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.
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.
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.
A continuous-wave (CW) photon-pair source operating in the L-band was successfully realized based on spontaneous four-wave mixing (SFWM) by using short length of a birefringent ZBLAN fiber. A type-III phase-matched SFWM process was utilized by pumping a birefringent ZBLAN fiber with pump photons polarized along the slow axis, where signal and idler photons were generated with polarizations aligned to the vertical axis of the birefringent ZBLAN fiber. The phase-matching curve was calculated, and the joint spectral intensity was analyzed under 1310 nm pumping conditions. Then, we experimentally generated signal and idler photons with center wavelengths of 1117 and 1578 nm using a 1310 nm CW laser as a pump light source and measured their quantum optical properties. The maximum coincidence-to-accidental ratio was calculated to be 22, resulting in a photon-pair brightness of 3.16 × 10 4 pairs/s/nm/mW. The results show the feasibility of using ZBLAN fibers in quantum optics as an efficient platform for generating a quantum light source.
Q uantum entanglement refers to the nonclassical dependency of physically separated quantum systems. It is an essential resource that must be freely available for implementing many of the novel functions of quantum information processing, such as database searching, clock synchronization, teleportation, computing and cryptography.1 The efficient generation and transmission of quantum entanglement is therefore of prime importance. With the ubiquitous standard optical fiber serving as the transmission medium and the widespread availability of efficient active and passive fiber devices, technological synergy between the generation and propagation components of the overall quantum network can be achieved by deploying sources of entanglement that rely on the nonlinearity of the fiber itself. Such fiber-based sources of quantum entanglement would also have the advantage of modal purity over their crystal counterparts,2,3 which is very important for realizing complex networks that involve many quantum operations. In our quest to develop fiber-based devices for quantum information processing, we have recently demonstrated an allfiber source of quantum-correlated photon pairs.4 The operating physical mechanism in this source is nondegenerate fourphoton mixing, wherein two pump photons scatter through the Kerr nonlinearity of the fiber to create simultaneous signal and idler photons. Our experiment is conducted near the zero-dispersion wavelength of standard dispersion-shifted fiber, where such scattering is enhanced owing to phase matching of the photon wave functions. It must be pointed out here that in a conventional wavelength-divisionmultiplexed classical optical communication line, one strives to suppress the fourphoton or four-wave mixing process, which otherwise causes cross talk between the wavelength channels and sets limits on the total data capacity of the communication line. In our experiment, the four-photon mixing takes place in a nonlinear-fiber Sagnac interferometer (NFSI), which we previously used to generate quantum-correlated twin beams in the fiber.5 The pump is a mode-locked train of ~3-ps-long pulses. The pulsed operation serves two important purposes: the NFSI amplifier can be operated at low average powers and the production of the signal/ idler fluorescence photons is confined to well-defined temporal windows, allowing use of a gated detection scheme to increase the signal-to-noise ratio. To measure the nonclassical (i.e., quantum) correlations between the signal and the idler photons, one must effectively suppress the pump photons from reaching the detectors. Since a typical pump pulse contains ~108 photons and we are interested in detecting ~0.01 signal (idler) photons/pulse, a pump-to-signal (idler) rejection ratio in excess of 100 dB is required. We achieved this specification by sending the fluorescence photons through a free-space double-grating spectral filter that separates the signal and idler photons from each other and from the pump photons not rejected by the NFSI. To demonstrate the quantum nature of the four-photon scattering at the single photon level, we assembled a photonFigure 1.C oincidence rates as a function of the single-photon rates in two different cases: signalidler fluorescence produced by a pump pulse (diamonds) and signal-idler fluorescence produced by two consecutive pump pulses (triangles). T he line represents the calculated accidental counts. T he inset shows a plot of the detected idler