and qiskit-ibm-runtime 0.45.0.
- Backend-Matched Noise Modeling: Baseline Aer simulations use NoiseModel.from_backend() with contemporaneous ibm fez calibration snapshots.The snapshots capture gate errors, readout errors, and T1/T2 times at execution.
- Backend-Matched Noise Modeling: Backend-matched noisy simulation is treated as a calibrated proxy for the noise floor rather than ground truth.The model may omit drift, crosstalk, leakage, and coherent errors not characterized by calibration data.
4 Results
The results compare ideal predictions, backend-matched noisy simulation, and hardware for coherence witnesses and population visibility. Hardware values degrade relative to ideal and noisy-simulation baselines but remain consistent with unitary evolution plus device noise.
- Interpretation of Results: Hardware visibility V = 0.8771 is lower than the backend-matched noisy simulation prediction V = 0.9381.The passage attributes the difference as potentially reflecting calibration drift, uncaptured noise, or transpilation effects.
- Interpretation of Results: Hardware coherence witnesses WX and WY show degradation beyond the backend-matched noisy simulation baseline.The supplied passages do not provide separate hardware values for WX and WY in this section.
- Interpretation of Results: All measured values remain consistent with unitary evolution plus device noise, with no anomalous deviations suggesting nonunitary physics observed.The results establish a baseline for future comparisons.
- Hardware–Simulation Comparisons: Figure 2 compares ideal, backend-matched noisy simulation, and hardware for coherence witnesses and population visibility.Panel (a) covers WX, WY, and Cmag; panel (b) covers visibility V.
5 Nonunitary Channel Constraint Pipeline
The pipeline converts witness measurements into bounds on parameterized nonunitary channels inserted at specified circuit locations. It uses predicted witness deviations and backend-matched uncertainty bands to identify detectable channel strengths.
- The pipeline defines a channel family, inserts it at a specified circuit point, and computes predicted V, WX, WY, and Cmag across λ.
- A channel strength λ is detectable when at least one witness deviation exceeds the combined shot-noise and device-noise uncertainty band.
- Observed hardware values constrain the channel strength to λ < λmax, where λmax is the detectability threshold.
- For phase-flip dephasing on the friend register after branch splitting, observables containing Pauli X or Y are attenuated by (1−2λ).
- Backend-matched noisy simulation supplies the detectability band and a conservative λmax for the chosen insertion point and channel family.
- Coherence witnesses are primary diagnostics because diagonal visibility can remain comparatively insensitive to dephasing depending on insertion location.
6 Interpretation
The experiment establishes consistency with unitary predictions within device noise while providing no interpretation-level discrimination. Its practical result is a methodology for constraining specific nonunitary channels, with coherence witnesses complementing visibility.
- The work does not uniquely test, confirm, or refute interpretations that reproduce standard quantum predictions for these circuits.
- The branch-transfer circuit behaves consistently with unitary predictions within device noise on IBM Quantum hardware.
- Coherence witnesses WX and WY provide off-diagonal sensitivity complementary to population-based visibility.
- The experiment provides a methodology for constraining specific nonunitary channel families relative to hardware noise floors.
- Future interpretation relevance would require statistically robust deviations persisting across multiple independently calibrated backends and varied transpilation or mitigation strategies.
7 Conclusions and Future Work
The paper presents a reproducible IBM-hardware proof of concept combining inter-branch circuit primitives, coherence diagnostics, backend-matched noise modeling, and a nonunitary-channel constraint pipeline. Future work expands replication across platforms, branch complexity, calibration conditions, and error mitigation.
- 7 Conclusions and Future Work: The work establishes baseline measurements and methodology without claiming interpretation-level conclusions.
- 7 Conclusions and Future Work: Follow-up studies aim to replicate the message-transfer primitive across superconducting, ion-trap, neutral-atom, photonic, and annealing-style platforms.
- 7 Conclusions and Future Work: Increasing branch divergence represents friend states with k-qubit bitstrings and turns branch swapping into longer X-strings for complexity-versus-coherence benchmarks.
- 7 Conclusions and Future Work: Multi-backend replication can assess reproducibility and identify backend-specific artifacts.
- 7 Conclusions and Future Work: Calibration-synchronized repeats can reduce drift effects by running experiments immediately after fresh calibration cycles.
- 7 Conclusions and Future Work: Randomized compiling, Pauli twirling, and zero-noise extrapolation may enable tighter constraints on nonunitary channels.
8 Reproducibility
The paper provides public source code, analysis artifacts, calibration metadata, job provenance, and archived release bundles for reproducing the reported workflow. The commands cover hardware execution, backend-matched simulation, figure generation, and environment verification.
- 8 Reproducibility: Source code, analysis notebooks, calibration metadata, and job provenance are publicly available and archived on Zenodo.
- 8 Reproducibility: The dedicated release bundle contains execution scripts, IBM Quantum job IDs, calibration snapshots, generated plots, and a SHA256 manifest.
- 8 Reproducibility: Independent verification requires Qiskit 2.3.0 and the packages listed in requirements.txt.
- 8 Reproducibility: The recorded workflow includes IBM hardware commands for coherence witnesses and visibility using 20000 shots on ibm_fez at optimization level 2.
- 8 Reproducibility: Backend-matched simulations use noise derived from ibm_fez for both coherence-witness and visibility modes.
- 8 Reproducibility: Analysis commands generate all figures, while supplementary diagnostics include measurement distributions and optimization-level sensitivity.