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Journal of the Optical Society of America B

Simple criterion for nonclassical two-mode states

Ching Tsung Lee

Published Mar 1, 1998• 12 citations
Publisher

Abstract

A sufficient condition for nonclassical two-mode states in terms of three probabilities, (n2+1)p(n1-1, n2+1)2n1p(n1, n2)+(n1+1)p(n1+1, n2-1)2n2p(n1, n2)<1, is established as a new type II criterion. This criterion is applied to the two-mode coherent state, the two-mode squeezed vacuum state, and the pair coherent state to show how easily it can be used. It is also applied to the photon-added two-mode thermal state and the mixing of two two-photon states by a symmetric beam splitter to show that the type II criterion is superior to its type I counterpart in at least these two examples.

DOI
10.1364/josab.15.001187

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A sufficient condition for nonclassical two-mode states in terms of three probabilities, (n2+1)p(n1-1, n2+1)2n1p(n1, n2)+(n1+1)p(n1+1, n2-1)2n2p(n1, n2)<1, is established as a new type II criterion. This criterion is applied to the two-mode coherent state, the two-mode squeezed vacuum state, and the pair coherent state to show how easily it can be used. It is also applied to the photon-added two-mode thermal state and the mixing of two two-photon states by a symmetric beam splitter to show that the type II criterion is superior to its type I counterpart in at least these two examples.
The concept of photon antibunching as a manifestation of nonclassical character in single-mode radiation is extended to two-mode radiation. A simple criterion for the existence of nonclassical effects in two-mode radiation is established. It implies that the intermode photon bunching, rather than antibunching, can play a key role in rendering two-mode radiation nonclassical. The two-mode squeezed vacuum states are used as simple examples to illustrate the essential point of the criterion. The criterion is then used to study the nonclassical properties of two types of two-mode squeezed states. A state generated by applying the squeeze operator to the two-mode vacuum first, followed by the displacement operator, is called a two-mode coherent squeezed state. If the order of the two operators is reversed, a two-mode squeezed coherent state is obtained. It is found that the latter has a much stronger tendency to become a nonclassical state than the former, assuming that the parameters involved are all the same. A measure of nonclassical depth of radiation is adopted. It is found that, according to this measure, the effect of squeezing in making two-mode coherent squeezed states nonclassical is not monotonic. Such phenomena do not occur in squeezed coherent states.
Nonclassicality of a single-mode field is equivalent to the rank of two-mode entanglement this field generates at the output of a beam-splitter [Phys. Rev. A 89, 052302 (2014)]. We derive a measure for the nonclassicality of a single-mode field using the two-mode inseparability measure of Simon-Peres-Horodecki. Degree of the single-mode nonclassicality requires the knowledge of only ⟨a^2⟩\langle \hat{a}^2\rangle⟨a^2⟩ and ⟨a^†a^⟩\langle\hat{a}^\dagger\hat{a}\rangle⟨a^†a^⟩. This condition/measure is both a necessary and sufficient condition for Gaussian single-mode states. We show that this measure increases with squeezing strength and display a jump at the critical coupling for a superradiant phase transition. Additionally, we derive a simple analytical condition, ∣⟨a^2⟩∣>⟨a^†a^⟩|\langle \hat{a}^2\rangle|>\langle\hat{a}^\dagger\hat{a}\rangle∣⟨a^2⟩∣>⟨a^†a^⟩, for nonclassicality from Duan-Giedke-Cirac-Zoller criterion. In a forthcoming study, we use the derived measure to show the following. Non-causal linear optical response of an optomechanical cavity emerges at the same critical cavity-mechanical coupling where output field becomes nonclassical.
The thermofield introduced by Takahashi and Umezawa [Collect. Phenom. 2, 55 (1975)] is used to represent the thermal noise; it is generated from the absolute vacuum by a so-called thermalizing operator and serves as the initial state for generating two-mode squeezed states by squeeze and displacement operators. The ones obtained by applying the squeeze operator first followed by the displacement operator are called coherent squeezed states, while the others obtained by applying the displacement operator first are called squeezed coherent states. A criterion for the existence of nonclassical photon statistics in a two-mode radiation introduced by us in a previous paper [Phys. Rev. A 42, 1609 (1990)] is then used to study the conditions for the two types of squeezed states to be nonclassical states. Several three-dimensional plots are presented to illustrate the effects of thermal noise on squeezed states.
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The properties of correlated two-mode SU(1, 1) coherent states are studied. These states are a generalization of the two-mode squeezed vacuum states and are generated by applying the two-mode squeeze operator to a state in which there are initially q photons in one mode, |q, 0〉, the squeezed vacuum (q = 0) obviously being a special case. For these states the photon-number distribution and the nonclassical properties of photon antibunching, enhanced phase fluctuations, violations of the Cauchy–Schwarz inequality, two-mode squeezing, and sum squeezing are studied. A Q-function quasi-probability distribution for these states is also presented.