Measuring the size of a quantum superposition of many-body states

Marquardt, Florian; Abel, Benjamin; Delft, Jan von

doi:10.1103/physreva.78.012109

Cited by 81 publications

(158 citation statements)

References 19 publications

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“…* pavel.sekatski@unige.ch Several criteria have been proposed recently to define the notion of macroscopicity [2][3][4][5][6][7][8]. Specifically, Korsbakken and coauthors [4] linked the macroscopicity of a superposition state carried by an ensemble of qubits with the ease of distinguishing its components when only a few qubits are analyzed.…”

Section: Introductionmentioning

confidence: 99%

Size of quantum superpositions as measured with classical detectors

2014

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We propose a criterion which defines whether a superposition of two photonic components is macroscopic. It is based on the ability to discriminate these components with a particular class of "classical" detectors, namely, a photon number measurement with a resolution coarse-grained by noise. We show how our criterion can be extended to a measure of the size of macroscopic superpositions by quantifying the amount of noise that can be tolerated and taking the distinctness of two Fock states differing by N photons as a reference. After applying our measure to several well-known examples, we demonstrate that the superpositions which meet our criterion are very sensitive to phase fluctuations. This suggests that quantifying the macroscopicity of a superposition state through the distinguishability of its components with "classical" detectors not only is a natural measure but also explains why it is difficult to observe superpositions at the macroscopic scale.

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Section: Introductionmentioning

confidence: 99%

Size of quantum superpositions as measured with classical detectors

2014

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“…These measures are called measures of quantum macrosopicity. To this end, various proposals have been suggested [10,[13][14][15][16][17][18][19][20]. The basic idea used in all these proposals is that macroscopic superposition entails a large amount of uncertainty when a suitably chosen macroscopic observable is measured.…”

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confidence: 99%

Scaling of macroscopic superpositions close to a quantum phase transition

Abad¹,

Karimipour²

2016

Phys. Rev. B

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It is well known that in a quantum phase transition (QPT), entanglement remains short ranged [Osterloh et al., Nature 416 608-610 (2005)]. We ask if there is a quantum property entailing the whole system which diverges near this point. Using the recently proposed measures of quantum macroscopisity, we show that near a quantum critical point, it is the effective size of macroscopic superposition between the two symmetry breaking states which grows to the scale of system size and its derivative with respect to the coupling shows both singular behavior and scaling properties.

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“…Although there is no commonly accepted single measure of macroscopic quantum states, one can identify a few similarities in some proposals. More specifically, many contributions [2,[10][11][12][13][14][15][16][17][18] explicitly or implicitly agree that a macroscopic quantum state should show large quantum coherence (or large quantum fluctuations) spread over a "reasonably chosen" spectrum. That is, a macroscopic quantum state should be in superposition of states that are far apart in the spectrum (like the biological cat being in a superposition of two distant parts in some kind of "vitality" spectrum).…”

Section: The Qfi As a Measure For Macroscopic Quantum Statesmentioning

confidence: 99%

Lower bounds on the size of general Schrödinger-cat states from experimental data

Fröwis¹

2017

J. Phys. A: Math. Theor.

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Experimental progress with meso-and macroscopic quantum states (i.e., general Schrödinger-cat states) was recently accompanied by theoretical proposals on how to measure the merit of these efforts. So far, experiment and theory were disconnected as theoretical analysis of actual experimental data was missing. Here, we consider a proposal for macroscopic quantum states that measures the extent of quantum coherence present in the system. For this, the quantum Fisher information is used. We calculate lower bounds from real experimental data. The results are expressed as an "effective size", that is, relative to "classical" reference states. We find remarkable numbers of up to 70 in photonic and atomic systems.

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Measuring the size of a quantum superposition of many-body states

Cited by 81 publications

References 19 publications

Size of quantum superpositions as measured with classical detectors

Size of quantum superpositions as measured with classical detectors

Scaling of macroscopic superpositions close to a quantum phase transition

Lower bounds on the size of general Schrödinger-cat states from experimental data

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