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

TL;DR

This paper demonstrates the implementation and benchmarking of a quantum circuit designed to estimate inter-branch communication witnesses on IBM Quantum hardware, analyzing its behavior under realistic noise conditions.

Contribution

It introduces a novel circuit-based approach for estimating communication witnesses in Wigner's friend scenarios using superconducting qubits, with experimental validation on real hardware.

Findings

Achieved high visibility and coherence witness values on IBM Quantum hardware.

Provided a reproducible pipeline for evaluating non-ideal channels relative to device noise.

Showed the circuit's sensitivity to certain noise classes despite limitations.

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.