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Acta Sinica Quantum Optica

Improvement of Continuous Variable Entanglement Enhancement Experiment

Xiaojun Jia

Published Jan 1, 2011• 1 citation
OpenAlex

Abstract

By using high finesse mode cleaner in the pump and injected filed and improving the accuracy in locking system,we improved the experimental system of continuous variable entanglement enhancement.The initial correlation degrees between amplitude and phase quadratures of EPR entanglement field are improved from(2.4±0.2) dB,(2.4±0.2) dB to(4.0±0.2) dB,(3.9±0.2) dB,and the quantum entanglement between amplitude and phase quadratures of the output field are improved from(3.0±0.2) dB,(3.0±0.2) dB to(5.6±0.2) dB,(5.5±0.2) dB,respectively.With respect to the past system,both the efficient and result of the entanglement enhancement are improved.

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By using high finesse mode cleaner in the pump and injected filed and improving the accuracy in locking system,we improved the experimental system of continuous variable entanglement enhancement.The initial correlation degrees between amplitude and phase quadratures of EPR entanglement field are improved from(2.4±0.2) dB,(2.4±0.2) dB to(4.0±0.2) dB,(3.9±0.2) dB,and the quantum entanglement between amplitude and phase quadratures of the output field are improved from(3.0±0.2) dB,(3.0±0.2) dB to(5.6±0.2) dB,(5.5±0.2) dB,respectively.With respect to the past system,both the efficient and result of the entanglement enhancement are improved.
We experimentally demonstrated that the quantum correlations of amplitude and phase quadratures between signal and idler beams produced from a non-degenerate optical parametric amplifier (NOPA) can be significantly improved by using a mode cleaner in the pump field and reducing the phase fluctuations in phase locking systems. Based on the two technical improvements the quantum entanglement measured with a two-mode homodyne detector is enhanced from approximately 4 dB to approximately 6 dB below the quantum noise limit using the same NOPA and nonlinear crystal.
Entanglement source with high entanglement degree is the guarantee for accomplishing the quantum information transmission and process with higher fidelity. Continuous variable Einstein-Podolsky-Rosen (EPR) entangled optical field with quantum correlation of amplitude and phase quadrature is a basic and important quantum resource in the quantum information science area, which can be obtained by a non-degenerate optical parametric amplifier (NOPA) operated below the threshold pump power. Because of the limitation of the imperfect performance of optical components in optical cavity, we should find efficient methods of implementing quantum manipulation to improve the entanglement degree of the entangled state of light. Connecting NOPA1 and NOPA2 in series, the entangled state of light output from the NOPA1 can be manipulated by NOPA2, and the entanglement degree can be enhanced under certain conditions. To improve the entanglement degree to a greater extent, the structure of the NOPA1 is chosen as a half-monolithic standing-wave optical resonator with the triple resonance of the pump and two subharmonic modes. The NOPA1 is able to output the entangled optical fields with an entanglement degree of 8.4 dB, which is the highest entanglement generated by a single device so far. The structure of the NOPA2 can be chosen as a standing-wave optical cavity or a four-mirror ring optical cavity. According to the different structures of the NOPA2, we theoretically design two kinds of optical systems with two cascaded cavities and compare the effects of the two optical systems on the continuous variable EPR entanglement cascaded enhancement in detail. Based on the above contrastive analysis, when the entanglement degree of the input optical fields is 8.4 dB and the transmissivity of the output coupler is lower, the structure of a four-mirror ring optical cavity for NOPA2 cannot enhance the entanglement degree, so the optical system including NOPA2 with standing wave cavity structure and the optical isolator with high transmission efficiency is appropriate. When the transmissivity of the output coupler and transmission efficiency of the optical isolator are higher, the structure of the NOPA2 should be chosen as a standing-wave optical cavity, otherwise the structure of the NOPA2 should be chosen as a four-mirror ring optical cavity. We also theoretically analyze the dependence of the correlation degree of output optical fields on physical parameters. The results show that under the conditions of higher input and output coupling efficiency, higher transmission efficiency and lower intro-cavity loss, the entangled state of light with higher entanglement degree can be obtained experimentally. This provides the reference for obtaining entangled optical fields with higher entanglement degree in the future.