photons as a function of the injected pump photons (hollow circles). A second-order polynomial is shown to fit the experimental data. T he contributions of the dark counts, linear scattering and quadratic scattering are plotted separately as well. counting apparatus for detecting the signal and idler photons in coincidence. This apparatus is based on commercial InGaAs avalanche photodiodes operating in the gated Geiger mode.4 In the inset in Fig. 1, we show the number of scattered photons, NS, versus the number of pump photons, NP, injected into the NFSI. We fit the experimental data with a second-order polynomial fit, including the number of dark counts during the gate interval, ND. The fit clearly shows that the quadratic scattering ( ∝NP) in the fiber can dominate over the residual linear scattering ( ∝NP) of the pump, owing to imperfect filtering. The main body of Fig. 1 shows the coincidence-counting results. The diamonds represent the rate of coincidence counts versus the rate of the signal (idler) photons generated during the same pump pulse. The triangles represent the measured coincidence rate versus the signalphoton count rate when the signal is delayed with respect to the idler by one pulse period. For two independent photon sources, each with a count rate RS <<1, the accidental coincidence rate RC is given by RC = RS 2, regardless of the photon statistics of the sources. This quadratic relation is plotted as the solid line in Fig. 1, which fits the delayed-coincidence data (triangles) very well. These measurements show that while the fluorescence photons produced by the adjacent pump pulses are independent, those coming from the same pump pulse show a strong correlation, which is a signature of their non-classical behavior. This work was supported by the Department of Defense Multidisciplinary University Research Initiative program administered by the Army Research Office under Grants DAAD19-00-1-0177 and DAAD19-00-1-0469, and by the U.S. Office of Naval Research under Grant N00014-91-J-1268.
We demonstrate an all-fiber broadband photon pair source based on four wave mixing (FWM) process in dispersion flattened highly non-linear fiber (DF-HNLF). The fiber exhibits a zero dispersion slope near 1550 nm, allowing phase-matched FWM over entire S, C, and L bands and thus efficient generation of signal and idler photons. A comparative theoretical study between conventional dispersion-shifted fiber (DSF) and DF-HNLF highlights the spectral range differences in the generation of photon pairs. We measure coincidence counts at three different sets of wavelengths. To study the effects of Raman scattering, which acts as noise source in these types of fibers, we calculate the correlationg(2)(τ)at different pump powers. We use stimulated emission tomography to characterize the generation of photon pairs across the S, C, and L bands. We show that DF-HNLF is an ideal medium for generating correlated photons over a broad spectral range (>100nm), making it suitable for frequency-multiplexed quantum communication systems. We estimate the photon pair generation efficiency to be 0.05mW−2/pulse.
We demonstrate an all-fiber photon pair source for the critical telecom C-band. We achieve high pair generation rates in excess of 10 MHz while maintaining coincidence-to-accidental ratios (CARs) greater than 100. This is one of the brightest and lowest-noise photon pair sources ever demonstrated. We achieve the high pair rate through CW-pumped spontaneous four-wave mixing in dispersion-shifted fiber. We achieve the high CAR by cooling the fiber to 4 K to suppress the Raman generation and detecting the photons with low jitter and low dark count superconducting single-photon detectors.
We demonstrate greatly improved results for the production of correlated photon-pairs using the four-photon scattering process in silica fiber. We achieve a true-coincidence-count to accidental-coincidence-count ratio greater than 10, when the photon-pair production rate is about 0.04 /pulse. This represents a four-fold improvement over our previous results. The contribution of spontaneous Raman scattering, the primary cause of uncorrelated photons that degrades the fidelity of this source, is reduced by decreasing the wavelength detuning between the correlated photons and the pump photons and by using polarizers to remove the cross-polarized Raman-scattered photons. Excess Raman scattering could be further suppressed by cooling the silica fiber. Even without cooling the fiber, the achieved 10 to 1 ratio of true-coincidence to accidental-coincidence makes the fiber source of correlated photon-pairs a useful tool for realizing various quantum-communication protocols.