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arXiv:2109.02298 · Physica Scripta

Tripartite Wigner’s friend scenario and its test

Dong Yan Ding, Can Wang, Y Q He, T Hou, Ting Gao, Fengli Yan

Published Jun 9, 2023• 2 citations
arXivHTMLPDF

Abstract

Abstract Wigner’s friend thought experiment is intended to reveal the inherent tension between unitary evolution and measurement collapse. Inspired by the Wigner’s friend experiment, Brukner derives a no-go theorem for observer-independent facts, focusing on two entangled observers in their respective laboratories. We construct an extended Wigner’s friend scenario, including three laboratories, namely, Alice’s laboratory, Bob’s laboratory and Charlie’s laboratory, where Alice, Bob and Charlie are standing outside the laboratories while their friends are placed inside their own laboratories. To realize this scenario, we create a universal quantum circuit, especially the circuit realization of the state evolution and measurement inside each laboratory, simple but novel. We carry out experiments on the IBM quantum computers to test the primary circuits, i.e. state preparation, state evolution and measurement inside each laboratory. Furthermore, we perform the simulation of the quantum circuit via Q# quantum programming on a classical computer.

DOI
10.1088/1402-4896/acdd33

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Abstract Wigner’s friend thought experiment is intended to reveal the inherent tension between unitary evolution and measurement collapse. Inspired by the Wigner’s friend experiment, Brukner derives a no-go theorem for observer-independent facts, focusing on two entangled observers in their respective laboratories. We construct an extended Wigner’s friend scenario, including three laboratories, namely, Alice’s laboratory, Bob’s laboratory and Charlie’s laboratory, where Alice, Bob and Charlie are standing outside the laboratories while their friends are placed inside their own laboratories. To realize this scenario, we create a universal quantum circuit, especially the circuit realization of the state evolution and measurement inside each laboratory, simple but novel. We carry out experiments on the IBM quantum computers to test the primary circuits, i.e. state preparation, state evolution and measurement inside each laboratory. Furthermore, we perform the simulation of the quantum circuit via Q# quantum programming on a classical computer.
Does quantum theory apply at all scales, including that of observers? New light on this fundamental question has recently been shed through a resurgence of interest in the long-standing Wigner’s friend paradox. This is a thought experiment addressing the quantum measurement problem—the difficulty of reconciling the (unitary, deterministic) evolution of isolated systems and the (non-unitary, probabilistic) state update after a measurement. Here, by building on a scenario with two separated but entangled friends introduced by Brukner, we prove that if quantum evolution is controllable on the scale of an observer, then one of ‘No-Superdeterminism’, ‘Locality’ or ‘Absoluteness of Observed Events’—that every observed event exists absolutely, not relatively—must be false. We show that although the violation of Bell-type inequalities in such scenarios is not in general sufficient to demonstrate the contradiction between those three assumptions, new inequalities can be derived, in a theory-independent manner, that are violated by quantum correlations. This is demonstrated in a proof-of-principle experiment where a photon’s path is deemed an observer. We discuss how this new theorem places strictly stronger constraints on physical reality than Bell’s theorem. For a scenario of two separated but entangled observers, inequalities are derived from three fundamental assumptions. An experiment shows that these inequalities can be violated if quantum evolution is controllable on the scale of an observer.
The Wigner's friend thought experiment raises the question of whether quantum theory can be applied consistently to systems that include observers. Extended Wigner's Friend Scenarios develop this question by considering several observers who may assign different descriptions to the same experiment. In the Local Friendliness (LF) framework, these scenarios lead to experimentally testable inequalities derived from assumptions such as Absoluteness of Observed Events and Local Agency. Motivated by the "Thoughtful" version of the Local Friendliness no-go theorem, which points towards future LF tests with human-level artificial agents implemented on quantum computers, this work implements the "friend" with explicit agent-like functionality within a reversible quantum circuit. Drawing from the literature on Artificial Intelligence, we construct rudimentary agent-like systems and embed them into a one-friend Extended Wigner's Friend Scenario. These agents store measurement outcomes, condition later operations on stored information, and, in the most structured case, use Born-rule probabilities to bet on the outcomes of their own future observations based on past observations. The circuits are simulated ideally and with an IBM-device noise model and executed on ibm_marrakesh. Ideal simulations reproduce the maximal quantum violation of a Local Friendliness inequality up to finite-shot fluctuations, while hardware runs show positive LF violations for all implemented agents. These results provide a first step towards more structured agent-like friend models in LF experiments on quantum computers.
