Comment on ‘A scattering quantum circuit for measuring Bell's time inequality: a nuclear magnetic resonance demonstration using maximally mixed states’

Knee, George C.; Gauger, Erik M.; Briggs, G. Andrew D.; Benjamin, Simon C.

doi:10.1088/1367-2630/14/5/058001

Cited by 9 publications

(11 citation statements)

References 10 publications

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“…Adapted from Knee et al [31]. has been criticised [136] (see also Ref. [137]) and some of these objections apply more generally to other NMR tests of the LGIs (see later in this section).…”

Section: Nuclear Spinsmentioning

confidence: 99%

Leggett–Garg inequalities

Emary¹,

Lambert²,

Nori³

2013

Rep. Prog. Phys.

407

400

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In contrast to the spatial Bell's inequalities which probe entanglement between spatially-separated systems, the Leggett-Garg inequalities test the correlations of a single system measured at different times. Violation of a genuine Leggett-Garg test implies either the absence of a realistic description of the system or the impossibility of measuring the system without disturbing it. Quantum mechanics violates the inequalities on both accounts and the original motivation for these inequalities was as a test for quantum coherence in macroscopic systems. The last few years has seen a number of experimental tests and violations of these inequalities in a variety of microscopic systems such as superconducting qubits, nuclear spins, and photons. In this article, we provide an introduction to the Leggett-Garg inequalities and review these latest experimental developments. We discuss important topics such as the significance of the non-invasive measurability assumption, the clumsiness loophole, and the role of weak measurements. Also covered are some recent theoretical proposals for the application of Leggett-Garg inequalities in quantum transport, quantum biology and nano-mechanical systems.

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“…Adapted from Knee et al [31]. has been criticised [136] (see also Ref. [137]) and some of these objections apply more generally to other NMR tests of the LGIs (see later in this section).…”

Section: Nuclear Spinsmentioning

confidence: 99%

Leggett–Garg inequalities

Emary¹,

Lambert²,

Nori³

2013

Rep. Prog. Phys.

407

400

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“…A possible objection to the above procedure concerns the degree to which NSIT conditions really characterize non-invasiveness. They were originally described as a statistical version of NIM [10] and with a mixed initial state can be satisfied essentially by averaging two situations which are individually invasive [40,41]. However, this is not a concern here since we are using ideal negative measurements which are ontically non-invasive.…”

Section: Macrorealistic Formulationmentioning

confidence: 99%

Leggett-Garg tests of macrorealism: Checks for noninvasiveness and generalizations to higher-order correlators

Halliwell

2019

Phys. Rev. A

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In the tests for macrorealism proposed by Leggett and Garg, the temporal correlation functions of a dichotomic variable Q must be measured in a non-invasive way to rule out alternative classical explanations of Leggett-Garg inequality violations. Ideal negative measurements, in which a null result is argued to be a non-invasive determination of the system's state, are often used. From a quantum-mechanical perspective, such a measurement collapses the wave function and will therefore typically be found to be invasive under any experimental check. Here, a simple modified ideal negative measurement protocol is described for measuring the correlation functions which is argued to be non-invasive from both classical and quantum perspectives and hence the non-invasiveness can then be checked experimentally, thereby permitting a quantitative measure of the degree of clumsiness of the measurement. It is also shown how this procedure may be extended to measure higher-order correlation functions and a number of higher-order conditions for macrorealism are derived. * Electronic address: j.halliwell@imperial.ac.uk

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“…The joint probabilities can thus be extracted by a final strong measurement or by a diagonal density matrix tomography 1 . Knee et al 2 argued that since CNOT operator flips the ancilla qubit if the system qubit is in state |1, the circuit is not quite noninvasive. They also proposed a simple variation, in which P(0, 0) and P(0, 1) are measured using the CNOT operator, while P(1, 0) and P(1, 1) are measured using an anti-CNOT operator (which flips the ancilla only if the system qubit is in state |1).…”

Section: Noninvasive Measurementsmentioning

confidence: 99%

Ancilla-Assisted Measurements on Quantum Ensembles: General Protocols and Applications in NMR Quantum Information Processing

Mahesh¹,

Shukla²,

Hegde³

et al. 2015

Current Science

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Quantum ensembles form easily accessible architectures for studying various phenomena in quantum physics, quantum information science and spectroscopy. Here we review some recent protocols for measurements in quantum ensembles by utilizing ancillary systems. We also illustrate these protocols experimentally via nuclear magnetic resonance techniques. In particular, we shall review noninvasive measurements, extracting expectation values of various operators, characterizations of quantum states and quantum processes, and finally quantum noise engineering.

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Comment on ‘A scattering quantum circuit for measuring Bell's time inequality: a nuclear magnetic resonance demonstration using maximally mixed states’

Cited by 9 publications

References 10 publications

Leggett–Garg inequalities

Leggett–Garg inequalities

Leggett-Garg tests of macrorealism: Checks for noninvasiveness and generalizations to higher-order correlators

Ancilla-Assisted Measurements on Quantum Ensembles: General Protocols and Applications in NMR Quantum Information Processing

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