Wigner’s Friend Circuit Achieves 0.877 Inter-Branch Communication Witness Fidelity on IBM

Scientists are probing the foundations of quantum mechanics with a novel circuit designed to detect correlations arising from multiple measurement contexts, as exemplified in the thought experiment known as Wigner’s Friend. Christopher Altman from Astradyne, alongside colleagues, have implemented and benchmarked this circuit on superconducting quantum hardware, demonstrating a five-qubit instance capable of revealing ‘inter-branch communication’ without physical signalling. Their research, detailed in a new paper, establishes a reproducible method for evaluating how effectively these subtle quantum effects can be observed amidst the noise inherent in real quantum devices, achieving visibility of 0.877 and significant coherence witness values. This work doesn’t attempt to resolve interpretations of quantum mechanics, but instead provides a crucial operational benchmark for assessing the detectability of non-ideal quantum channels and pushing the boundaries of quantum hardware capabilities.

Wigner’s Friend Protocol and Quantum Witnesses

Scientists have demonstrated a five-qubit implementation of a circuit designed to estimate inter-branch communication witnesses, crucial for exploring foundational questions in quantum mechanics. The research team realized a Wigner’s-friend-style protocol as an inter-register message-transfer pattern within a single quantum circuit, meticulously evaluating its behaviour under realistic device noise and compilation constraints. This innovative approach avoids physical signalling, instead focusing on correlations arising from branch-conditioned evolution of an observer subsystem, dependent on a control qubit, followed by a controlled transfer operation that probes these correlations. Executing the circuit 20,000times on the ibm_fez backend, the team observed a population-based visibility of 0.877, alongside coherence witnesses of 0.840 and -0.811 along orthogonal axes, and a phase-sensitive magnitude of approximately 1.17.
The study unveils a reproducible operational constraint pipeline for evaluating the detectability of non-ideal channels relative to calibrated device noise, offering a powerful tool for future investigations. The circuit encodes a branch-conditioned evolution where a “Wigner’s friend” (observer subsystem) evolves differently based on the state of a control qubit, followed by a controlled transfer operation that probes correlations between these conditional measurement contexts. Researchers measured multi-qubit Pauli-parity correlators, WX and WY, sensitive to off-diagonal coherences, complementing the standard population-based visibility metric, V. By comparing hardware results with Qiskit Aer simulations using noise models derived from contemporaneous ibm_fez calibration data, the team achieved backend-matched noise modelling for enhanced accuracy and reliability.

This work establishes a methodology for constraining parameterized dephasing channels relative to observed witness values and device-noise bands, providing a concrete framework for future experiments. The team meticulously documented the experiment with full provenance, including job IDs, calibration snapshots, and software versions, ensuring independent verification and extension of their findings. The five-qubit instance utilizes a specific qubit mapping: Q (control qubit), R (reference/ancilla), F (friend register), P (probe qubit), and an auxiliary qubit for the branch-transfer primitive, with logical indices mapped to physical qubit indices recorded in the reproducibility bundle. The visibility metric, V, was extracted from Z-basis measurement populations, calculated as P(R = 0 | P = 1) −P(R = 1 | P = 1), providing a population-based diagnostic of the system.
Experiments show that while the visibility metric is insensitive to certain dephasing effects, the coherence witnesses offer complementary sensitivity to off-diagonal noise, enhancing the diagnostic capabilities of the protocol. The research does not attempt to test or discriminate among interpretations of quantum mechanics, such as the Many-Worlds interpretation or Copenhagen-style collapse models, as these all predict identical measurement statistics for unitary circuits. Instead, the team frames the experiment as a demonstration that branch-transfer primitives behave consistently with unitary predictions on real hardware, establishing a foundation for constraining specific nonunitary perturbations in future studies. This breakthrough opens avenues for exploring the boundaries between quantum and classical realms, and refining our understanding of measurement and observer perspectives in quantum mechanics.

Five-Qubit Witness of Wigner’s-Friend Correlations reveals quantum reality

Scientists engineered a five-qubit circuit to investigate operational inter-branch communication witnesses, focusing on correlations arising from compiled Wigner’s-friend circuits. The study realized this protocol as an inter-register message-transfer pattern contained within a single circuit, eschewing physical signaling for an internal, controlled process, a crucial design choice for isolating the core phenomenon under investigation. Researchers meticulously constructed the circuit to encode branch-conditioned evolution of an observer subsystem, dynamically altering its behaviour based on a control qubit, followed by a controlled transfer operation designed to reveal correlations between different measurement contexts. Experiments employed the ibm_fez backend, executing the circuit with 20,000 shots to gather statistically significant data, a substantial number ensuring reliable observation of subtle quantum effects.

The team measured population-based visibility, achieving a value of 0.877, alongside coherence witnesses registering at 0.840 and -0.811 along orthogonal axes, and a phase-sensitive magnitude of approximately 1.17. This precise quantification of key metrics allowed for detailed analysis of the circuit’s performance under realistic device noise and compilation constraints. The methodology deliberately avoids interpretations of quantum mechanics, instead establishing a reproducible pipeline for evaluating the detectability of non-ideal channels relative to calibrated device noise, a critical step towards robust quantum experimentation. The circuit utilized five qubits, logically assigned as follows: Q (control qubit), R (reference/ancilla), F (friend register), P (probe qubit), and an auxiliary qubit, the latter participating in the transfer but remaining unmeasured for coherence diagnostics.

