Increasing relative nonclassicality quantified by standard entanglement potentials by dissipation and unbalanced beam splitting

TL;DR

This paper investigates how dissipation and unbalanced beam splitting can enhance the quantification of nonclassicality in quantum states by manipulating entanglement potentials, revealing new ways to measure and increase nonclassical features.

Contribution

It demonstrates that local losses and unbalanced beam splitters can increase the measurable nonclassicality, challenging the assumption that a balanced lossless beam splitter captures all nonclassicality.

Findings

Maximal nonclassicality can be increased using tunable beam splitters or amplitude damping.

Dephasing can increase the nonclassicality measured by negativity for a given REEP.

Partially-mixed states can be more nonclassical than pure or fully mixed states.

Abstract

If a single-mode nonclassical light is combined with the vacuum on a beam splitter, then the output state is entangled. As proposed in [Phys. Rev. Lett. 94, 173602 (2005)], by measuring the output-state entanglement for a balanced lossless beam splitter, one can quantify the input-state nonclassicality. These measures of nonclassicality (referred to as entanglement potentials) can be based, in principle, on various entanglement measures, leading to the negativity (NP) and concurrence (CP) potentials, and the potential for the relative entropy of entanglement (REEP). We search for the maximal nonclassicality, which can be achieved by comparing two entanglement measures for arbitrary two-qubit states and those which can be generated from a photon-number qubit via a balanced lossless beam-splitter, where the qubit basis states are the vacuum and single-photon states. Surprisingly, we find that the maximal nonclassicality, measured by the REEP for a given value of the NP, can be increased (if NP<0.527) by using either a tunable beam splitter or by amplitude damping of the output state of the balanced beam splitter. We also show that the maximal nonclassicality, measured by the NP for a given value of the REEP, can be increased by dephasing. The entanglement itself is not increased by these local losses, but the possible ratios of different measures are affected. Moreover, we show that partially-mixed states can be more nonclassical than both pure states and completely-mixed states, by comparing the NP for a given value of the REEP. This implies that not all entanglement measures can be used as entanglement potentials. Alternatively, a single balanced lossless beam splitter is not always transferring the whole nonclassicality of its input state into the entanglement of its output modes. Applying a lossy beam splitter can solve this problem at least for the cases analyzed in this paper.