Fluctuations

Devon Bowen

T. Palm, P. Nalbach

We study the nonequilibrium dynamics of a quantum system under the influence of two noncommuting fluctuation sources, i.e., purely dephasing fluctuations and relaxational fluctuations. We find that increasing purely dephasing fluctuations suppress increasing relaxation in the quantum system. This effect is further enhanced when both fluctuation sources are fully correlated. These effects arise for medium to strong primary fluctuations already when the secondary fluctuations are weak due to their noncommuting coupling to the quantum system. Dephasing, in contrast, is increased by increasing any of the two fluctuations. Fully correlated fluctuations result in overdamping at much lower system-bath coupling than uncorrelated noncommuting fluctuations. In total, we observe that treating subdominant secondary environmental fluctuations perturbatively leads, as neglecting them, to erroneous conclusions.

David Wick

Tatsuya Tomaru

A fluctuation-characterization method using an interferometer is ordinarily classically formulated by considering frequency fluctuations. However, it is preferable to treat it quantum mechanically because the method measures q-number quadrature fluctuations at the same time. We formulated an interferometric fluctuation-characterization method by treating quadrature fluctuations as q-number variables and frequency fluctuations as a c-number stochastic variable, and we gave the power spectral density of the fluctuations for pulse light. This method measures fluctuations that are calibrated with shot noise level, and it makes it possible to estimate the fluctuation angle in phase space, which is the quantity that determines transmission characteristics in optical communications. Fluctuations in a laser diode were characterized as an example of measurement.

Daniel F. Walls, Matthew J. Collett, Alistair S. Lane +1 more

Current laser systems have phase and amplitude fluctuations which are partially quantum in origin. The amplitude fluctuations of a highly stabilized laser are shot noise limited, that is they have quantum fluctuations due to the Poissonian fluctuations of photons in a coherent state. These photon number fluctuations have been shown to have an origin in the incoherent pumping mechanism used for standard lasers. It has recently been demonstrated experimentally that by suppressing the amplitude fluctuations of the pump a laser output with reduced amplitude fluctuations is possible. The phase fluctuations in a laser are due to phase diffusion and are substantially above the quantum limit. Techniques to reduce the laser phase fluctuations have been proposed making use of correlated spontaneous emission.