The objectivity and invariance of quantum predictions (original) (raw)

Entanglement and the Measurement Problem

Quantum Engineering, 2022

The entangled “measurement state” (MS), predicted by von Neumann to arise during quantum measurement, seems to display paradoxical properties such as multiple macroscopic outcomes. But analysis of interferometry experiments using entangled photon pairs shows that entangled states differ surprisingly from simple superposition states. Based on standard quantum theory, this paper shows that the MS (i) does not represent multiple detector readings but instead represents nonparadoxical multiple statistical correlations between system states and detector readings, (ii) implies that exactly one outcome actually occurs, and (iii) implies that when one outcome occurs, the other possible outcomes simultaneously collapse nonlocally. Point (iii) resolves an issue first raised in 1927 by Einstein who demonstrated that quantum theory requires instantaneous state collapse. This conundrum’s resolution requires nonlocal correlations, which from today’s perspective suggests the MS should be an entang...

Observation and Quantum Objectivity

Philosophy of Science, 2013

The paradox of Wigner's friend challenges the objectivity of description in quantum theory. A pragmatist interpretation can meet this challenge by judicious appeal to decoherence. On this interpretation, quantum theory provides situated agents with resources for predicting and explaining what happens in the physical world-not conscious observations of it. Even in Wigner's friend scenarios, differently situated agents agree on the objective content of statements about the values of physical magnitudes. In more realistic circumstances quantum Darwinism also permits differently situated agents equal observational access to evaluate their truth. In this view, quantum theory has nothing to say about consciousness or conscious experiences of observers. But it does prompt us to reexamine the significance even of everyday claims about the physical world.

Subjective probability and quantum certainty

Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics, 2007

In the Bayesian approach to quantum mechanics, probabilities-and thus quantum states-represent an agent's degrees of belief, rather than corresponding to objective properties of physical systems. In this paper we investigate the concept of certainty in quantum mechanics. Particularly, we show how the probability-1 predictions derived from pure quantum states highlight a fundamental difference between our Bayesian approach, on the one hand, and Copenhagen and similar interpretations on the other. We first review the main arguments for the general claim that probabilities always represent degrees of belief. We then argue that a quantum state prepared by some physical device always depends on an agent's prior beliefs, implying that the probability-1 predictions derived from that state also depend on the agent's prior beliefs. Quantum certainty is therefore always some agent's certainty. Conversely, if facts about an experimental setup could imply agent-independent certainty for a measurement outcome, as in many Copenhagen-like interpretations, that outcome would effectively correspond to a preëxisting system property. The idea that measurement outcomes occurring with certainty correspond to preëxisting system properties is, however, in conflict with locality. We emphasize this by giving a version of an argument of Stairs [A. Stairs, Phil. Sci. 50, 578 (1983)], which applies the Kochen-Specker theorem to an entangled bipartite system.

Quantum Theory and the Limits of Objectivity

Foundations of Physics

Three recent arguments seek to show that the universal applicability of unitary quantum theory is inconsistent with the assumption that a wellconducted measurement always has a definite physical outcome. In this paper I restate and analyze these arguments. The import of the first two is diminished by their dependence on assumptions about the outcomes of counterfactual measurements. But the third argument establishes its intended conclusion. Even if every well-conducted quantum measurement we ever make will have a definite physical outcome, this argument should make us reconsider the objectivity of that outcome.

On quantum entanglement, counterfactuals, causality and dispositions

Synthese Special Issue "Metaphysics of Entanglement", 2017

The existence of non-local correlations between outcomes of measurements in quantum entangled systems strongly suggests that we are dealing with some form of causation here. An assessment of this conjecture in the context of the collapse interpretation of quantum mechanics is the primary goal of this paper. Following the counterfactual approach to causation, I argue that the details of the underlying causal mechanism which could explain the non-local correlations in entangled states strongly depend on the adopted semantics for counterfactuals. Several relativistically-invariant interpretations of spatiotemporal counterfactual conditionals are discussed, and the corresponding causal stories describing interactions between parts of an entangled system are evaluated. It is observed that the most controversial feature of the postulated causal connections is not so much their non-local character as a peculiar type of circularity that affects them.

TRACES OF OBJECTIVITY: Causality and Probabilities in Quantum Physics

Objectivity does not necessarily require deterministic causation, unlike in certain interpretations of Kant's epistemology. But the very structure of the probabilities used by a physical theory is capable of bearing the trace of a constitution of objectivity in Kant’s sense. Accordingly, an examination of the differences between classical and quantum probabilities is performed. It is shown that quantum probabilities carry the mark of the contextuality of the phenomena to which they apply. Conversely, certain conditions that have the form of Bell’s inequalities carry the mark of decontextualization. In other terms, quantum theories include both a sign of the limit of objectivity, and a method to make an objective use of this limit.

Complementary relationships between entanglement and measurement

Academia Quantum, 2024

Complementary relationships exist among interference properties of particles such as pattern visibility, predictability, and distinguishability. Additionally relationships between average information gain G ̄ and measurement disturbance F for entangled spin pairs are well established. This article examines whether a similar complementary relationship exists between entanglement and measurement. For qubit systems, both measurements on a single system and measurements on a bipartite system are considered in regard to entanglement. It is proven that E ̄ + D ≤ 1 holds, where E ̄ is the average entanglement after a measurement is made and D is a measure of the measurement disturbance of a single measurement. Assuming measurements on a bipartite system shared by Alice and Bob, it is shown that E ̄ + G ̄ ≤ 1, where G ̄ is the maximum average information gain that Bob can obtain regarding Alice’s result. These results are generalized to arbitrary initial mixed states and non-Hermitian operators. In the case of maximally entangled initial states, it is found that D ≤ EL and G ̄ ≤ EL, where EL is the loss of entanglement due to measurement by Alice. We conclude that the amount of disturbance and average information gain one can achieve is strictly limited by entanglement.

A Few Insights into Quantum Entanglement 2

(Typo Corrections and minor revisions 5-10-23) The conventional analysis of both quantum product states and quantum entanglement is shown to be consistent with a local, hidden variable (LHV) model, where two spatially separated observers make independent local measurements on local wave functions that share a common random hidden source variable. A conventional quantum mechanical LHV derivation also suggests that four quanta are required to truly measure a "zero spin" singlet state, with two quanta detected by each observer. In contrast, Bell local hidden variable (BLHV) models and inequalities assume one quantum detection by each observer, which does accurately model product states, but NOT entangled states. It is also shown that quantum entanglement can be viewed as an interference phenomenon, and can be factored into a "disentangled" product of local wave functions at the two spatially separated observers. Experimental measurements of quantum entanglement appear to be measuring Bell product states, and yet see quantum entanglement; which may suggest a non-local hidden variable (NLHV) process, where a detection by one observer instantaneously modifies the wave function in transit to the other observer. However, this proposed non-local process has serious potential flaws. Alternatively, it is shown that "coincidence of clicks" measurements on local, hidden variable (LHV) entangled or product states can approximate the experimentally reported entangled behavior. Additional experiments could potentially discriminate between these interpretations of the experimental data.