Leggett-Garg macrorealism and the quantum nondisturbance conditions

Uola, Roope; Vitagliano, Giuseppe; Budroni, Costantino

doi:10.1103/physreva.100.042117

Cited by 39 publications

(30 citation statements)

References 87 publications

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“…In addition to the standard assumption of realism, i.e., that physical variables have a definite value at any instant of time, macrorealist theories assume that such values can be measured in a noninvasive way. The latter is a strong condition that makes LG experiments challenging due to the clumsiness loophole [23]: A clumsy measurement alone may be responsible for the violation of the LG inequality, and considerable effort is needed to close it [24][25][26][27][28]. It is clear, then, that any information processing task where the information carrier is modified by sequential operations cannot be discussed in terms of LG inequalities.…”

Section: Introductionmentioning

confidence: 99%

Temporal correlations in the simplest measurement sequences

Vieira

Budroni

2022

Quantum

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We investigate temporal correlations in the simplest measurement scenario, i.e., that of a physical system on which the same measurement is performed at different times, producing a sequence of dichotomic outcomes. The resource for generating such sequences is the internal dimension, or memory, of the system. We characterize the minimum memory requirements for sequences to be obtained deterministically, and numerically investigate the probabilistic behavior below this memory threshold, in both classical and quantum scenarios. A particular class of sequences is found to offer an upper-bound for all other sequences, which suggests a nontrivial universal upper-bound of 1/e for the classical probability of realization of any sequence below this memory threshold. We further present evidence that no such nontrivial bound exists in the quantum case.

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Section: Introductionmentioning

confidence: 99%

Temporal correlations in the simplest measurement sequences

Vieira

Budroni

2022

Quantum

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“…We comment that violations of Leggett-Garg inequalities have been predicted and tested for a range of superposition states (e.g. [46][47][48][49][50][51][52][53][54][55][56][57]) and alternative procedures exist to justify NIM.…”

Section: B Leggett-garg Inequality and Violationsmentioning

confidence: 99%

Macroscopic delayed-choice and retrocausality: quantum eraser, Leggett-Garg and dimension witness tests with cat states

Thenabadu,

Reid

2021

Preprint

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We propose delayed choice experiments carried out with macroscopic qubits, realised as macroscopically-distinct coherent states |α and | − α . Quantum superpositions of |α and | − α are created via a unitary interaction U (θ) based on a nonlinear Hamiltonian, in analogy with polarising beam splitters used in photonic experiments. Macroscopic delayed-choice experiments give a compelling reason to develop interpretations not allowing macroscopic retrocausality (MrC). We therefore consider weak macroscopic realism (wMR), which specifies a hidden variable λ θ to determine the macroscopic qubit value (analogous to 'which-way' information), independent of any future measurement setting φ. Using entangled cat states, we demonstrate a quantum eraser where the choice to measure a which-way or wave-type property is delayed. Consistency with wMR is possible, if we interpret the macroscopic qubit value to be determined by λ θ without specification of the state at the level of order , where fringes manifest. We then demonstrate violations of a delayed-choice Leggett-Garg inequality, and of the dimension witness inequality applied to the Wheeler-Chaves-Lemos-Pienaar experiment, where measurements need only distinguish the macroscopic qubit states. This negates all two-dimensional non-retrocausal models, thereby suggesting MrC. However, one can interpret consistently with wMR, thus avoiding conclusions of MrC, by noting extra dimensions, and by noting that the violations require further unitary dynamics U for each system. The violations are then explained as failure of deterministic macroscopic realism (dMR), which specifies validity of λ θ prior to the dynamics U (θ) determining the measurement setting θ. Finally, although there is consistency with wMR for macroscopic observations, we demonstrate Einstein-Podolsky-Rosen-type paradoxes at a microscopic level, based on fringe distributions.

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“…Within this concept the violation of KC or its corresponding LG-type inequality will simply imply a violation of macro-realism. In addition, it has been proven recently that KC or nosignaling-in-time is equivalent to non-disturbance property of the intermediate measurement, defined when the observable associated with the latter measurement observable is invariant under non-selective measurements at all possible intermediate time steps [61].…”

Section: Leggett-garg Inequalitymentioning

confidence: 99%

Statistics of projective measurement on a quantum probe as a witness of noncommutativity of algebra of a probed system

Sakuldee¹,

Cywiński²

2021

Preprint

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We consider a quantum probe P undergoing pure dephasing due to its interaction with a quantum system S. The dynamics of P is then described by a well-defined sub-algebra of operators of S, i.e. the "accessible" algebra on S from the point of view of P. We consider sequences of n measurements on P, and investigate the relationship between Kolmogorov consistency of probabilities of obtaining sequences of results with various n, and commutativity of the accessible algebra. For a finitedimensional S we find conditions under which the Kolmogorov consistency of measurement on P, given that the state of S can be arbitrarily prepared, is equivalent to the commutativity of this algebra. These allow us to describe witnesses of non-classicality (understood here as noncommutativity) of part of S that affects the probe. For P being a qubit, the witness is particularly simple: observation of breaking of Kolmogorov consistency of sequential measurements on a qubit coupled to S means that the accessible algebra of S is non-commutative.

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Leggett-Garg macrorealism and the quantum nondisturbance conditions

Cited by 39 publications

References 87 publications

Temporal correlations in the simplest measurement sequences

Temporal correlations in the simplest measurement sequences

Macroscopic delayed-choice and retrocausality: quantum eraser, Leggett-Garg and dimension witness tests with cat states

Statistics of projective measurement on a quantum probe as a witness of noncommutativity of algebra of a probed system

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