Logical qubit assignments were meticulously mapped to physical qubit indices on the ibm_fez device, with a complete record of this mapping preserved in a reproducibility bundle for verification and future replication. The core of the experiment involved three distinct stages: preparation of branch-conditioned evolution, application of the controlled message-transfer primitive, and final measurement in either the computational basis (for visibility) or rotated bases (for coherence witnesses), a carefully sequenced process designed to isolate and amplify the desired quantum correlations. Visibility, V, was extracted from Z-basis measurement populations using the formula V = P(R = 0 | P = 1) −P(R = 1 | P = 1), providing a population-based diagnostic of the system’s coherence. Recognizing the limitations of visibility, its insensitivity to off-diagonal density matrix elements, scientists supplemented this metric with direct coherence measurements using four-qubit Pauli-parity correlators, WX and WY, calculated from expectation values of Pauli operators applied to the (Q, R, F, P) qubit subset. To measure WX, Hadamard gates were applied to each qubit before Z-basis measurement, and the expectation value computed from bitstring counts as WX = (Neven −Nodd) / (Neven + Nodd), where Neven and Nodd represent the number of shots with even and odd total parity, respectively, a sophisticated technique for extracting coherence information from noisy quantum data.

Five-Qubit Branch Transfer and Witness Measurements

Scientists achieved a population-based visibility of 0.877 when implementing and benchmarking a circuit family for estimating operational inter-branch communication witnesses on hardware. The team realized a five-qubit instance of the protocol as an inter-register message-transfer pattern, evaluating its behaviour under realistic device noise and compilation constraints. Experiments revealed coherence witnesses of 0.840 and -0.811 along orthogonal axes, alongside a phase-sensitive magnitude of approximately 1.17, all obtained from 20000 shots on the ibm_fez backend. These measurements demonstrate the successful implementation of a branch-transfer primitive consistent with unitary predictions on real hardware.

Results demonstrate the capability to measure multi-qubit Pauli-parity correlators, sensitive to off-diagonal coherences potentially missed by standard population-based visibility metrics. The work details a reproducible operational constraint pipeline for evaluating detectability of non-ideal channels relative to calibrated device noise, providing full provenance with job IDs, calibration snapshots, software versions, and SHA256 hashes for all artifacts. Data shows the circuit encodes branch-conditioned evolution of an observer subsystem, dependent on a control qubit, followed by a controlled transfer operation probing correlations between conditional measurement contexts. This methodology establishes a foundation for constraining specific nonunitary perturbations in future experiments designed to test alternative models of quantum mechanics.

The study utilized five qubits, assigning logical indices to the control qubit (Q), reference qubit (R), friend register (F), probe qubit (P), and an auxiliary qubit. Measurements confirm that the coherence witnesses WX and WY, four-qubit Pauli correlators measured on (Q, R, F, P), provide complementary sensitivity to off-diagonal noise, addressing a limitation of the visibility metric V which is insensitive to certain dephasing classes. Tests prove the visibility metric V, extracted from Z-basis measurement populations, is calculated as P(R = 0 | P = 1) −P(R = 1 | P = 1), offering a population-based diagnostic of the system. Furthermore, scientists measured WX and WY using a basis rotation procedure involving Hadamard and inverse phase gates applied before Z-basis measurement, calculating expectation values from bitstring counts. The phase-sensitive magnitude, Cmag, defined as q W 2X + W 2 Y, was determined to be approximately 1.17, offering insight into the coherence properties of the system. The breakthrough delivers a concrete circuit template suitable for near-term quantum processors, enabling independent verification and establishing a methodology for future investigations into nonunitary models.

Quantum Witnessing Achieves High Fidelity on IBM

Scientists have successfully implemented and benchmarked a circuit family designed to estimate operational inter-branch communication witnesses on quantum hardware. This research demonstrates a five-qubit instance of the protocol, realized as an inter-register message-transfer pattern within a single circuit, and evaluates its performance under realistic device noise and compilation constraints. The experiment, conducted on the ibm_fez backend, yielded a population-based visibility of 0.877, coherence witnesses of 0.840 and -0.811, and a phase-sensitive magnitude of approximately 1.17. The findings establish a reproducible operational constraint pipeline for evaluating the detectability of non-ideal channels relative to calibrated device noise, providing baseline measurements and methodology without drawing interpretive conclusions.

Researchers acknowledge that this work does not attempt to test or discriminate among interpretations of quantum mechanics, but rather focuses on establishing a robust experimental framework. Limitations include the specific hardware used and the scope of noise models considered, though the authors highlight the importance of exhaustive exclusion of hardware artifacts and systematic variation of circuit parameters. Future work will focus on replicating the experiment across multiple quantum computing modalities, including superconducting, ion-trap, neutral-atom, photonic, and annealing-style hardware, to quantify a cross-modality “compilation tax” and develop a generalized connectivity scaling law. Further extensions will explore branch divergence scaling and calibration-synchronized repeats, alongside the application of error mitigation techniques to refine the constraints on nonunitary channels.