A strict experimental test of macroscopic realism in a superconducting flux qubit

Knee, George C.; Kakuyanagi, Kosuke; Yeh, Mao Chuang; Matsuzaki, Yuichiro; Toida, Hiraku; Yamaguchi, Hiroshi; Saito, Shiro; Leggett, Anthony J.; Munro, William J.

doi:10.1038/ncomms13253

Cited by 136 publications

(192 citation statements)

References 31 publications

(40 reference statements)

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“…It has recently been applied to a number of systems [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. Unfortunately, a clumsiness loophole [23] can thoroughly undermine the significance of any violation of the Leggett-Garg inequality.…”

Section: Introductionmentioning

confidence: 99%

Violation of noninvasive macrorealism by a superconducting qubit: Implementation of a Leggett-Garg test that addresses the clumsiness loophole

Huffman

Mizel

2017

Phys. Rev. A

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The Leggett-Garg inequality holds for any macrorealistic system that is being measured noninvasively. A violation of the inequality can signal that a system does not conform to our primal intuition about the physical world. Alternatively, a violation can simply indicate that "clumsy" experimental technique led to invasive measurements. Here, we consider a recent Leggett-Garg test designed to try to rule out the mundane second possibility. We tailor this Leggett-Garg test to the IBM 5Q Quantum Experience system and find compelling evidence that qubit Q2 of the system cannot be described by noninvasive macrorealism.

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Section: Introductionmentioning

confidence: 99%

Violation of noninvasive macrorealism by a superconducting qubit: Implementation of a Leggett-Garg test that addresses the clumsiness loophole

Huffman

Mizel

2017

Phys. Rev. A

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“…There is a connection between large QFI witnessed via equation (5) and the violation of LeggettGarg inequalities [31] with "hardly-invasive" measurements [27]. It turns out that, for some important instances, the (quasi-)optimal measurement for equation (5) is simple, at least for moderate system sizes.…”

Section: Lower Bounds On the Qfimentioning

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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“…The largest objects to date for which matter-wave interference fringes have been observed are molecules consisting of 430 atoms, with a total mass of 6910 atomic mass units (amu) [12], and interference experiments with even larger molecules appear possible [13][14][15]. Another possible test ground are superconducting qubits, where superpositions of current involving several thousand electrons have been inferred [33].Other methods to detect the breakdown of classical physics have been proposed [34][35][36], and some of them have been verified experimentally by measurements on weak light fields [35,37]. Recently, Kot et al [38] have extended the results of Ref.…”

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confidence: 99%

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confidence: 99%

Strictly nonclassical behavior of a mesoscopic system

Vendeiro

Chen

et al. 2017

Phys. Rev. A

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We experimentally demonstrate the strictly nonclassical behavior in a many-atom system using a recently derived criterion [E. Kot et al., Phys. Rev. Lett. 108, 233601 (2012)] that explicitly does not make use of quantum mechanics. We thereby show that the magnetic-moment distribution measured by McConnell et al. [Nature (London) 519, 439 (2015)] in a system with a total mass of 2.6 × 10 5 atomic mass units is inconsistent with classical physics. Notably, the strictly nonclassical behavior affects an area in phase space 10 3 times larger than the Planck quantumh. DOI: 10.1103/PhysRevA.95.030105 Ever since the advent of modern quantum theory almost a century ago, one perplexing question has been the boundary between quantum mechanics and classical physics. Considering just two particles, Bell showed [1,2] that reasonable assumptions consistent with classical physics lead to predictions that are inconsistent with the measurement results [3][4][5][6][7]. For macroscopic systems, Schrödinger's gedanken experiment of a simultaneously dead and alive cat highlights the question about the quantum-classical boundary in larger and larger systems [8][9][10][11][12][13][14][15]. Within the conceptual framework of quantum mechanics, one can establish definite and quantitative criteria for entanglement [16][17][18][19][20], and hence potential nonclassicality. However, one may argue that an experiment never confirms a theory as valid, but only fails to violate it. Therefore, mere consistency with results derived from quantum mechanics does not rule out a description within classical physics. It is therefore interesting to identify and experimentally test criteria that are formulated within classical physics, without assuming the validity or concepts of quantum mechanics [21].In quantum optics, criteria that distinguish nonclassical states of light from classical ones [22][23][24][25][26][27][28][29] were developed early, and tested successfully in experiments [30,31], such as antibunching [22,23,30], Klyshko's criterion [24,25], and nonclassical statistical properties [22,[26][27][28][29]31]. Some of these demonstrations [30,31] have become standard methods to verify classes of nonclassical states, such as the singlephoton states [30], and sub-Poissonian states [31].More tests have been performed on atomic and molecular systems. One such approach is to test the wave properties of larger and larger molecules, and one day perhaps even living objects, through double-slit interference experiments [32]. The largest objects to date for which matter-wave interference fringes have been observed are molecules consisting of 430 atoms, with a total mass of 6910 atomic mass units (amu) [12], and interference experiments with even larger molecules appear possible [13][14][15]. Another possible test ground are superconducting qubits, where superpositions of current involving several thousand electrons have been inferred [33].Other methods to detect the breakdown of classical physics have been proposed [34][35][36], and some of them have...

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A strict experimental test of macroscopic realism in a superconducting flux qubit

Cited by 136 publications

References 31 publications

Violation of noninvasive macrorealism by a superconducting qubit: Implementation of a Leggett-Garg test that addresses the clumsiness loophole

Violation of noninvasive macrorealism by a superconducting qubit: Implementation of a Leggett-Garg test that addresses the clumsiness loophole

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

Strictly nonclassical behavior of a mesoscopic system

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