The precise interpretation of quantum mechanics is still open to discussion. A number of non-intercompatible interpretations, with a not unsignificant respective following, are at the time of writing still viable. When Eugene Wigner stated his titular thought experiment he managed to highlight how varying assumptions and accompanying mechanisms can have implications even on a macro-scale. Caslav Brukner expands the thought experiment of Wigner's Friend in his recent paper and derives a theorem using the setup. In our paper, we empirically validate the theorem derived in Brukner's paper, employing a setup based on his extended Wigner's Friend experiment. More precisely, we realise this setup on a 5-qubit Quantum Computer and achieve a violation of a Bell-type inequality, close to what is theoretically possible. This seems to imply that local observer-independent facts are not possible within the current framework of quantum mechanics.
How can we address the idea of observers in quantum mechanics? The long-standing Wigner’s friend thought experiment provides a conceptual framework for thinking about this question. It considers the the difficulty of reconciling unitary, evolution of systems and non-unitary state update after a making observations. Following from a recent suggested protocol with two friends that share and entangled state, we prove and investigate, in a photonic experiment, a strong no-go theorem for this Wigner’s friend scenario. Specifically, if quantum mechanics applies at the scale of observers, then there are constraints on the set of assumptions: locality; no-superdeterminism; and absoluteness of observed events. We note that this new theorem places strictly stronger constraints on physical reality than Bell’s theorem.
Wigner's friend paradoxes highlight contradictions between measurements made by friends inside a laboratory and superobservers outside the laboratory, who have access to an entangled state of the measurement apparatus. Where there are two separated laboratories, the contradictions lead to no-go theorems for observer-independent facts, thus challenging concepts of objectivity. Here, we examine the paradoxes from the perspective of establishing consistency with macroscopic realism. We present versions of the Brukner-Wigner's-friend and Frauchiger-Renner paradoxes in which the spin-1/21/21/2 systems measured by the friends correspond to two macroscopically distinct states. The local unitary operations ${U}_{{\ensuremath{\theta}}_{j}}$ that determine the measurement settings ${\ensuremath{\theta}}_{j}$ at each laboratory are carried out using nonlinear interactions, which are tailored to ensure that measurements need only distinguish between the macroscopically distinct states. The macroscopic paradoxes are perplexing, seemingly suggesting there is no objectivity in a macroscopic limit. However, we demonstrate consistency with a contextual weak form of macroscopic realism (wMR): The premise wMR asserts that each system jjj can be considered to have a definite spin outcome ${\ensuremath{\lambda}}_{{\ensuremath{\theta}}_{j}}$, at the time after the system has undergone the local unitary rotation ${U}_{{\ensuremath{\theta}}_{j}}$ to prepare it in a suitable measurement basis. We further show that the paradoxical outcomes imply failure of deterministic macroscopic local realism and arise when there are unitary interactions ${U}_{{\ensuremath{\theta}}_{j}}$ occurring due to a change of measurement setting at both laboratories, with respect to the state prepared by each friend. In models which validate wMR, there is a breakdown of a subset of the assumptions that constitute the Bell-locality premise. A similar interpretation involving a contextual weak form of local realism exists for the original paradoxes.
When Eugene Wigner conceived his Gedanken experiment in 1961 [1], he argued that in quantum theory two observers, Wigner and his friend, can experience two fundamentally different descriptions of reality. Yet, only six decades later, this question has been rigorously tackled independently by Brukner [2] and Frauchiger and Renner [3], leveraging on the Bell's theorem and on the Hardy's Paradox respectively. They exploit a new extended Wigner's friend scenario, whereby two superobservers (Alice and Bob) and two observers (Alice and Bob's friends) are now entangled, and record their own facts shown to be subjective to the observer who established them.