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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Theoretical study on affecting factors on squeezing level in the generation and detection of squeezed light

Fei Feng, Jiang M. Xu, Jing T. Ma, Zun L. Liu

Published Feb 28, 2017• 0 citations
Publisher

Abstract

We analyze the influence factors that affect the observed degree of squeezing in generation and detection stages theoretically, including the fluctuations of pump power, the intra-cavity loss of the optical parametric amplifier (OPA) cavity , the phase fluctuation on balanced homodyning stage, the propagation efficiency, the mode matching efficiency, the out-put mirror transmission of the OPA cavity and the quantum efficiency of the detector. The theoretical results show that different loss sources appearing at various stages of the squeezed light generation and detection are far more dominant and in the end limit the squeezing level achievable. The analysis of these parameters will be extremely useful for subsequent experiments.

DOI
10.1117/12.2268974

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We analyze the influence factors that affect the observed degree of squeezing in generation and detection stages theoretically, including the fluctuations of pump power, the intra-cavity loss of the optical parametric amplifier (OPA) cavity , the phase fluctuation on balanced homodyning stage, the propagation efficiency, the mode matching efficiency, the out-put mirror transmission of the OPA cavity and the quantum efficiency of the detector. The theoretical results show that different loss sources appearing at various stages of the squeezed light generation and detection are far more dominant and in the end limit the squeezing level achievable. The analysis of these parameters will be extremely useful for subsequent experiments.
We demonstrate the methods for increasing the observed squeezing level in the generation of squeezed states of light at 1064 nm with periodically poled KTiOPO4 (PPKTP) crystal. We analyze the technical limits to the reduction of noise in the squeezed quadrature theoretically, including the intra-cavity loss of the optical parametric amplifier (OPA) cavity, the normalized amplitude, the detection frequency, and the mode matching on the balanced homodyning stage. To observe a high degree of squeezing, we improve the quality of mode matching into the OPA cavity and the mode cleaning (MC) cavity experimentally. By optimizing mode matching of the light into cavities, the finesse of the cavities would be higher in practice and the desirable spatial mode can be realized. As the intensity noise of the laser light reach the shot noise limit above the frequency of 1 MHz, which is sufficient for our experiment, so the MC cavity is incorporated to filter higher-order transverse modes of the local oscillator (LO) beam for the balanced homodyne detection. The experimental and theoretical results show that it is essential to optimize the mode matching efficiency to generate and detect high degree of squeezing otherwise an inefficiency mode matching will throw away the squeezing and transform the squeezed field into a vacuum field.
Nowadays, the realization of quantum computations and communications based on continuous variables has attracted a significant attention due to a substantial expansion of the system dimensionality. The main progress in this area is attributed to the implementation of multimode systems based on squeezed states of light. One of the simplest ways to generate such states relies upon their producing in a single-pass optical parametric amplifier (OPA) using ultrafast pumping. However, for homodyne detection of such multimode states, the profile of the local oscillator (LO) must perfectly match the profile of the measured mode. Usually, this is not the case; therefore a proper treatment of multimode squeezing is required. In this work, we study both theoretically and experimentally the multimode squeezed light generated in type-0 and type-II OPA. We characterize such sources and investigate the degree of squeezing in dependence on the LO spectral profile, employing a pulse shaping technique. The theoretical analysis is performed using the Schmidt-mode theory. This work might have a significant impact on the realization of multimode quantum protocols
