Wigner's Friend as a Circuit: Inter-Branch Communication Witness Benchmarks on Superconducting Quantum Hardware

TL;DR

This work operationalizes Wigner's friend as a circuit for inter-branch communication on superconducting hardware, introducing coherence witnesses $W_X$, $W_Y$, and the phase-sensitive magnitude $C_ ext{mag}$ alongside the population-based visibility $V$. Using a five-qubit branch-transfer circuit implemented on IBM Quantum hardware, it demonstrates a reproducible constraint pipeline that compares hardware results to backend-matched noise models and quantifies potential nonunitary perturbations. The results show hardware values consistent with unitary evolution under device noise, providing a practical baseline and a method to bound nonunitary channels via the observed witnesses. By delivering full provenance, a rigorous noise-model framework, and a plan for cross-modality replication, the work offers a concrete toolkit for future tests of nonideal dynamics and interpretations while clarifying the operational scope of such experiments.

Abstract

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.

Figures (3)

• Figure 1: Branch-transfer circuit schematic (five-qubit primitive) as transpiled and executed on hardware. Coherence-witness measurements act on the four-qubit subset $(Q,R,F,P)$; the auxiliary qubit participates in the controlled transfer but is not measured in coherence-witness mode. Qubit labels correspond to logical register assignments; physical qubit indices after transpilation are recorded in the reproducibility bundle.
• Figure 2: Primary hardware vs. simulation comparisons: (a) coherence witnesses across ideal, backend-matched noisy simulation, and hardware; (b) population visibility across the same conditions. Provenance and SHA256 hashes are recorded in assets/bundles/wigner-friend-v2b/MANIFEST.json.
• Figure 3: Dephasing-channel forecast for the branch-transfer circuit under (a) ideal and (b) backend-matched noisy conditions.