Single photon sources (SPSs) play important roles in quantum communication and quantum information processing. Spontaneous four wave mixing (SFWM) in optical fibers provides a promising way to realize practical heralded single photon sources (HSPSs), since it is compatible with current techniques of optical communications. In this paper, two independent HSPSs at 1.5μm band are realized in one polarization maintaining dispersion shifted fiber (PM-DSF) utilizing its large birefringence. When pulsed pump light passes through an optical fiber, two kinds of SFWM will take place simultaneously. One is scalar processes, in which two annihilated pump photons and generated photon pair are all polarized along the same fiber polarization axis. The other is vector processes, in which two annihilated pump photons are polarized along different fiber polarization axes, either for the two photons of the generated pair. In the PM-DSF, the large birefringence generates obvious walk-off effect on the two pump polarization components, which leads to an effective suppression of the vector processes. Hence, by proper pump polarization, correlated photon pairs (CPPs) with different polarization directions can be generated independently by the two scalar processes, which can be used to realize two independent HSPSs. The indistinguishability of the heralded photons generated by the two independent sources is demonstrated by an experiment of Hong-Ou-Mandel (HOM) interference. Using a fiber coupler as the beam splitter, a visibility of HOM dip of 76% is achieved, showing their potential on quantum information.
Single photon sources (SPSs) play important roles in quantum communication and quantum information processing. Spontaneous four wave mixing (SFWM) in optical fibers provides a promising way to realize practical heralded single photon sources (HSPSs), since it is compatible with current techniques of optical communications. In this paper, two independent HSPSs at 1.5μm band are realized in one polarization maintaining dispersion shifted fiber (PM-DSF) utilizing its large birefringence. When pulsed pump light passes through an optical fiber, two kinds of SFWM will take place simultaneously. One is scalar processes, in which two annihilated pump photons and generated photon pair are all polarized along the same fiber polarization axis. The other is vector processes, in which two annihilated pump photons are polarized along different fiber polarization axes, either for the two photons of the generated pair. In the PM-DSF, the large birefringence generates obvious walk-off effect on the two pump polarization components, which leads to an effective suppression of the vector processes. Hence, by proper pump polarization, correlated photon pairs (CPPs) with different polarization directions can be generated independently by the two scalar processes, which can be used to realize two independent HSPSs. The indistinguishability of the heralded photons generated by the two independent sources is demonstrated by an experiment of Hong-Ou-Mandel (HOM) interference. Using a fiber coupler as the beam splitter, a visibility of HOM dip of 76% is achieved, showing their potential on quantum information.
In this paper, a heralded single photon source (HSPS) at 1.5 μm with two independent orthogonally polarized outputs is realized based on a piece of polarization maintaining dispersion shifted fiber (PM-DSF). The HSPS is based on two scalar spontaneous four wave mixing (SFWM) processes along the two fiber polarization axes, while two vector SFWM processes are suppressed due to the high birefringence in the PM-DSF. The preparation efficiencies of the two independent outputs are about 73.7% and 69.1%, respectively, under a second-order correlation function g(2)(0) of 0.059. The indistinguishability between the two independent heralded single photons is demonstrated by Hong-Ou-Mandel (HOM) interference with a visibility of 78.9%, showing its great potential in quantum optics experiments and applications of quantum information.
Spontaneous four-wave mixing (SFWM) in optical fibers is an important way to generate correlated/entangled photon pairs. When the pulsed pump light passes through the optical fiber, two kinds of SFWM will take place simultaneously. One is scalar scattering processes, in which two annihilated pump photons and generated photon pair are all polarized along the same fiber polarization axis. The other is vector scattering processes, in which two annihilated pump photons are polarized along different fiber polarization axes, either to the two photons of the generated pair. If the fiber has large group birefringence, the intensity of vector scattering processes will be suppressed at the phase matching frequencies of the scalar scattering processes. On the other hand, the walk-off effect of the pump pulse components polarized along the two fiber polarization axes also suppresses the vector scattering processes. Hence, by proper pump polarization and signal/idle frequency selection, photon pairs can be generated only by the two independent scalar scattering processes in optical fibers with birefringence, which provide a simple way to realize polarization entangled photon pair generation. In this paper, related experiments based on the high nonlinearity microstructure fiber (HN-MSF) with group birefringence and polarization maintained dispersion shifted fiber (PM-DSF) are introduced, showing their potential on developing practical quantum light sources.
In this Letter, a linear scheme to generate polarization entanglement at 1.5 um based on commercial polarization maintained dispersion shifted fiber (PM-DSF) is proposed. The birefringent walk-off effect of the pulsed pump light in the PM-DSF provides an effective way to suppress the vector scattering processes of spontaneous four wave mixing. A 90 degree offset of fiber polarization axes is introduced at the midpoint of the fiber to realize the quantum superposition of the two correlated photon states generated by the two scalar processes on different fiber polarization axes, leading to polarization entanglement generation. Experiments of the indistinguishable property on single side and two-photon interference in two non-orthogonal polarization bases are demonstrated. A two photon interference fringe visibility of 89\pm3% is achieved without subtracting the background counts, demonstrating its great potential in developing highly efficient and stable fiber based polarization-entangled quantum light source at optical communication band.
