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Modern Physics Letters B

TIME EVOLUTION OF THE NONCLASSICALITY FOR THE FOCK STATES AND THEIR SUPERPOSITION STATES BY THE WIGNER FUNCTION

KUN SI, NING-LI ZHU, H. Y. Jia

Published Mar 31, 2011• 3 citations
Publisher

Abstract

The Wigner functions of the Fock states and their superposition states have negative value clearly. We focus on discussing the time evolution of the Fock states and their superposition states by the Wigner function under the coherent drive (pumping laser) and dissipation channels (cavity radiation). The results show that the negative region of their Wigner function gradually diminishes as the time kt or the gain coefficient g increasing. In addition, the disappearing time is related to the value of g - k. The loss of non-classical features becomes slower when g increases with the k > g; there is the "frozen zone" when k = g; then the loss of non-classical features becomes faster when g increases with k < g.

DOI
10.1142/s021798491102636x

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The Wigner functions of the Fock states and their superposition states have negative value clearly. We focus on discussing the time evolution of the Fock states and their superposition states by the Wigner function under the coherent drive (pumping laser) and dissipation channels (cavity radiation). The results show that the negative region of their Wigner function gradually diminishes as the time kt or the gain coefficient g increasing. In addition, the disappearing time is related to the value of g - k. The loss of non-classical features becomes slower when g increases with the k > g; there is the "frozen zone" when k = g; then the loss of non-classical features becomes faster when g increases with k < g.
Master equations of density operators can be concisely solved by virtue of thermo entangled state representation and the technique of integration within an ordered product of operators.The solution of master equations of density operators is given.We get the Wigner function evolution formula in phase space and photon number distribution formula of the state,which is a superposition of vacuum state and single-photon state initially in laser process.By numerical calculation we plot its Wigner function and photon number distribution evolution curves,when cavity gain constant and cavity loss constant are both equal to certain values.The effect of decoherence on nonclassical property of the state is discussed by comparing negative area size and the depth of negativity of Wigner function.Numerical results show that the negativity area size and the depth of negativity of its Wigner function decrease and then vanish with dissipation.In addition,we calculate the variation of negative volume of Wigner function with dissipation time.We find that it decreases with dissipation time increasing.These results show that the nonclassical properties of the state are weakened and then disappear with dissipation time increasing.
Detections and manipulations of quantum optical state at single-photon level have received much attention in the current experiments. Here, by numerically calculating the time-evolved Wigner functions, we investigate the dynamics of the typical non-classical state, i.e., few-photon superposition states in a dissipating optical microcavity. It is shown that the negativity of their Wigner function vanishes with dissipation. But this does not imply that all the non-classical features of the dissipative quantum state disappear. In fact, it is shown that the value of the second-order correlation function g(2)(0) (which serves usually as the standard criterion of a typical non-classical effect, i.e., the anti-bunching of photons, if g(2)(0)g(2A)(0) varies with the cavity dissipation and thus could be used to describe the physical effects of the dissipative cavity. Finally, we discuss the experimental feasibility of our proposal with a practically-existing cavity QED system.
We study the nonclassicality and decoherence in photon-loss channel of generalized squeezed Fock state, which is related to the squeezed operator with three real parameters. By using the Weyl ordering invariance under similarity transformations and the technique of integration within an ordered product of operators, we obtain the compact Wigner function of the generalized squeezed Fock state (Laguree-Gaussian function). According to the integration formula of the time evolution of the Wigner function, the Wigner expresssion in photon-loss channel is analytically deduced. In particular, we discussed the nonclassicality from the negativity of the Wigner function.
By numerically calculating the time-evolved Wigner functions, we investigate the dynamics of a few-photon superposed (e.g., up to two ones) state in a dissipating cavity. It is shown that, the negativity of the Wigner function of the photonic state unquestionably vanishes with the cavity's dissipation. As a consequence, the nonclassical effects related to the negativity of the Wigner function should be weakened gradually. However, it is found that the value of the second-order correlation function g(2)(0)g^{(2)}(0)g(2)(0) (which serves usually as the standard criterion of a typical nonclassical effect, i.e., g(2)(0)<1g^{(2)}(0)<1g(2)(0)<1 implies that the photon is anti-bunching) is a dynamical invariant during the dissipative process of the cavity. This feature is also proven analytically and suggests that g(2)(0)g^{(2)}(0)g(2)(0) might not be a good physical parameter to describe the photonic decays. Alternatively, we find that the anti-normal-order correlation function g(2A)(0)g^{(2A)}(0)g(2A)(0) changes with the cavity's dissipation and thus is more suitable to describe the dissipative-dependent cavity. Finally, we propose an experimental approach to test the above arguments with a practically-existing cavity QED system.
In this paper, the decoherence of the quantum excitation (the photon-added case) of even/odd coherent states, in a photon-loss channel is studied via the evaluation of the negative part of the Wigner quasi-distribution function in terms of the rescaled time ( ). We first obtain the explicit form of the time-dependent Wigner functions of the states considered, for arbitrary values of the photon excitations. To achieve this purpose, an equivalent Fokker–Planck-like equation which describes the time evolution of the Wigner function in the framework of the standard master equation of the density matrix is used. Then, by analyzing the time variation of the corresponding Wigner functions, the loss of the nonclassicality behavior and, hence, the decay of the negative part of the Wigner functions as a result of decoherence are clearly shown. In particular, we derive the threshold value of the rescaled time for single-photon-added even and odd coherent states; through this it is observed that the associated Wigner functions become positive whenever the rescaled time exceeds that value, i.e., the decoherence has become complete. By virtue of numerical calculations, it is also illustrated that the negative volume of the corresponding Wigner functions decreases with increasing time. In addition, the states with larger values of the excitation number m generally (but not absolutely always) possess smaller values of the rescaled threshold time.