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Journal of the Optical Society of America B

Quantum description of a PT-symmetric nonlinear directional coupler

Vlasta Peřinová, A. Lukš, J. Křepelka

Published Mar 5, 2019• 11 citations
Publisher

Abstract

We treat a PT-symmetric directional coupler with Kerr nonlinearity in both the single-mode waveguides. We investigate the quantum propagation of a coherent state through this device and simplify the appropriate description to a semiclassical one. Through numerical calculation we find that, for lower gain rates, the oscillatory behavior of the coupler changes only a little under the influence of the supplied noise. But for higher gain rates, the noise causes the coupler to behave like above the threshold of the increase of the photon number in one mode. In the coupler without the cross-Kerr interaction, this behavior is more pronounced. In this connection, the first- and second-order correlation functions exhibit a temporary oscillatory behavior for higher gain rates.

DOI
10.1364/josab.36.000855

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We treat a PT-symmetric directional coupler with Kerr nonlinearity in both the single-mode waveguides. We investigate the quantum propagation of a coherent state through this device and simplify the appropriate description to a semiclassical one. Through numerical calculation we find that, for lower gain rates, the oscillatory behavior of the coupler changes only a little under the influence of the supplied noise. But for higher gain rates, the noise causes the coupler to behave like above the threshold of the increase of the photon number in one mode. In the coupler without the cross-Kerr interaction, this behavior is more pronounced. In this connection, the first- and second-order correlation functions exhibit a temporary oscillatory behavior for higher gain rates.
Quantum behavior of two oscillator modes, with mutually balanced gain and loss and coupled via linear coupling (including energy conserving as well as energy non-conserving terms) and nonlinear cross-Kerr effect, is investigated. Stationary states are found and their stability analysis is given. Exceptional points for PT -symmetric cases are identified. Quantum dynamics treated by the model of linear operator corrections to a classical solution reveals nonclassical properties of individual modes (squeezing) as well as their entanglement.
It is generally difficult to study the dynamical properties of a quantum system with both inherent quantum noises and nonperturbative nonlinearity. Due to the possibly drastic intensity increase of an input coherent light in gain-loss waveguide couplers with parity-time (PT)(\mathcal{PT})(PT) symmetry, the Kerr effect from a nonlinearity added into the system can be greatly enhanced and is expected to create macroscopic entangled states of the output light fields with huge photon numbers. Meanwhile, quantum noises also coexist with the amplification and dissipation of the light fields. Under the interplay between the quantum noises and nonlinearity, the quantum dynamical behaviors of the systems become rather complicated. However, the important quantum noise effects have been mostly neglected in previous studies about nonlinear PT\mathcal{PT}PT-symmetric systems. Here we present a solution to this nonperturbative quantum nonlinear problem, showing the real-time evolution of the system observables. The enhanced Kerr nonlinearity is found to give rise to a previously unknown decoherence effect that is irrelevant to the quantum noises and imposes a limit on the emergence of macroscopic nonclassicality. In contrast to what happens in linear systems, the quantum noises exert significant impact on the system dynamics and can create nonclassical light field states in conjunction with the enhanced Kerr nonlinearity. This study on the noise involved in quantum nonlinear dynamics of coupled gain-loss waveguides can help to better understand the quantum noise effects in many nonlinear systems.
We investigate a codirectional nonlinear coupler composed of two Kerr nonlinear waveguides. Unlike the conventional device, the linear coupling between the guides is supposed to be a variable function of the propagation distance. We calculate quantum statistical and dynamical properties of the Kerr nonlinear coupler with a coherent input and analyse the influence of coupling variation on oscillations in mean photon number. The possibility to control the switching characteristics and principal squeezing effect by adjusting the form of coupling function is shown.
The quantum effects in a four-mode Kerr nonlinear directional coupler (NLDC) are investigated using the phase-space representation. The four-mode Kerr NLDC device consists of two closely connected Kerr waveguides where the first channel is traversed by one mode while the second channel is traversed by three modes. The cavity effect on the propagation characteristics of the nonclassical light is also examined. The introduction of a cavity supports amplification of the nonlinearities at the expense of losses in the model. The dynamical quantum behaviour of the system is investigated by integrating the Langevin equations derived in both positive P and Wigner representations numerically. The characteristics of the mean fields and quadrature variances are explored by integrating the stochastic equations numerically over many trajectories. In comparison with a conventional two-mode device, we have found that the cavityless model provides better squeezing and the amplitude oscillation is amplified by controlling the initial state of the coherent light in the first channel. Moreover, the range of squeezing is extended as a result of multimode interaction. In addition, confining the system inside an optical cavity with highly reflective mirrors introduces rapid amplification in the oscillation of the field dynamics.
Quantum statistical properties of interacting optical fields in Kerr nonlinear couplers may be regarded as one of the most fundamental problems yet to be solved in quantum optics. As an analytical solution to this system is not always possible, a semi-analytic or direct numerical solution must be employed in order to obtain a more complete description of their evolution. This study investigates the quantum properties of four-mode couplers composed of two and four Kerr nonlinear waveguides, both with and without cavity set up via phase space representation. In these systems, the electromagnetic fields are described by their Hamiltonians, while the time-evolution of the systems are described by the Von-Neumann equation. Following standard techniques, the master equations for the density matrices of the systems are obtained and further converted to the corresponding classical Fokker- Planck equations using both positive P and Wigner representations. The corresponding set of Langevin Stochastic equations is then obtained from the Fokker-Planck equations by means of Ito rules. Finally, the systems are integrated numerically and averaged over many trajectories to get the relevant information. The effects of self-action nonlinearity, cross-action coupling and the initial energy of the coherent light on the evolution of the field quadrature variances are investigated. Furthermore, the influences of multimode interaction on the dynamics of the squeezed states are discussed. In this research, we show that the system may produce squeezed states in both quadrature variances for all calculated parameters. The multimode two channel model can provide better squeezing in contrast to the conventional device. The amplitude oscillation of maximal squeezing may be amplified by manipulation of the initial state of the coherent light in the first mode, and the range may be extended through multimode interaction. When the model is contained in a cavity, the system is able to generate better amplitude oscillations and squeezing range with leaf-revival-collapse-like behaviour; with the whole oscillation able to evolute below the standard quantum limit Regular photon transition and self trapping effects of the interacting modes in the four channels model are simulated, and squeezing is predicted for various initial intensities of the coherent field and nonlinear coupling parameter g. The cavity setup caused the system to undergo fluctuation amplification and the potential for different types of squeezing to be generated is observed. Depending on how we choose the interaction parameter values, the system may generate better squeezing in comparison to the cavity-less model.
In the framework of Hamiltonian formalism nonclassical effects of an optical field propagating inside a directional coupler containing nondegenerate parametric amplification have been studied. We investigate the effect of switching between the input modes and the out going fields from the coupler. Particular attention has been paid to two mode squeezing, second-order correlation function, quasiprobability distribution functions, and photon-number distribution. Incident number and coherent states are considered. It has been shown that when one of the modes enters the coupler in the Fock state and the other modes are in vacuum states, the coupler can serve as a generator for coherent state. Furthermore, regimes for generation and transmission of squeezed and/or sub-Poissonian light are found.