In this Letter, a linear scheme to generate polarization entanglement at 1.5 μm based on commercial polarization maintained dispersion shifted fiber (PM-DSF) is proposed. The birefringent walk-off effect of the pulsed pump light in the PM-DSF provides an effective way to suppress the vector scattering processes of spontaneous four-wave mixing. A 90 deg offset of fiber polarization axes is introduced at the midpoint of the fiber to realize the quantum superposition of the two correlated photon states generated by the two scalar processes on different fiber polarization axes, leading to polarization entanglement generation. Experiments of the indistinguishable property on single-side and two-photon interference in two nonorthogonal polarization bases are demonstrated. A two-photon interference fringe visibility of 89±3% is achieved without subtracting the background counts, demonstrating its great potential in developing highly a efficient and stable fiber based polarization-entangled quantum light source at the optical communication band.
Interference between independent photon sources is the key technique to realize complex quantum systems for more sophisticated applications such as multi-photon entanglement generation and quantum teleportation. Here, we report Hong-Ou-Mandel interference (HOMI) between two independent 1.55 m all-fiber photon pair sources over two 100 GHz dense wave division multiplexing (DWDM) channels, whose visibility reaches 53.2%8.4% (82.9%5.3%) without (with) back ground counts subtracted. In addition, we theoretically describe in detail the single photon spectral purity of the photon source generated in dispersion shifted fiber (DSF), simulate the influences of the pulse width and filter bandwidth on the purity, and obtain the optimized condition. The optimized pump pulse width is 8 ps and filter bandwidth is about 40 GHz or less. A home-made 1550.1 nm mode-locked fiber laser source, whose pulse width and repetition rate are 25 ps and 27.9 MHz respectively, acts as a pump of photon source. A tunable attenuator is used to adjust the pump power of the photon source, and the broad band background fluorescence photons are filtered out by cascade 100 GHz DWDM filters. The clean pump beam is divided into two equal parts by the 50 : 50 optical coupler to pump two 300 m DSFs (cooled by liquid nitrogen) to generate independent photon sources. Then the strong pump beam and noise photon from Raman scattering in orthogonal polarization are removed by 2 groups of 200 GHz DWDM filters and fiber polarization rotator and polarizer. Then two 100 GHz DWDMs are used for separating photons at correlated channel pairs. The relative delay between the two independent photons is adjusted by tunable fiber delay line. Photons from the same channels are combined in a second beam splitter for interference, and the other two photons are used as trigger signals. The two triggered photons are detected by two free running InGaAs avalanched single photon detectors (APD1, APD4, ID Quanta, ID220, 20% detection efficiency, 3 s dead time, dark count rate 4k cps), and the outputs of detectors APD1 and APD4 are used to trigger two single-photon detectors running in the gated mode (APD2, APD3, Qasky, Hefei, China, 100 MHz, free gating single photon detectors, 20% detection efficiency, dark count probability 410-5 per gate) for twophoton coincidence measurement. Detection output signals from APD2 and APD3 are sent to our coincidence count device (Pico quanta, TimeHarp 260, 1.6 ns coincidence window) for four-photon coincidence measurement. Before measuring the HOMI, we obtain a maximum-coincidence-to-accidental-coincidence ratio (CAR) of 131 by cooling the fiber in liquid nitrogen when the pump power is 23 W. There are a few remarks we want to point out.Firstly, the photon sources are not operated at the optimized pump pulse width for pure single photon generation, but narrow band 100 GHz filters are used in the experiments to increase the purity of the sources. Secondly, single photon detectors used in our experiment have lower detection efficiency and much higher dark counts than nano-wire single photon detectors, if we have high-performance nano-wire single photon detector, experimental results will be greatly improved due to the four-fold coincidences and dark coincidences scaling quadruplicate with the detection efficiency and dark count probability of a single detector. Thirdly, we use relatively high pump power for each DSF (0.12 mW) to reduce measurement time for photon coincidence, which will lead to a very poor raw visibility certainly. Finally, though only a 100 GHz channel pair is used in our experiment, we can use other channels for multiplexing such interference processes to improve the channel capacity in future quantum communication tasks theoretically. Our study shows greatly promising integrated optical elements for future scalable quantum information processing.
Single photons are essential resource for quantum communication and quantum information processing, which can carry quantum information to distant locations. A promising scheme for single photon generation is the heralded single photon source (HSPS), which is based on the generation of correlated photon pairs (CPPs). Utilizing the quantum correlation property of the CPPs, one photon of the CPP is detected providing an electrical signal to herald the other photon as a single photon output. Recently, 1.5 μm CPP generation through spontaneously four wave-mixing (SFWM) in fiber has focused much attention, which provides a practical way to realize 1.5 μm fiber-based HSPS. The quality of a HSPS is described by the preparation efficiency and g(2)(0). In the fiber-based HSPS, the preparation efficiency is determined by the loss of the filtering and splitting system and the noise photons generated by spontaneously Raman scattering (SpRS). Considering the impact of the SpRS can be reduced by cooling the fiber and optimizing the frequency detuning of filtering and splitting system, the loss of the filtering and splitting system may give a theoretical up-limit of the preparation efficiency. In this paper, using commercial fiber components, we realize a high quality HSPS based on cooled fiber with a preparation efficiency of 80% under a g(2)(0) of0.06, showing its great potential in the application of quantum information technology.