Observation of photon-pair generation in the normal group-velocity-dispersion regime with slight detuning from the pump wavelength

Abstract

A fiber-based photon-pair source in the telecom C-band is suitable for quantum information science including quantum communications.Spontaneous four-wave mixing effects are known to create photon pairs that are slightly detuned from the pump wavelength only in the anomalous groupvelocity-dispersion (GVD) regime.Here, we achieve high-quality photon-pair generation slightly detuned from the pump wavelength in the normal GVD regime through a dispersion shifted fiber, for the first time.The photon pairs in C-band exhibit strong temporal correlation with each other and excellent heralded anti-bunching property.This photon-pair generation scheme can be exploited as telecom-band quantum light sources for quantum information applications.A non-classical light source is one of the key elements of quantum information processing systems, and several types of these sources have been investigated, including molecules [1], trapped atoms [2], quantum dots [3], diamond vacancy centers [4], spontaneous parametric down-conversion (SPDC) [5], and spontaneous fourwave mixing (SFWM) [6]. Among these processes, the SPDC and the SFWM processes create quantumcorrelated photon pairs from one and two pump photons, respectively, and both processes require satisfying the energy conservation as well as phase matching [7].Typically, the nonlinear coefficient of SPDC in second-order nonlinear crystals is larger than that of SFWM in amorphous single mode optical fibers as SFWM is a third-order nonlinear process.In the case of several hundred meters long single-mode fibers, however, the SFWM conversion efficiency from pump beam to photon pairs can be similar with that of the SPDC process in nonlinear crystals.In addition, the fiber based photon-pair sources are able to be directly coupled to fiber systems with negligible coupling loss, but waveguide-to-fiber coupling efficiency of a periodically poled SPDC crystal is reported about 50% [8].The SFWM process has been studied intensively for the fiber-based photon generation [6, 9-20] for quantum information applications including quantum key distribution.Photon-pair generation via SFWM is active near the zero group-velocity-dispersion (GVD) wavelength, λ ZGVD , in the anomalous GVD regime [6, 9-15] or normal GVD regime [16][17][18][19][20].In the normal GVD regime, the wavelength of the generated photon pairs is significantly detuned from the pump wavelength.In this case, Raman noise can be avoided, but the generated photon pair do not exhibit telecom-band wavelength together.On the other hand, in the anomalous GVD regime, the wavelength of the generated photon pair is slightly detuned from the pump in the telecom C-band and is strongly power dependent [16].Since the wavelength of the generated pair is sensitive to the λ ZGVD of the generation medium, several types of commercial fibers which have λ ZGVD in the telecom C-band have been investigated as the telecom C-band pair sources.Even if commercial optical fibers have small tolerances in their physical properties, it is common to have sizeable λ ZGVD variance along an optical fiber [21,22].Therefore, the precise measurement of the GVD spectrum is required, but it is often tricky with short length optical fibers.One way to overcome this problem is engineering the dispersion of a commercial fiber by heating and pulling the fiber, but this method is an ambitious research field [19,20,23].On the other hand, if a slightly detuned pair can be generated in the normal GVD regime, the constraint for the λ ZGVD of medium can be weakened, but it has not been reported to the best of our knowledge because phases are mis-matched in this regime due to the Kerr effects of the media.