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arXiv:1606.00178 · Physical Review A

Enhanced optical squeezing from a degenerate parametric amplifier via time-delayed coherent feedback

Nikolett Német, A. S. Parkins

Published Aug 1, 2016• 48 citations
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Abstract

Squeezing levels of a degenerate parametric amplifier could be substantially increased by performing a conceptually simple modification to its operation. The study presents a detailed analysis of the influence of time-delayed feedback, i.e. feeding one of the output fields of the system back to the input channel after a significant delay.

DOI
10.1103/physreva.94.023809

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Squeezing levels of a degenerate parametric amplifier could be substantially increased by performing a conceptually simple modification to its operation. The study presents a detailed analysis of the influence of time-delayed feedback, i.e. feeding one of the output fields of the system back to the input channel after a significant delay.
Quantum coherent feedback control is a measurement-free control method fully preserving quantum coherence. In this paper we show how time-delayed quantum coherent feedback can be used to control the degree of squeezing in the output field of a cavity containing a degenerate parametric oscillator. We focus on the specific situation of Pyragas-type feedback control where time-delayed signals are fed back directly into the quantum system. Our results show how time-delayed feedback can enhance or decrease the degree of squeezing as a function of time delay and feedback strength.
The theory of quantum feedback networks has recently been developed with the aim of showing how quantum input-output components may be connected together so as to control, stabilize, or enhance the performance of one of the subcomponents. In this paper, we show how the degree to which an idealized component (a degenerate parametric amplifier in the strong-coupling regime) can squeeze input fields may be enhanced by placing the component in loop in a simple feedback mechanism involving a beam splitter. We study the spectral properties of output fields, placing particular emphasis on the elastic and inelastic components of the power density.
The squeezing level of light on atomic resonance produced by a degenerate optical parametric amplifier (DOPA) is now limited by blue or violet pump-light-induced loss, and it is difficult to improve the squeezing by simply reducing the loss of DOPA. In this study, two promising schemes, namely, the cascaded DOPA (C-DOPA) and the coherent feedback-controlled DOPA (CFC-DOPA), are theoretically discussed. For a ideal case, the CFC-DOPA can realize infinite squeezing output but requires almost infinite accuracy in phase locking. For a practical case with feasible physical parameters of realistic systems, the performance of squeezing enhancement for the C-DOPA and CFC-DOPA is compared. The C-DOPA shows advantages in the megahertz frequency range, while the CFC-DOPA shows advantages in the low-frequency range. At a general analysis frequency of 2 MHz, C-DOPA shows better robustness against the loss of the system. As a price, it requires a smaller instability tolerance for phase locking. With the C-DOPA, the squeezing on Rb atomic resonance can be pushed from − 5 d B to − 7 d B . Our analytical results can provide a valuable reference for designing experimental devices to efficiently produce high-quality squeezed light on atomic resonance, which has potential applications in quantum information processing and quantum metrology.
The purpose of this article is to present an efficient coherent feedback control technique for squeezing enhancement of degenerate parametric amplifiers (DPAs). The feedback controllers to be designed are in the form of linear quantum stochastic systems which can be described by quantum feedback networks theory recently developed. The problem of squeezing enhancement is formulated as a non-convex constrained nonlinear programming problem, whose solution yields the coherent feedback controllers to be designed. Compared with the previous study, the proposed approach reveals clear advantage when DPAs are lossy, as is always the case in practice. Based on the recently developed quantum network synthesis theory, we show how the designed coherent feedback controllers can be implemented by means of quantum optical devices.
A study of an open nondegenerate parametric oscillator is presented below threshold in the undepleted pump regime with time-delayed coherent feedback. The figure of merit is the two-mode squeezing spectrum measuring the strength of continuous-variable Einstein-Podolsky-Rosen-type entanglement between the down-converted modes in the output field as a function of frequency. Exploring different areas of the parameter space reveals the possibility of significantly enhanced entanglement with tunable characteristic frequency. This is a result of the emerging dynamical features that are only accessible due to delayed feedback. The system is very sensitive to phase matching, which can be adjusted by various parameters of the system and the feedback loop. In this way, sharp, resonancelike features are obtained that show a level of robustness against extraneous losses.