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Physical Review A

Quantum simulations of nonlinear optical damping: An exact solution for the stochastic differential equations and an interpretation of ‘‘spiking’’

I. J. D. Craig, K. J. McNeil

Published Jun 1, 1989• 15 citations
Publisher

Abstract

This paper considers the ``positive-P'' description of nonlinear optical damping where one-photon and two-photon loss mechanisms are allowed. An exact solution of the corresponding stochastic differential equations is presented that graphically shows the breakdown of the positive-P representation found in earlier numerical-simulation work. The problem of ``spiking'' in simulations of nonlinear optical processes is then addressed, using the nonlinear damping process as an example. It is pointed out that although ``spiking'' can be exascerbated at high levels of quantum noise by numerical inaccuracy, the tendency to spike is in fact an analytic property of the stochastic variables.

DOI
10.1103/physreva.39.6267

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This paper considers the positive-P'' description of nonlinear optical damping where one-photon and two-photon loss mechanisms are allowed. An exact solution of the corresponding stochastic differential equations is presented that graphically shows the breakdown of the positive-P representation found in earlier numerical-simulation work. The problem of spiking'' in simulations of nonlinear optical processes is then addressed, using the nonlinear damping process as an example. It is pointed out that although ``spiking'' can be exascerbated at high levels of quantum noise by numerical inaccuracy, the tendency to spike is in fact an analytic property of the stochastic variables.
We present here an extensive study of the technique of numerical simulations, applied to the solution of a system of equations obtained by the positive P representation. The model we introduce to aid in these investigations has the distinct advantage of an immediate physical interpretation. In these simulations we experience the divergent trajectory or ``spike,'' noted by other authors; however, due to the simplicity of our equations, we are able to categorize and explain these spikes. Finally, we report a disturbing anomaly in some of the simulation results which indicates a possible problem with the use of the positive P representation.
The problem of the formulation and simulation of quantized optical systems using the positive-P representation is considered. We show that feasible computational strategies are determined mainly by the eigenstructure of the system of stochastic differential equations that correspond to the positive-P Fokker-Planck equation. We demonstrate analytically that the stability and accuracy of numerical solutions are generally much improved by adopting time-centered implicit schemes. These findings are then related to the specific problem of second-harmonic generation. It is shown that the eigenstructure of the stochastic system possesses interrelationships that can lead to severe difficulties in numerical simulations. In particular we show how the problem of ``spiking'' encountered at high levels of quantum noise can be related to the numerical ``stiffness'' of the problem. We also demonstrate the advantages of time-centered implicit methods by highlighting the distortions that can occur in the Euler-type simulation of limit cycle solutions. Of physical interest is the fact that the phase-space morphology of the quantized second-harmonic generator admits families of limit cycles. This is in marked contrast to the purely classical picture in which, for a given parameter set, there exists only a single limit cycle. Finally we demonstrate by direct analytic solution that, under the conditions of adiabatic elimination, the positive-P representation for the second-harmonic generator with zero driving field fails, in the sense that it produces nonphysical results. This failure is in all respects analogous to the breakdown encountered in the problem of nonlinear optical damping.
The positive P representation is a very successful tool in quantum optics. However, the usual assumption of negligible boundary terms in the time-evolution equations is not always valid. We explore the range of validity of the time-evolution equations both analytically and by numerical investigation of a number of specific examples. We present practical ways of verifying the validity of the use of the positive P representation and find that the standard time-evolution equation can become invalid when nonlinear terms (at unit photon number) are large relative to the damping rate. This is very much larger than is normally the case in nonlinear optics, except possibly near resonances. We are able to show that when the positive P representation is invalid, the boundary terms, normally neglected in an integration by parts, become non-negligible. When numerical simulations are carried out using the positive P representation, specific checks given in this paper should be carried out to verify the compactness of the distribution. In conclusion, we find that (apart from special cases) this technique of quantum time evolution is typically asymptotically valid in the limit of small nonlinearity, rather than exact.
Stochastic differential equations for absorptive optical bistability derived using the positive P representation are solved numerically. The stochastic model is formulated in ten dimensions, twice the dimensions of the space of physical observables. Extra dimensions are needed to include the quantum character of the noise. Ensemble averages are computed for the field amplitude, the incoherent intensity, and the second-order correlation function of the transmitted light. Comparisons with linearized theory are made and detailed results are given in the vicinity of the critical point where nonlinear effects are paramount. The scaling of fluctuations with system size at the critical point is investigated. Transitions between deterministic steady states are studied in a system just beyond the threshold for bistability. A potential difficulty with the use of the positive P representation for modeling nonlinear quantum-statistical systems is identified.
Generation of light with nonclassical properties, such as squeezed light, has gathered a lot of attention because of possible uses in such areas as quantum state engineering, quantum imaging, continuous variable quantum computing, and detection of gravitational waves. Squeezed light is generally produced by transporting light through a nonlinear medium. One such media is Kerr-nonlinear optical fibres. We aim to find the parameters for a fibre polarization squeezing setup by studying the dependencies of quantum noise suppression on the duration of the pulse and the input power. This is performed by modeling the light field propagation using the split-step Fourier method to numerically solve the stochastic nonlinear Schrödinger equation obtained by using the Wigner representation. This equation includes such physical effects as damping, quantum loss noise, dispersion, Kerr and Raman nonlinearities, and stochastic Raman noise.
Abstract The stochastic Schrödinger equation, a novel method for describing the dissipative dynamics of open quantum systems interacting with their environment, is applied to some nonlinear optical processes, namely nonlinear absorption and second‐harmonic generation, the latter also in the range of instabilities and classical chaotic behaviour. Where possible the results are compared with numerical simulations of the positive P representation, which in some cases are found to give unreliable results.