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Bandaoti guangdian

Noise in Generation of Photon Pairs in Highly Nonlinear Fiber and Its Suppression

Yunxia Liu

Published Jan 1, 2006• 0 citations
OpenAlex

Abstract

It is important to generate high quality photon pairs in the highly nonlinear optical fiber for practical applications of quantum information.Based on analysis of the principle of the generation of photon pairs by means of four-wave-mixing in optical fiber,main factors influencing noise generation of photon pairs have been presented.Several methods to suppress the noise have been discussed.With a cooled DSF,a short pulse and a narrow filtering bandwidth,signal and idler photons that generate with greater wavelength separation space from the bump,and high quality photon pairs with a better signal to noise ratio can be achieved in the highly nonlinear fiber.

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It is important to generate high quality photon pairs in the highly nonlinear optical fiber for practical applications of quantum information.Based on analysis of the principle of the generation of photon pairs by means of four-wave-mixing in optical fiber,main factors influencing noise generation of photon pairs have been presented.Several methods to suppress the noise have been discussed.With a cooled DSF,a short pulse and a narrow filtering bandwidth,signal and idler photons that generate with greater wavelength separation space from the bump,and high quality photon pairs with a better signal to noise ratio can be achieved in the highly nonlinear fiber.
Spontaneous four-wave mixing in a dispersion-shifted fiber (DSF) is a promising approach for generating quantum-correlated photon pairs in the 1.5 microm band. However, it has been reported that noise photons generated by the spontaneous Raman scattering process degrade the quantum correlation of the generated photons. This paper describes the characteristics of quantum-correlated photon pair generation in a DSF cooled by liquid nitrogen. With this technique, the number of noise photons was sufficiently suppressed and the ratio of true coincidence to accidental coincidence was increased to ~30.
A one-meter-long high nonlinear photonic crystal fiber is pumped by laser pulse train at room temperature, and the generated correlated idler and signal photons are centred at 830 nm and 1411 nm, respectively. The full widths at half maximum of the broad band filters of the two channel are 15 nm and 35 nm, respectively. The fitting results of single channel photon counts reveal that almost all the photons originate from spontaneous four wave mixing and the influence of Raman scattering is eliminated. When the production rate is 0.0085 photon per pulse, the ratio between coincidence rate and accidental coincidence rate is measured to be 102, which not only confirms the low noise property of the photon pair, but also shows that the photon pair are naturally narrow band, and the collecting efficiency in our experiment is high. Moreover, these high purity photon pairs connect different optical bands, and have potential applications in quantum information technologies.
We generate photon pairs at telecom wavelength through a spontaneous four-wave mixing process in a short 10 m of highly nonlinear fiber. We use a counterpropagating scheme to generate a correlated and entangled photon pair. We observe coincidence to accidental-coincidence ratio of 29±3 at room temperature (300 K) and as high as 130±5 when the fiber is cooled to liquid-nitrogen temperature (77 K). Two-photon interference with visibility >98% (>92%) and the violation of Bell's inequality by >12 (≈5) standard deviation are observed at 77 K (300 K), respectively, without subtracting accidental-coincidence count. We obtain a photon-pair production rate about factor 3(2) higher than a 300 m dispersion-shifted fiber at 300 K (77 K).
Using a Sagnac fiber loop functions as a deterministic splitter of photon pairs produced by the frequency degenerate four wave mixing, we show that the background noise of the degenerate photon pairs is contributed by both Raman scattering and frequency non-degenerate four wave mixing. To improve the purity of photon pairs in the high gain regime, in addition to suppressing the noise photons by cooling the nonlinear fiber and by optimizing the detuning between the frequencies of the pump and photon pairs, the walk-off effect of the two pulsed pump fields should be mitigated by managing the dispersion of the fiber. Our investigation is not only the first step towards the generation of multi-mode squeezed vacuum in fiber optical parametric amplifiers pumped with pulsed lights, but also contributes to improving the purity of the fiber sources of degenerate photon pairs.
Single photon sources are a key element for quantum computing, quantum key distribution (QKD) and quantum communications. In particular, producing single photons at telecommunications wavelengths is valuable for QKD protocols and would enable realizing the quantum internet. The preferred method for their generation has long been spontaneous down conversion in bulk crystals, which suffers from connection loss to fiber networks. In-fiber spontaneous four-wave mixing provides a viable alternative as a photon pair source due to being compatible with existing fiber networks. We present an all-fiber photon pair source based on degenerate four-wave mixing in a 400 m Highly-Nonlinear fiber, with signal and idler wavelengths generated at 1552.5 nm and 1557 nm respectively. The source consists of CW pump laser operating at 1554.75 nm, which is slightly detuned from the zero group velocity dispersion wavelength into the normal dispersion regime. After pair generation in the highly-nonlinear fiber, three arrayed waveguide gratings are employed to spatially separate signal and idler, and provides a 120 dB pump power reduction. Firstly the source is modelled and experimentally characterized in the well known classical regime of stimulated four-wave mixing. The effect of fiber cooling on spontaneous Raman scattering is modelled and characterized, and a 30% reduction in spontaneous emission is found when cooling the fiber to -77°C. In the low power regime the coincidence to accidental count ratio is simulated and measured. An increase in the coincidence to accidental count ratio is observed when cooling the fiber.