Continuous variable (CV) quantum entanglement is a fundamental resource of CV quantum communication and quantum computation. It is useful in a wide variety of applications, including quantum teleportation, quantum dense coding, quantum key distribution, and high-precision quantum measurement. In this paper, we generate CV quantum entanglement at a telecommunication wavelength of 1342 nm by using a nondegenerate optical parametric amplifier (NOPA) with a type-Ⅱ periodically poled KTiOPO4 (PPKTP) crystal. A home-made continuous-wave single-frequency dual-wavelength (671 nm and 1342 nm) Nd:YVO4/LiB3O5 laser is achieved with output powers of 1.5 W (671 nm) and 1.3 W (1342 nm). Then a mode cleaner (MC1) with a fineness of 400 and linewidth of 0.75 MHz and a mode cleaner MC2 with a fineness of 400 and linewidth of 0.75 MHz are used to filter the noises of laser at 1342 nm and 671 nm, respectively. By using MCs, the intensity noise of laser reaches a shot noise level (SNL) for analysis frequencies higher than 1.0 MHz, and the phase noise of laser reaches an SNL for analysis frequencies higher than 1.3 MHz. Utilizing this kind of low noise single-frequency 671 nm laser as a pump, a doubly-resonant optical parametric oscillator with a threshold of 325 mW is realised. When the low noise single-frequency 1342 nm laser is injected as a signal and the relative phase between the pump and injected signal is locked to , the NOPA is operated at deamplification. After optimizing the temperature of the type-Ⅱ PPKTP crystal and at a pump power of 260 mW, Einstein-Podolsky-Rosen (EPR)-entangled beams with quantum correlation of 3.0 dB for both the amplitude and phase quadratures are experimentally generated. The strength of EPR-entangled beams is relatively low. It is maybe due to the low nonlinear conversion efficiency and large absorption of the type-Ⅱ PPKTP crystal at 671 nm and 1342 nm. The generated CV quantum entanglement at 1.34 m has lower transmission loss and smaller phase diffusion effect in a silica fiber. The research contributes to a high quality quantum source for the CV quantum communication based on existing telecommunication fiber networks.
A maximal joint quadrature squeezing of -6.8±0.4 dB is experimentally obtained by a scheme of cascaded four-wave mixing (FWM) processes, which gives strong proof about the inseparability or entanglement between output of the twin beams from the system. Here joint quadrature is the difference between the two quadratures of the twin beam output from the cascaded FWM processes. This result is enhanced by about 3.1 dB, compared with the one of the single FWM process. We also study the gain dependence of the entanglement enhancement in this cascaded system. Theoretical predictions with the considerations of the losses in the experiment are also studied, and a similar trend in the low-gain regime can be found between the experimental results and the theoretical predictions. The scheme of cascaded FWM processes, which can be used to improve or even manipulate the degree of the entanglement between the output fields from the single FWM process, may find its applications in the continuous-variable quantum communication protocols.
We demonstrate the experimental generation of quadrature amplitude entanglement in the 1550 nm band by using a fiber optical parametric amplifier. The measured noise variances of the difference and sum of the quadrature amplitudes of the pulsed signal and idler twin beams fall below the shot noise limit by about 1 and 0.8 dB (4.2 and 3.6 dB after the correction for efficiency), respectively, showing that the inseparability criterion of Einstein-Podolsky-Rosen entanglement I<2 is satisfied. Our investigation reveals that the quality of the measured entanglement can be further improved by increasing the transmission efficiency of the twin beams and by optimizing the temporal mode matching of the two sets of homodyne detection systems.
In this work, we propose a new simple entanglement measurement scheme, which can be utilized to take a measurement for the bright EPR beams generated by a nondegenerate optical parametric amplifier. Owing to the output signal and idler modes being frequency degenerate and in phase, one need not take a measurement for the signal and idler modes, respectively. By virtue of a quarter-wave plate and a half-wave plate, and then inserting a polarizing beam splitter, one can realize the measurements of the correlated phase quadratures and anti-correlated amplitude quadratures, simultaneously. Therefore, phase-locking and local oscillators are avoided.