arXiv subjects

Jesse Fulton

Publications and source records attributed to Jesse Fulton.

3 recordsLinked to original sources

arXiv2022

Wigner's Friend paradoxes: consistency with weak-contextual and weak-macroscopic realism models

Wigner's friend paradoxes highlight contradictions between measurements made by Friends inside a laboratory and superobservers outside a laboratory, who have access to an entangled state of the measurement apparatus. 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-friend and Frauchiger-Renner paradoxes in which the spin-$1/2$ system measured by the Friends corresponds to two macroscopically distinct states. The local unitary operations $U_{\theta}$ that determine the measurement setting $\theta$ are carried out using nonlinear interactions, thereby ensuring 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 the system can be considered to have a definite spin outcome $\lambda_{\theta}$, at the time after the system has undergone the unitary rotation $U_{\theta}$ to prepare it in a suitable pointer basis. We further show that the paradoxical outcomes imply failure of deterministic macroscopic local realism, and arise when there are unitary interactions $U_{\theta}$ occurring due to a change of measurement setting at both sites, 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 weak contextual form of realism exists for the original paradoxes.

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arXiv2022

An Einstein-Podolsky-Rosen argument based on weak forms of local realism not falsifiable by GHZ or Bell experiments

The Einstein-Podolsky-Rosen (EPR) paradox gives an argument for the incompleteness of quantum mechanics based on the premises of local realism. A general view is that the argument is compromised, because EPR's premises are falsified by Greenberger-Horne-Zeilinger (GHZ) and Bell experiments. In this paper, we present an EPR argument based on premises not falsifiable by these experiments. We propose macroscopic EPR and GHZ experiments using spins $S_\theta$ defined by two macroscopically distinct states. The analyzers that realize the unitary operations $U_\theta$ determining the measurement settings $\theta$ are devices that create macroscopic superposition states. For a system with two macroscopically distinct states available, macroscopic realism (MR) posits a predetermined outcome for a measurement $S_\theta$ distinguishing between the states. Deterministic macroscopic realism (dMR) posits MR for the system prior to the interaction $U_\theta$. Weak macroscopic realism (wMR) posits MR for the system after $U_\theta$, at the time $t_f$ (when the system is prepared for a final "pointer" measurement), the outcome of $S_\theta$ not being changed by interactions that might occur at a remote system $B$. The premise also posits that if the outcome for $S_\theta^A$ of a system $A$ can be predicted by a pointer measurement on a system $B$ defined after the interaction fixing the setting at $B$, then the outcome for $S_\theta^A$ is determined at this time. The GHZ predictions negate dMR but are consistent with wMR. Yet, an EPR paradox arises based on wMR for the set-up proposed by Schr\"odinger, where one measures two complementary spins simultaneously, "one by direct, the other by indirect" measurement. We revisit the original EPR paradox and find similarly that an EPR argument can be based on a weak form of local realism not falsifiable by GHZ or Bell tests.

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arXiv2021

Weak versus deterministic macroscopic realism, and Einstein-Podolsky-Rosen's elements of reality

Violation of Leggett-Garg inequalities allows proof of the incompatibility between quantum mechanics and the combined premises (called macrorealism) of macroscopic realism (MR) and non-invasive measurability (NIM). Arguments can be given that the incompatibility arises because MR fails $-$ or else, that NIM fails. In this paper, we consider a strong failure of macrorealism, involving superpositions of coherent states, where the NIM premise is replaced by Bell-locality. We follow recent work and propose validity of a subset of Einstein-Podolsky-Rosen (EPR) and Leggett-Garg premises, referred to as \emph{weak macroscopic realism} (wMR). In finding consistency with wMR, we identify that the Leggett-Garg inequalities are violated because of failure of both MR and NIM, but also that both are valid in a less restrictive sense. Weak MR is distinguished from \emph{deterministic macroscopic realism} (dMR) by recognizing that a measurement involves a reversible unitary interaction that establishes the measurement setting. Weak MR posits a predetermined value for the measurement outcome, for the system defined at the time after the interaction, when the measurement setting is experimentally specified. An extended definition of wMR considers the element of reality defined by EPR for a system A, where one can predict with certainty the outcome of a measurement on A, by measurement on a system B. Weak MR posits that the element of reality exists once the unitary interaction determining the setting at B has occurred. We show compatibility of systems violating Leggett-Garg inequalities with wMR, but point out that dMR is falsifiable. We compare with other MR models, and give an argument for wMR on the basis that wMR resolves inconsistencies pointed out by Leggett and Garg between failure of macrorealism and assumptions intrinsic to quantum measurement theory.

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