Waveguide-based optical parametric amplification (OPA) is advantageous in generating broadband or pulsed squeezed vacuum. To enhance the squeezing level, it is crucial to clarify its limiting factors. In this study, we develop a numerical method which allows us to derive the squeezing level in waveguide OPA. Based on the developed method, we investigate the effect of the higher-order mode of the pump light, which generates noise due to the OPA between the fundamental mode and the higher-order mode. We also investigate the effect of optical loss in the waveguide. Specifically, we formulated coupled equations that take into account the degenerate OPA between the pump and signal both in the fundamental mode, and the non-degenerate OPA between the pump in the higher-order mode, the signal in the fundamental mode, and the idler in the higher-order mode. By solving the coupled equations, we can express the quantum state as a linear combination of annihilation and creation operators, and the squeezing level can be derived from its coefficients. The analysis allowed us to quantify the effect of OPA on the fundamental and higher-order modes, and the requirement for mode matching of pump light with the fundamental mode. We also analyzed the effect of loss in the waveguide based on the same procedure. The results show the loss in the waveguide to have less effect than the same loss at the output of the waveguide. These results provide important design criteria of waveguide OPA for squeezing.
We investigate the effects of cavity detuning on squeezing in the generation of the squeezed light at 1064 nm with a degenerate optical parametric amplifier (OPA) based on a periodically poled KTiOPO4 crystal. We theoretically analyze several effects that lead to cavity detuning, including the displacement and tilt of the incident laser light and the cavity length fluctuations of the OPA. To reduce the influence of the cavity detuning and increase the degree of generated quantum noise reduction, a side-of-fringe locking technique is applied to achieve the resonance between the OPA cavity and the injected laser beam. The experimental results show that the transmitted power through the locked cavity is increased and the cavity detuning is greatly suppressed. After locking of the cavity, we could expect to get a value for the control loop with an accuracy of 10−9 and the locking time is not less than 2 h, which paves the way for the preparation of the squeezed light.
A squeezed state with higher-order sidebands is a valuable quantum resource for channel multiplexing quantum communication. However, balanced homodyne detection used in nonclassical light detection has a trade-off performance between the detection bandwidth and clearance, in which the verification of a highly squeezing factor faces a challenge. Here, we construct two optical parametric amplifiers with cavity enhancement; one is for the generation of a -10.5 dB squeezed vacuum state, and the other is for all-optical phase-sensitive parametric homodyne detection. Finally, -6.5 dB squeezing at the carrier with 17 pairs of squeezing sidebands (bandwidth of 156 GHz) is directly and simultaneously observed. In particular, for the cavity-enhanced parametric oscillation and detection processes, we analyze the limiting factors of the detectable bandwidth and measurement deviation from the generated value, which indicates that the length difference and propagation loss between two optical parametric amplifiers should be as small as possible to improve the detection performance. The experimental results confirm our theoretical analysis.
Useful applications for squeezed states of light have been theorized for decades, however, only recently have advancements in the technology allowed for experimental investigations into applications of squeezed light. Most experiments which involve squeezing could be benefited by having a squeezing source which produces stable, highly squeezed states. This thesis investigates the generation of squeezed states of light which exhibit these properties, stability and high levels of squeezing. The key investigations are; • Initially a model is produced which estimates the expected level of squeezing in a singly or doubly resonant cavity. The properties of the cavity are altered by varying the reflectivity of one of the cavity mirrors for either the pump or seed frequencies. The effects of increasing pump power and introducing additional losses into the cavity are investigated as well as the optimum resonance conditions for maximum squeezing. • Built 3 squeezers with 3 different nonlinear mediums. A maximum squeezing value was found for each of the crystals and any effects which degraded or improved the results were investigated. Using PPKTP 5.8±0.1dB of squeezing was directly observed. Using PPSLT 4.9±0.1dB of squeezing was directly observed. PPSLT was found to have an unusually high threshold power, indicating that the crystal may have had a nonlinear coefficient much lower than expected. Using 1.7% MgO:PPSLN 1.1±0.2dB of squeezing was observed. The poor squeezing results were due to an anomaly in the material which affected its reflectivity and is investigated in the thesis. • The intra-cavity loss introduced to the system through the addition of each of the crystals was inferred from measurements of the cavity finesse, and the effect of this loss is explored. • Demonstrated the usefulness of a wedge in periodically poled materials to reach the optimum resonance condition in the squeezing setup. It is shown that the wedge is capable of cancelling the dispersion introduced in a round trip of the cavity, with no discernible negative effects. These investigations are used to comment on the suitability of each of these crystals for future experiments involving squeezing and possible modifications to improve the results.