We present a source of noncollinear correlated photon pairs in the standard 1550 nm telecommunication band. They are generated by a spontaneous parametric down-conversion process and emitted in a cone because of type-I noncollinear phase matching. Within the band, the source gives a completely flexible choice of the frequencies of the photon pairs, and correlation properties related to spatial momentum as well as energy and time can easily be utilized. We characterize the source by measuring the spatial intensity distribution of the down-converted light and by performing coincidence counting.
©2006 Optical Society of America
Correlated photon pairs are essential ingredients in quantum information processing, as well as in fundamental tests of quantum mechanics [1, 2]. They have been widely used for various applications including quantum cryptography, quantum communications, and optical realizations of quantum logic gates.
The generation of correlated photon pairs in the 1550 nm telecom band is of particular interest for future applications with low-loss standard optical fibers. In recent experiments, correlated photon pairs in the telecom band have been generated by using a spontaneous parametric down-conversion (SPDC) process in a nonlinear crystal [3, 4], periodically poled lithium niobate waveguides [5, 6, 7], and periodically poled silica fibers , and also by using a four-wave mixing process in optical fibers [9, 10, 11, 12]. Those experiments typically used collinear interactions in a nonlinear medium, so that each pair consisted of a signal photon and an idler photon in the same spatial mode. Hence, it was impossible to separate the signal and idler photons when they were also in identical frequency and polarization modes. Indeed, in most previous experiments in the telecom band, a signal photon and an idler photon at different frequencies were generated and then separated by the use of a WDM coupler or a diffraction grating.
Another important method for generating correlated photon pairs is the use of a noncollinear interaction in a nonlinear medium, which has been widely exploited in the SPDC process . With the noncollinear interaction, a pair of photons can be generated in different spatial modes, and the separation problem is resolved. Besides the separation problem, a noncollinear photon pair source has also proved useful in many other applications. For instance, polarization-entangled , momentum-entangled , and frequency-entangled  twin photon states can be directly prepared by selecting appropriate spatial modes from the down-converted emission cones . Multidimensional entanglement of the orbital angular momentum states [18, 19] and quantum images produced by twin photons entangled in orbital angular momentum  have been demonstrated in the spatial modes of a noncollinear type-I SPDC. A single-photon on-demand source using a noncollinear type-I down-converter has also been suggested . However, to date, the noncollinear generation of correlated photon pairs has not yet been exploited in the telecom band.
In this paper we present the first, to the best of our knowledge, noncollinear correlated photon pair source in the 1550 nm standard telecom band using a type-I SPDC process. In this case, two photons of a pair with identical polarizations are emitted on opposite sides of an axis defined by the direction of a pump beam. Thus, the separation problem vanishes and there is no restriction on choosing the operating frequencies within the whole telecom band. The spatial correlation properties of the photon pairs can also be easily utilized in the telecom band.
2. Experimental setup
The experimental setups are shown in Fig. 1. Ultrashort laser pulses from a mode-locked Ti:Sapphire laser at a wavelength of 775 nm and an average power of 615 mW (with a pulse duration of 150 fs and a repetition rate of 75 MHz) are used to pump a lithium triborate (LBO) nonlinear crystal. The pump beam is focused to a beam waist of about 100 μm at the LBO crystal position. The crystal is cut to have collinear degenerate type-I phase matching when the pump beam is nearly orthogonal to the crystal surface. The Rayleigh range of the pump mode at the crystal location is about 10 mm, which is greater than the crystal thickness of 6 mm. We use interference filters (F) centered at 1550 nm with 10 nm bandwidth. The down-converted photon pairs with a spectral bandwidth determined by the interference filter are detected by the InGaAs/InP avalanche photodiode (APD) modules operating in a gated Geiger mode (id Quantique id-200). The trigger signals to the APD modules are derived from the pump laser after lowering the signal rate from 75 MHz to 3.95 MHz.
