Nonclassicality and entanglement in continuous variable systems

Sooryansh Asthana, V. Ravishankar

In a recent work [S. Asthana. New Journal of Physics 24.5 (2022): 053026], we have shown the interrelation of different nonclassical correlations in multiqubit systems with quantum coherence in a single logical qubit. In this work, we generalize it to higher-dimensional systems. For this, we take different choices of logical qudits and logical continuous-variable (cv) systems in terms of their constituent physical qudits and physical cv systems. Thereafter, we show reciprocity between conditions for coherence (in logical qudits and logical cv systems) and conditions for nonlocality and entanglement (in their underlying constituent qudits and cv systems). This shows that a single nonclassicality condition detects different types of nonclassicalities in different physical systems. Thereby, it reflects the interrelations of different nonclassical features of states belonging to Hilbert spaces of nonidentical dimensions.

Qingqing Sun, Muhammad Suhail Zubairy

W. Vogel

The problem of characterizing nonclassicality and entanglement in terms of continuous variables is considered. Necessary and sufficient conditions for the nonclassicality, that is the P-functions fails to be a probability, are formulated in terms of measurable characteristic functions or moments. Applications for highly sensitive measurements are discussed. Conditions for entanglement are formulated and the nonclassical correlations of multimode radiation fields are studied. The needed correlation functions can be observed by balanced homodyne correlation measurements.

Jun-Yi Wu

A linear-optics network is a multimode interferometer system, where indistinguishable photon inputs can create nonclassical interference that cannot be simulated with classical computers. Such nonclassical interference implies the existence of entanglement among its subsystems if we divide its modes into different parties. Entanglement in such systems is naturally encoded in multirail (multimode) quantum registers. For bipartite entanglement, a generation and detection scheme with multirail encoding was theoretically proposed [Wu and Hofmann, New J. Phys. 19, 103032 (2017)] and experimentally realized [Kiyohara et al., Optica 7, 1517 (2020)]. In this paper, we will take a step further to establish a theory for the detection of multirail-encoded discrete-variable genuine multipartite entanglement (GME) in fixed local-photon-number subspaces of linear-optics networks. We also propose a scheme for GME generation with both discrete-variable (single photons) and continuous-variable (squeezed states) light sources. This scheme allows us to reveal the discrete-variable GME in continuous-variable systems. The effect of photon losses is also numerically analyzed for the generation scheme based on continuous-variable inputs.

Frank E. S. Steinhoff