Probing nonclassicality with matrices of phase-space distributions

Martin Bohmann1,2, Elizabeth Agudelo1, and Jan Sperling3

1Institute for Quantum Optics and Quantum Information - IQOQI Vienna, Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austria
2QSTAR, INO-CNR, and LENS, Largo Enrico Fermi 2, I-50125 Firenze, Italy
3Integrated Quantum Optics Group, Applied Physics, Paderborn University, 33098 Paderborn, Germany

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We devise a method to certify nonclassical features via correlations of phase-space distributions by unifying the notions of quasiprobabilities and matrices of correlation functions. Our approach complements and extends recent results that were based on Chebyshev's integral inequality [65]. The method developed here correlates arbitrary phase-space functions at arbitrary points in phase space, including multimode scenarios and higher-order correlations. Furthermore, our approach provides necessary and sufficient nonclassicality criteria, applies to phase-space functions beyond $s$-parametrized ones, and is accessible in experiments. To demonstrate the power of our technique, the quantum characteristics of discrete- and continuous-variable, single- and multimode, as well as pure and mixed states are certified only employing second-order correlations and Husimi functions, which always resemble a classical probability distribution. Moreover, nonlinear generalizations of our approach are studied. Therefore, a versatile and broadly applicable framework is devised to uncover quantum properties in terms of matrices of phase-space distributions.

The intuitively accessible representation of quantum effects via quasiprobabilities, defying the nonnegativity requirement of classical probabilities, is a common technique to identify quantum features. However, the complexity of the reconstruction of such distributions increases with their sensitivity to uncover nonclassical signatures. Conversely, approaches based on correlation functions are experimentally available but less intuitive.

The method devised in our paper overcomes such disadvantageous features by unifying both aforementioned techniques. That is, quasiprobabilities can be correlated to unveil nonclassical properties even if the individual distributions are not sensitive enough to identify quantum properties. For example, it is shown that this necessary and sufficient approach applies to discrete- and continuous-variable, single- and multimode, pure and mixed states of light using phase-space distributions that can never become negative.

Thereby, we demonstrate the usefulness of our novel method to certify quantum characteristics in a practical manner that formthe basis for current and future quantum technologies.

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