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arXiv:2209.06236

Thought experiments in a quantum computer

Nuriya Nurgalieva, Simon Mathis, Lídia del Rio, Renato Renner

Published Sep 13, 2022• 3 citations
arXivHTMLPDF

Abstract

We introduce a software package that allows users to design and run simulations of thought experiments in quantum theory. In particular, it covers cases where several reasoning agents are modelled as quantum systems, such as Wigner's friend experiment. Users can customize the protocol of the experiment, the inner workings of agents (including a quantum circuit that models their reasoning process), the abstract logical system used (which may or not allow agents to combine premises and make inferences about each other's reasoning), and the interpretation of quantum theory used by different agents. Our open-source software is written in a quantum programming language, ProjectQ, and runs on classical or quantum hardware. As an example, we model the Frauchiger-Renner extended Wigner's friend thought experiment, where agents are allowed to measure each other's physical memories, and make inferences about each other's reasoning.

DOI
10.3929/ethz-b-000595570

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We introduce a software package that allows users to design and run simulations of thought experiments in quantum theory. In particular, it covers cases where several reasoning agents are modelled as quantum systems, such as Wigner's friend experiment. Users can customize the protocol of the experiment, the inner workings of agents (including a quantum circuit that models their reasoning process), the abstract logical system used (which may or not allow agents to combine premises and make inferences about each other's reasoning), and the interpretation of quantum theory used by different agents. Our open-source software is written in a quantum programming language, ProjectQ, and runs on classical or quantum hardware. As an example, we model the Frauchiger-Renner extended Wigner's friend thought experiment, where agents are allowed to measure each other's physical memories, and make inferences about each other's reasoning.
Quantum mechanics is one of our most successful physical theories; its predictions agree with experimental observations to an extremely high accuracy. However, the bare formalism of quantum theory does not provide straightforward answers to seemingly simple questions: for example, how should one model systems that include agents who are themselves using quantum theory? These foundational questions may be investigated with a theorist's tool – the thought experiment. Its purpose is to turn debates about the interpretation of quantum theory into actual physics questions. In this article, we give a state-of-the-art overview on quantum thought experiments involving observers, from the basic Wigner's friend to the recent Frauchiger-Renner setup, clarifying their interpretational significance and responding to objections and criticism on the way.
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 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 thesis implements the “friend” with explicit agent-like functionality within a reversible quantum circuit. Several such agents are constructed and embedded into a one-friend Extended Wigner's Friend Scenario, following recent work that uses quantum computers as a platform for LF violations with more sophisticated “friend” systems. 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. The circuits are implemented in Qiskit, simulated ideally and with IBM-device noise models, and executed on ibm_marrakesh. In ideal simulations, all agent implementations reproduce the maximal quantum violation of a Local Friendliness inequality up to finite-shot fluctuations. Hardware runs show positive Local Friendliness violations for all implemented agents, with decreasing violation strength consistent with greater noise accumulation. The observed violations also remain above an estimated bound obtained by considering relaxations of a Local Friendliness assumption. These results provide a first step towards more structured agent-like friend models in LF experiments on quantum computers.
Abstract We take a fresh look at Wigner’s Friend thought-experiment and some of its more recent variants and extensions, such as the Frauchiger-Renner (FR) Paradox. We discuss various solutions proposed in the literature, focusing on a few questions: What is the correct epistemic interpretation of the multiplicity of state assignments in these scenarios? Under which conditions can one include classical observers into the quantum state descriptions, in a way that is still compatible with traditional Quantum Mechanics? Under which conditions can one system be admitted as an additional ‘observer’ from the perspective of another background observer? When can the standard axioms of multi-agent Epistemic Logic (that allow “knowledge transfer” between agents) be applied to quantum-physical observers? In the last part of the paper, we propose a new answer to these questions, sketch a particular formal implementation of this answer, and apply it to obtain a principled solution to Wigner Friend-type paradoxes.
We introduce ProjectQ, an open source software effort for quantum computing. The first release features a compiler framework capable of targeting various types of hardware, a high-performance simulator with emulation capabilities, and compiler plug-ins for circuit drawing and resource estimation. We introduce our Python-embedded domain-specific language, present the features, and provide example implementations for quantum algorithms. The framework allows testing of quantum algorithms through simulation and enables running them on actual quantum hardware using a back-end connecting to the IBM Quantum Experience cloud service. Through extension mechanisms, users can provide back-ends to further quantum hardware, and scientists working on quantum compilation can provide plug-ins for additional compilation, optimization, gate synthesis, and layout strategies.
The Wigner's friend thought experiment was intended to illustrate the difficulty one has in describing an agent as a quantum system when that agent performs a measurement. While it does pose a challenge to the orthodox interpretation of quantum theory, most modern interpretations have no trouble in resolving the difficulty. Recently, a number of extensions of Wigner's ideas have been proposed. We provide a gentle introduction to six such arguments, modifying the specifics of many of them so that they are as simple and unified as possible. In particular, we show that all of the arguments hinge on assumptions about correlations between measurement outcomes that are not accessible to any observer, even in principle. We then provide a critical analysis of each argument, focusing especially on how well one can motivate the required assumptions regarding these inaccessible correlations. Although we argue that some of these assumptions are not entirely well-motivated, all of the arguments do shed light on the nature of quantum theory, especially when concerning the description of agents and their measurements.