Visible light photon counters feature noise-free avalanche multiplication and narrow pulse height distribution for single photon detection events. Such a well-defined pulse height distribution for a single photon detection event, combined with the fact that the avalanche multiplication is confined to a small area of the whole detector, opens up the possibility for the simultaneous detection of two photons. In this letter, we investigated this capability using twin photons generated by parametric down conversion, and present a high quantum efficiency (ϳ47%) detection of two photons with good time resolution (ϳ2 ns), which can be distinguished from a single-photon incidence with a small bit-error rate (ϳ0.63%).

A high-quantum-efficiency single-photon counting system has been developed. In this system, single photons were detected by a visible light photon counter operated at 6.9 K. The visible light photon counter is a solid state device that makes use of avalanches across a shallow impurity conduction band in silicon. Threefold tight shielding and viewports that worked as infrared blocking filters were used to eliminate the dark count caused by room-temperature radiation. Corrected quantum efficiencies as high as 88.2%Ϯ5% ͑at 694 nm͒ were observed, which we believe is the highest reported value for a single-photon detector. The dark count increased as the exponential of the quantum efficiency with changing temperature or bias voltage, and was 2.0ϫ10 4 cps at the highest quantum efficiency. © 1999 American Institute of Physics. ͓S0003-6951͑99͒01308-X͔ Photon counting methods have been widely applied to the accurate measurement of very weak lights, and have recently been used in quantum key distribution systems which encode a data bit to a single photon. The sensitivity of the measurement and the data transmission efficiency depends on the quantum efficiency of photon counters. Therefore, improvements in quantum efficiency have been the main issue in the development of photon counters. Photomultiplier tubes ͑PMTs͒ have been commonly used as the photon counter, however, their quantum efficiency is 25% at its most sensitive wavelength and 15% in the near infrared region. Silicon avalanche photodiodes ͑APDs͒ operating in Geiger mode have become popular, and a quantum efficiency of 76% was recorded with the APD, 1 which is the highest value so far. One of the important applications for such a highquantum-efficiency photon counter is the loophole free test of Bell's inequality. [2][3][4][5] In order to close the loophole, the quantum efficiency of the system has to be higher than 83% when an ordinary Einstein-Podorsky-Rosen ͑EPR͒ pair source is used. 6,7 The loophole free test has not been performed yet, and the lack of a highly efficient single-photon counting system over the threshold is one of the reasons.The visible light photon counter ͑VLPC͒ is an alternative detector, which is a solid state device using the avalanche multiplication effect of electrons in an impurity band in silicon. [8][9][10] The quantum efficiency of a VLPC was estimated to be 93%, but the measured value as a system was less than 70%. 1 The difficulty was that, in order to cope with both ''high quantum efficiency'' and ''low dark counts,'' a special shielding system which has high transmittance for the desired wavelength but filters out room-temperature radiation sufficiently is required. Infrared photons of the radiation up to 28 m in wavelength can excite electrons in the shallow impurity band and might cause large dark count ͑up to 10 15 cps). We developed the cryostat system shown in Fig. 1. Threefold shields at 77, 4, and 6.5 K were used to reduce the thermal photons reaching the detector. We put felt between the shields to block the background...

Si solid state photomultipliers utilize impact ionization of shallow impurity donor levels to create an avalanche multiplication when triggered by a photoexcited hole. The distribution of pulse height from a single photon detection event shows narrow dispersion, which implies that the avalanche multiplication process in these devices is inherently noise-free. We have measured the excess noise factor using two different techniques, digital pulse height analysis and analog noise power measurement. The results demonstrate nearly noise-free avalanche multiplication accomplished in these devices.

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