Optimality of Gaussian Discord

Pirandola, Stefano; Spedalieri, Gaetana; Braunstein, Samuel L.; Cerf, Nicolas J.; Lloyd, Seth

doi:10.1103/physrevlett.113.140405

Cited by 82 publications

(122 citation statements)

References 33 publications

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“…(1), where the minimization in J ← (AB) is restricted to Gaussian measurements. The Gaussian discord coincides with unrestricted discord (1) for some states considered by us [10], which confirms the relevance of its use. All nonproduct bipartite Gaussian states have been shown to have nonzero Gaussian discord [8,11] but many of them are termed classical according to the conventional nonclassicality criterion.…”

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confidence: 76%

Quantum nature of Gaussian discord: Experimental evidence and role of system-environment correlations

et al. 2015

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We provide experimental evidence of quantum features in bipartite states classified as entirely classical according to a conventional criterion based on the Glauber P function but possessing nonzero Gaussian quantum discord. Their quantum nature is experimentally revealed by acting locally on one part of the discordant state. We experimentally verify and investigate the effect of discord increase under the action of local loss and link it to the entanglement with the environment. Adding an environmental system purifying the state, we unveil the flow of quantum correlations within a global pure system using the Koashi-Winter inequality. For a discordant state generated by splitting a state in which the initial squeezing is destroyed by random displacements, we demonstrate the recovery of entanglement highlighting the role of system-environment correlations. As quantum information science develops towards quantum information technology, the question of the efficient use and optimization of resources becomes a burning issue. So far, quantum information processing (QIP) has been mostly thought of as entanglement-enabled technology. Quantum cryptography is an exception, but even there the so-called effective entanglement between the parties plays a decisive role [1,2]. With the advent of new quantum computation paradigms [3] interest in more generic and even nonentangled QIP resources has emerged [4]. Unlike entanglement, the new resources, commonly dubbed as quantum correlations, reside in all states which do not diagonalize in any local product basis. Entanglement and quantum correlations are equivalent notions only for pure states. Quantumness of correlations in separable states is fundamentally related to the noncommutativity of observables, nonorthogonality of states, and properties of quantum measurements, whereas entanglement can be seen as a consequence of the quantum superposition principle. Correlated mixed states are a lucid illustration of the fact that the quantum-classical divide is actually purpose-oriented and that such states, long considered unsuitable for QIP, may become a robust and efficient quantum tool.In what follows, we will use quantum discord [5] for quantification of quantum correlations. For two systems A and B, quantum discord is defined as the difference,between quantum mutual information I(AB) = S(A) + S(B) − S(AB) encompassing all correlations present in the system, and the one-way classical correlation, which is operationally related to the amount of perfect classical correlations which can be extracted from the system [6]. Here, S is the von Neumann entropy of the respective state, H {ˆ i } (A|B) is the conditional entropy with measurement on B, and the infimum is taken over all possible measurements {ˆ i }.In this Rapid Communication, we focus on bipartite mixed Gaussian states relevant in the context of continuousvariable quantum information [7]. The respective correlation quantifier is then Gaussian quantum discord [8,9] defined by Eq. (1), where the minimization in J ← (AB) is r...

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confidence: 76%

Quantum nature of Gaussian discord: Experimental evidence and role of system-environment correlations

et al. 2015

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“…Its quantum discord can be computed exactly according to the results of Ref. [37], and it coincides with its Gaussian discord [38,39]. Expressing the state in terms of the non-local source modes c + and c − , the covariance matrix reads which represents the product of two thermal states with different mean photon number N + = (1 + w)N and N − = (1 − w)N .…”

Section: Correlated Thermal Sourcesmentioning

confidence: 83%

Ultimate Precision Bound of Quantum and Subwavelength Imaging

2016

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We determine the ultimate potential of quantum imaging for boosting the resolution of a far-field, diffractionlimited, linear imaging device within the paraxial approximation. First we show that the problem of estimating the separation between two point-like sources is equivalent to the estimation of the loss parameters of two lossy bosonic channels, i.e., the transmissivities of two beam splitters. Using this representation, we establish the ultimate precision bound for resolving two point-like sources in an arbitrary quantum state, with a simple formula for the specific case of two thermal sources. We find that the precision bound scales with the number of collected photons according to the standard quantum limit. Then we determine the sources whose separation can be estimated optimally, finding that quantum-correlated sources (entangled or discordant) can be super-resolved at the sub-Rayleigh scale. Our results set the upper bounds on any present or future imaging technology, from astronomical observation to microscopy, which is based on quantum detection as well as source engineering. Introduction. Quantum imaging aims at harnessing quantum features of light to obtain optical images of high resolution beyond the boundary of classical optics. Its range of potential applications is very broad, from telescopy to microscopy and medical diagnosis, and has motivated a substantial research activity [1][2][3][4][5][6][7][8][9][10]. Typically, quantum imaging is scrutinized to outperform classical imaging in two ways. First, to resolve details below the Rayleigh length (sub-Rayleigh imaging). Second, to improve the way the precision scales with the number of photons, by exploiting non-classical states of light. It is well known that a collective state of N quantum particles has an effective wavelength that is N times smaller than individual particles [11][12][13][14][15][16][17][18]. If N independent photons are measured one expects that the blurring of the image scales as 1/ √ N (known as standard quantum limit or shot-noise limit), while for N entangled photons one can sometimes achieve a 1/N scaling (known as the Heisenberg limit).

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“…Furthermore, since our source is in a mixed state (more precisely, a two-mode squeezed thermal state), we also quantify its normalized quantum discord [24,37] DðwjoÞ=n w , which captures the quantum correlations carried by each microwave photon. As we can see from Fig.…”

Section: Fig 1 (Color Onlinementioning

confidence: 99%

Microwave Quantum Illumination

et al. 2015

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Quantum illumination is a quantum-optical sensing technique in which an entangled source is exploited to improve the detection of a low-reflectivity object that is immersed in a bright thermal background. Here, we describe and analyze a system for applying this technique at microwave frequencies, a more appropriate spectral region for target detection than the optical, due to the naturally occurring bright thermal background in the microwave regime. We use an electro-optomechanical converter to entangle microwave signal and optical idler fields, with the former being sent to probe the target region and the latter being retained at the source. The microwave radiation collected from the target region is then phase conjugated and upconverted into an optical field that is combined with the retained idler in a joint-detection quantum measurement. The error probability of this microwave quantum-illumination system, or quantum radar, is shown to be superior to that of any classical microwave radar of equal transmitted energy.

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Optimality of Gaussian Discord

Cited by 82 publications

References 33 publications

Quantum nature of Gaussian discord: Experimental evidence and role of system-environment correlations

Quantum nature of Gaussian discord: Experimental evidence and role of system-environment correlations

Ultimate Precision Bound of Quantum and Subwavelength Imaging

Microwave Quantum Illumination

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