Two kinds of experiments were performed with these setups to investigate the correlated photon pair generation in the 1550 nm telecom band. Firstly, spatial (momentum) distributions of the down-converted light were measured for collinear and noncollinear cases. Secondly, coincidence-counting experiments of the photon pairs were performed for a noncollinear case. Figure 1(a) shows the experimental setup used for the measurement of the spatial distributions. The collinear and noncollinear conditions were set by adjusting the phase matching angle, which was achieved by tilting the crystal, in the horizontal plane, away from the configuration in which the pump beam is incident normally to the crystal surface. The down-converted light at 1550 nm wavelength, which emerges with a vertical polarization (ordinary ray) because of type-I phase matching, was first sent through a dichroic mirror (DM) and subsequently collimated with a lens L 1 of focal length 20 cm. A fiber-optic collimator mounted on a motorized XY-translation stage was then used to scan the resulting spatial distribution of the down-converted light. To obtain sufficient angular resolution, the effective focal length of the aspheric lens L 2 of the fiber-optic collimator was chosen to be 4.60 mm. Thus, in this setting, the angular width of the spatial mode coupled into the standard single-mode fiber (SMF) is estimated to be about 0.34°, while the estimated Gaussian angular spread of the down-converted light emerging from the region of the crystal illuminated by the pump beam is about 1.13 °.
3. Experimental results and analysis
Measurement results of the spatial intensity distributions are shown in Fig. 2. In the collinear case of Fig. 2(a), the crystal is tilted away from the normal incidence configuration by +0.32 °. The down-converted light vanishes when the tilting angle of the crystal is smaller. Figure 2(b) shows the noncollinear case with a tilting angle of +6.65°, resulting in the emission of the down-converted light in a cone. We note that the right side of the intensity distribution is slightly broader than the left side because of the pump beam (extraordinary ray) walk-off inside the down-conversion crystal.
The modified experimental setup shown in Fig. 1(b) allows coincidence counting of signal and idler photons for the noncollinear case of Fig. 2(b). Each of the signal and idler photons is reflected by a mirror (M) after passing through the dichroic mirror and then collimated with a lens L 3 of focal length 20 cm. In this case of coincidence counting, it is also desirable to optimize the collection of correlated photon pairs [22, 23, 24, 25]. The effective focal length of the aspheric lenses of the fiber-optic collimators was thus changed to 15.29 mm. A simple calculation shows that the angular width of the spatial mode coupled into the single-mode fiber roughly matches the estimated Gaussian angular spread of 1.13 ° for the down-converted light in this setting . After changing the aspheric lenses, we typically observe an enhancement of 2.7 times in the single-count rate, resulting in 2800 counts/s for the signal and idler modes when the average pump power is set at 615 mW. Meanwhile the coincidence-count rate is about 29 counts/s, and the ratio of true coincidence counts to accidental coincidence counts is estimated to be about 14. We point out that this ratio can be significantly improved by decreasing the pump power. For instance, the ratio is about 22 when the pump power is 364 mW and 57 for 172 mW.
The probability that a pump pulse creates a correlated photon pair in the signal and idler modes was also measured using the side peaks method . According to this method, the pair creation probability Ppair is given by the ratio of side to main coincidence-count peak. Figure 3 shows the measurement results for the pair creation probability as a function of the average pump power. The overall collection efficiency C is then estimated to be around 12% from the measured pair creation probability using
where N is the single-count rate, f is the trigger signal rate to the detector, and η is the detector quantum efficiency, which is about 10%.
In conclusion, we have presented, for the first time to our knowledge, a noncollinear correlated photon pair source in the 1550 nm standard telecommunication band using a type-I SPDC process. Compared with previous collinear schemes, this source gives a completely flexible choice of the frequencies of photon pairs within the whole telecom band. In addition, the spatial (momentum) correlation properties as well as energy and time correlations of the photon pairs can be easily utilized. With these important advantages, the present source should find various applications in quantum cryptography, quantum communications, and quantum information processing.
The authors acknowledge support from the Korea Ministry of Information and Communication and Korea Ministry of Science and Technology. The authors also thank S.-K. Choi for helpful discussions. One of us (J.K.) was supported by Korea Research Foundation Grant (KRF-2002-070-C00029).
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26. We consider only the Gaussian spread of the down-converted light, because in this noncollinear type-I SPDC it is significantly larger than the extra divergence caused by the SPDC phase-matching conditions over the wavelength range of interest.