We have experimentally investigated and shown evidence of correlated photon-pair generation based on coupled intracavity four-wave mixing (FWM) in a dual-wavelength erbium-doped fiber ring laser operating in the fiber-optic communicationC-band regime. In the coupled FWM process, the pump photons are both involved in the absorption and stimulated emission, which generate signal and idler wavelengths simultaneously. The correlation experiment was performed with various photon count levels in the order of hundreds of photons/s at each channel. The photon number of channel was less than 0.1/pulse and the maximum coincidence to single counts ratio was approximately 12% with a Gaussian profile in the accumulated counts.
Correlated Photon-Pair Source for Secured C-Band Quantum Communications
We have experimentally investigated and shown evidence of correlated photon-pair generation based on coupled intracavity four-wave mixing (FWM) in a dual-wavelength erbium-doped fiber ring laser operating in the fiber-optic communicationC-band regime. In the coupled FWM process, the pump photons are both involved in the absorption and stimulated emission, which generate signal and idler wavelengths simultaneously. The correlation experiment was performed with various photon count levels in the order of hundreds of photons/s at each channel. The photon number of channel was less than 0.1/pulse and the maximum coincidence to single counts ratio was approximately 12% with a Gaussian profile in the accumulated counts.
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.
Quantum correlations, where two or more parties exhibit strong correlations in a particular degree of freedom are important resources for fundamental science [1], such as the exploration of non-locality and entanglement, as well as non-classical technologies including quantum computation, quantum key distribution and quantum metrology. One typical way to generate correlated photon pairs is to use the spontaneous four-wave mixing process, with demonstrations on various platforms [2]. So far, single-photon stimulated four-wave mixing (FWM) has been demonstrated in optical fibers [3]. Here we show induced correlation by single photon seeded FWM through exploiting the orthogonal polarization modes in an integrated ring resonator [4]. We allow two different four-wave mixing processes to occur simultaneously with the same signal frequency mode but different idler frequency modes (see Fig.1(a)). The single-photon induced FWM process, named here seeded FWM, was created through the link of type-0 SFWM and type-2 SFWM. In the type-0 process, two excitation photons from the same field are converted into two new photons, signal and idler, with the same polarization as the pump field. While in the type-2 FWM, two annihilated photons, each one from a different polarized pump field, namely TE and TM, generate an orthogonal-polarized photon pair. More importantly, spatial and temporal overlapping of two processes and their sharing of one common resonance lead to correlation between the other two signal photons, one from each process. For example, an idler photon created in type-0 FWM seeds the type-2 FWM process or vice versa, i.e. the TM idler photon in Fig. 1, induces correlation between the signal TM and TE photons. This is proven by the observation of a clear coincidence peak for both signal photons in Fig.1(b). Furthermore, we confirmed our observation via power-dependent correlation measurements (see Fig.1 (c)). We summarized the true coincidence count (C) and its coincidence to accidental ratio (CAR) when only the TE pump power is increased while maintaining the TM mode pump power (black squares), and both pump powers are increased with a constant ratio (blue dots). We found that the seeded FWM scales with the square of both pump powers (C ∝ PjePtm) and that the CAR reach its maxima of 1.5 at the available pump power, indicating thermal statistics. Our findings are relevant to the fundamental understanding of spontaneous parametric effects as well as single-photon induced processes and single photon cloning.
Quantum-correlated photon pairs with tunable wavelength are highly desirable in a number of quantum technologies. In this letter, we demonstrate a wavelength tunable source of the correlated photon pairs based on pulse-pumped spontaneous four-wave mixing in photonic-crystal fiber. The wavelengths of the correlated signal and idler photons can be continuously tuned at room temperature by adjusting the wavelength of pump originating from an ytterbium-doped fiber laser. The tuning range of the idler photons is 757-880 nm and the range of the signal photons is 1310-1610 nm, which covers the 1310- and 1550-nm telecom bands.
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.
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.