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Optics Letters

Long-distance distribution of the telecom band intensity difference squeezing generated in a fiber optical parametric amplifier

Yuhong Liu, Nan Huo, Jiamin Li, Xiaoying Li

Published Nov 5, 2018• 25 citations
Publisher

Abstract

Using an all-fiber source of pulsed twin beams, with the record intensity difference squeezing of 6.1±0.15 dB for fiber optical parametric amplifiers, we experimentally distribute the pulsed continuous variable (CV) quantum state over a long distance. After separating the signal and idler twin beams by 26 km single-mode fibers, the measured intensity difference noise is still below the shot noise limit by 2 dB. The result shows that the distribution process is only affected by the vacuum noise originated from the fiber transmission loss. The extra noise contributed by the acoustic wave Brillouin scattering and Raman scattering in 26 km transmission fibers has not been observed. The investigation is beneficial for studying long-distance CV quantum communication.

DOI
10.1364/ol.43.005559

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Using an all-fiber source of pulsed twin beams, with the record intensity difference squeezing of 6.1±0.15 dB for fiber optical parametric amplifiers, we experimentally distribute the pulsed continuous variable (CV) quantum state over a long distance. After separating the signal and idler twin beams by 26 km single-mode fibers, the measured intensity difference noise is still below the shot noise limit by 2 dB. The result shows that the distribution process is only affected by the vacuum noise originated from the fiber transmission loss. The extra noise contributed by the acoustic wave Brillouin scattering and Raman scattering in 26 km transmission fibers has not been observed. The investigation is beneficial for studying long-distance CV quantum communication.
Motivated by the pursuit of a simple system to produce a continuous variable non-classical light source for long-distance quantum communication, we construct an all-fiber source of pulsed twin beams in the 1550 nm band by using a high gain fiber optical parametric amplifier. The intensity-difference noise of the twin beams is below the shot noise limit by 3.1 dB (10.4 dB after correction for losses). A detailed study reveals a number of limiting factors for noise reduction. Therefore, further noise reduction will be feasible once care is taken for these limiting factors.
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.
We theoretically investigate the quantum-noise properties of a pulse-pumped high-gain-fiber optical parametric amplifiers (FOPAs) by using the Bogoliubov transformation in multiple frequency modes to describe the evolution of the nondegenerate signal and idler twin beams. The results show that the noise figure of the FOPA is generally greater than the 3 dB quantum limit unless the joint spectral function is factorable and the spectrum of the input signal well matches the gain spectrum in the signal band. However, the intensity-difference noise of the twin beams, which weakly depends on the joint spectral function, can be significantly less than the shot-noise limit when the temporal modes of the pump and the input signal are properly matched. Moreover, to closely resemble real experimental conditions, the quantum noise of twin beams generated from a broadband FOPA is numerically studied by taking the various kinds of experimental imperfections into account. Our study is not only useful for developing a compact fiber source of twin beams, but also helpful for understanding the quantum-noise limit of a pulse-pumped FOPA in the fiber communication system.
Continuous variable (CV) quantum entanglement is an essential resource for quantum computation and communication protocols. The use of CV quantum entanglement at a telecommunication wavelength of 1.5m in combination with existing fiber telecommunication networks offers the possibility to implement long-distance quantum communication protocols like quantum key distribution (QKD) and applications such as quantum repeaters, quantum teleportation in the future. In spite of the fact that the optical power attenuation of light in a standard telecommunication fiber is lowest at a wavelength of 1.5m, the entangled states will interact with fiber channels and the disentanglement will occur. It is one of the important factors restricting the development of long distance quantum information. In this paper, CV entangled state at 1.5m telecommunication band is obtained by using a type-II periodically poled KTP (PPKTP) crystal inside a nondegenerate optical parametric amplifier (NOPA). A wedged PPKTP is used for implementing frequency-down-conversion of the pump field to generate the optically entangled state and achieving the dispersion compensation between the pump and the subharmonic waves. By controlling the temperature and the length of the PPKTP crystal, a triply resonant optical parametric oscillator with a threshold of 80 mW is realized. Einstein-PodolskyRosen (EPR)-entangled beams with quantum correlation of 8.3 dB for both the amplitude and phase quadratures are experimentally generated by using a single NOPA at a pump power of 40 mW and an injected signal power of 10 mW when the relative phase between the pump and injected signal is locked to . The generated entangled state is coupled into a single-mode optical fiber, and the transmission characteristics of the generated EPR entangled beams through standard single-mode fibers are investigated experimentally and theoretically. A fiber polarization controller is used to compensate for the polarization state variation induced by random fluctuations of birefringence of the single mode fiber when the light propagates along the fiber, and to keep the polarization of light linear at the fiber output. A 0.21 dB quantum entanglement could still be observed for the EPR-entangled beams transmitted through a 50-km-long single-mode fiber. The theoretical prediction considering the excess noise in fiber channel is in good agreement with the experimental result. The generated CV quantum entanglement is highly suitable for the required experiments, such as CV measurement-device-independence QKD based on standard fibers, owing to the fact that the tolerance of the excess noise in the quantum channel can be enhanced significantly with respect to a coherent state if EPR-entangled beams are used.
Multimode squeezed states are essential resources in quantum information processing and quantum metrology with continuous variables. Here we present the experimental generation of squeezed vacuum via the degenerate four wave mixing realized by pumping a piece of dispersion shifted fiber with mode-locked ultrafast pulse trains. The noise fluctuation is lower than the shot noise limit by 1.1 ± 0.08 dB (1.95 ± 0.17 dB after correction for detection losses). The detailed investigation illustrates that the results can be further improved by suppressing Raman scattering and by reshaping the spectrum of the local oscillator to achieve the required mode-matching of the homodyne detection system. Our study is useful for developing a compact fiber source of multi-mode squeezed vacuum.