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Optics Letters

Generation of continuous variable quantum entanglement using a fiber optical parametric amplifier

Xueshi Guo, Nannan Liu, Yuhong Liu, Xiaoying Li, Z. Y. Ou

Published Feb 1, 2016• 25 citations
Publisher

Abstract

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.

DOI
10.1364/ol.41.000653

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Acta Physica Sinica•Jan 1, 2017•6 citations
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.
Mode matching plays an important role in measuring the continuous variable entanglement. For the signal and idler twin beams generated by a pulse pumped fiber optical parametric amplifier (FOPA), the spatial mode matching is automatically achieved in single mode fiber, but the temporal mode property is complicated because it is highly sensitive to the dispersion and the gain of the FOPA. We study the temporal mode structure and derive the input-output relation for each temporal mode of signal and idler beams after decomposing the joint spectral function of twin beams with the singular-value decomposition method. We analyze the measurement of the quadrature-amplitude entanglement, and find mode matching between the multi-mode twin beams and the local oscillators of homodyne detection systems is crucial to achieve a high degree of entanglement. The results show that the noise contributed by the temporal modes nonorthogonal to local oscillator may be much larger than the vacuum noise, so the mode mis-match can not be accounted for by merely introducing an effective loss. Our study will be useful for developing a source of high quality continuous variable entanglement by using the FOPA.
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.
We report on the generation of a continuous variable Einstein-Podolsky-Rosen (EPR) entanglement using an optical fiber interferometer. The Kerr nonlinearity in the fiber is exploited for the generation of two independent squeezed beams. These interfere at a beam splitter and EPR entanglement is obtained between the output beams. The correlation of the amplitude (phase) quadratures is measured to be 4.0+/-0.2 (4.0+/-0.4) dB below the quantum noise limit. The sum criterion for these squeezing variances 0.80+/-0.03<2 verifies the nonseparability of the state. The product of the inferred uncertainties for one beam (0.64+/-0.08) is well below the EPR limit of unity.
Balanced homodyne detection relies on a beam splitter to superpose the weak signal input and strong local oscillator. However, recent investigation shows that a high gain phase sensitive amplifier (PSA) can be viewed as homodyne detector, in which the strong pump of PSA serves as the local oscillator [1]. Here, we analyze a new method of measuring the continuous variable entanglement by assisting a balanced homodyne detector with the PSA and implement it experimentally. Before measuring quadrature amplitude with the balanced homodyne detectors, two entangled fields generated from a pulse pumped fiber optical parametric amplifier are simultaneously coupled into the PSA. We find that the normalized noise for both the difference and sum of the quadrature amplitudes of the two entangled fields fall below the shot noise limit by about 4.6 dB, which is the record degree of entanglement measured in optical fiber systems. The experimental results illustrate that the advantages of the new measurement method include but not limit to tolerance to detection loss and characterizing entanglement with only one homodyne detector. The influence of mode-mismatching due to multi-mode property of entanglement on the measured noise reduction can also be greatly mitigated, indicating the new method is advantageous over the traditional measurement in multi-mode case.
Mode matching plays an important role in measuring the continuous variable entanglement. For the signal and idler twin beams generated by a pulse pumped fiber optical parametric amplifier (FOPA), the spatial mode matching is automatically achieved in single mode fiber, but the temporal mode property is complicated because it is highly sensitive to the dispersion and the gain of the FOPA. We study the temporal mode structure and derive the input-output relation for each temporal mode of signal and idler beams after decomposing the joint spectral function of twin beams with the singular-value decomposition method. We analyze the measurement of the quadrature-amplitude entanglement, and find mode matching between the multi-mode twin beams and the local oscillators of homodyne detection systems is crucial to achieve a high degree of entanglement. The results show that the noise contributed by the temporal modes nonorthogonal to local oscillator may be much larger than the vacuum noise, so the mode mis-match can not be accounted for by merely introducing an effective loss. Our study will be useful for developing a source of high quality continuous variable entanglement by using the FOPA.
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.