We discuss Wiseman, Cavalcanti and Rieffel's "thoughtful" local friendliness no-go theorem and the experimental programme they propose to test local friendliness inequalities. We argue that, to prove the theorem, the assumptions need to be strengthened to exclude the possibility of variable numbers of thoughtful agents existing in different phases of the experiment. We argue further that this possibility may arise naturally, even in one-world versions of quantum theory. We also query whether the motivations they give for their assumptions hold up well under their definition of "thoughtfulness" as displaying human-level cognitive ability, and suggest that their justification requires replacing "thoughtfulness" by "consciousness" or "conscious thoughtfulness".
We discuss Wiseman, Cavalcanti and Rieffel's "thoughtful" local friendliness no-go theorem and the experimental programme they propose to test local friendliness inequalities. We argue that, to prove the theorem, the assumptions need to be strengthened to exclude the possibility of variable numbers of thoughtful agents existing in different phases of the experiment. We argue further that this possibility may arise naturally, even in one-world versions of quantum theory. We also query whether the motivations they give for their assumptions hold up well under their definition of "thoughtfulness" as displaying human-level cognitive ability, and suggest that their justification requires replacing "thoughtfulness" by "consciousness" or "conscious thoughtfulness".
A recent paper by two of us and co-workers \cite{CQD20}, based on an extended Wigner's friend scenario, demonstrated that certain empirical correlations predicted by quantum theory (QT) violate inequalities derived from a set of metaphysical assumptions we called "Local Friendliness" (LF). These assumptions are strictly weaker than those used for deriving Bell inequalities. Crucial to the theorem was the premise that a quantum system with reversible evolution could be an observer (colloquially, a "friend"). However, that paper was noncommittal on what would constitute an observer for the purpose of an experiment. Here, we present a new LF no-go theorem which takes seriously the idea that a system's having thoughts is a sufficient condition for it to be an observer. Our new derivation of the LF inequalities uses four metaphysical assumptions, three of which are thought-related, including one that is explicitly called "Friendliness". These four assumptions, in conjunction, allow one to derive LF inequalities for experiments involving the type of system that "Friendliness" refers to. In addition to these four metaphysical assumptions, this new no-go theorem requires two assumptions about what is technologically feasible: Human-Level Artificial Intelligence, and Universal Quantum Computing which is fast and large scale. The latter is often motivated by the belief that QT is universal, but this is not an assumption of the theorem. The intent of the new theorem is to give a clear goal for future experimentalists, and a clear motivation for trying to achieve that goal. We review various approaches to QT in light of our theorem. The popular stance that "quantum theory needs no interpretation" does not question any of our assumptions and so is ruled out. Finally, we quantitatively discuss how difficult the experiment we envisage would be, and briefly discuss milestones on the paths towards it.
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.
Recently, the local friendliness (LF) no-go theorem has attracted considerable interest due to its deep foundational implications. This no-go theorem applies to scenarios which combine Bell experiments with Wigner's friend-type setups, containing spacelike separated superobservers assumed to be capable of performing quantum operations on a local observer, also known as their ``friend.'' The theorem shows that a set of assumptions termed ``local friendliness''---weaker than the set of assumptions required for proving Bell's theorem---constrains the space of probabilistic behaviors in extended Wigner's friend scenarios to the ``local friendliness (LF) polytope,'' and that quantum correlations can, in principle, violate those constraints. In this work, we extend the study of local friendliness to multipartite scenarios---with arbitrary but finite numbers of superobservers, friends, measurements, and outcomes---and fully characterize the corresponding LF polytopes, their relationships with each other, with the Bell polytope, the no-signaling polytope, and the quantum set of correlations. In particular, we identify, via constructive proofs, all the canonical LF scenarios where the LF polytope coincides with the Bell polytope---hence identifying all the scenarios where ``genuine LF inequalities'' may arise. Among other results, we also show that whenever the LF polytope is distinct from the Bell and the no-signaling polytope, it also is neither contained in nor contains the quantum set of correlations. Apart from the foundational motivation of better understanding the constraints from local friendliness, our results may also find use in a priori unrelated contexts of device-independent information processing, where the LF polytopes have also been shown to arise, under the name of ``partially deterministic polytopes.''
Nonclassical causal modeling was developed in order to explain violations of Bell inequalities while adhering to relativistic causal structure and faithfulness ---that is, avoiding fine-tuned causal explanations. Recently, a no-go theorem that can be viewed as being stronger than Bell's theorem has been derived, based on extensions of the Wigner's friend thought experiment: the Local Friendliness (LF) no-go theorem. Here we show that the LF no-go theorem poses formidable challenges for the field of causal modeling, even when nonclassical and/or cyclic causal explanations are considered. We first recast the LF inequalities, one of the key elements of the LF no-go theorem, as special cases of monogamy relations stemming from a statistical marginal problem. We then further recast LF inequalities as causal compatibility inequalities stemming from a nonclassical causal marginal problem, for a causal structure implied by well-motivated causal-metaphysical assumptions. We find that the LF inequalities emerge from this causal structure even when one allows the latent causes of observed events to admit post-quantum descriptions, such as in a generalized probabilistic theory or in an even more exotic theory. We further prove that no nonclassical causal model can explain violations of LF inequalities without violating the No Fine-Tuning principle. Finally, we note that these obstacles cannot be overcome even if one appeals to cyclic causal models, and we discuss potential directions for further extensions of the causal modeling framework.
Extended Wigner's friend no-go theorems provide a modern lens for investigating the measurement problem, by making precise the challenges that arise when one attempts to model agents as dynamical quantum systems. Most such no-go theorems studied to date, such as the Frauchiger-Renner argument and the Local Friendliness argument, are explicitly constructed using quantum correlations that violate Bell inequalities. In this work, we show that such correlations are not necessary for having extended Wigner's friend paradoxes, by constructing a no-go theorem utilizing a proof of the failure of noncontextuality. The argument hinges on a novel metaphysical assumption (which we term Commutation Irrelevance) that is a natural extension of a key assumption going into the Frauchiger and Renner's no-go theorem.