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ATESIT Résumé de rapport

Project ID: IST-2000-29681
Financé au titre de: FP5-IST
Pays: Italy

Realization of a photon-number-resolving detector

The objective of realizing a photon-number-resolving detector didn’t give a conclusive result within the development of the ATESIT project. The relevance of this objective resides on the fact that linear optics computation requires photon number resolving detectors to achieve the scalability of computation. The excitation of a photocatode with an evanescent wawe represented originally a new kind of coupling between radiation and photocatode to enhance the quantum efficiency qe. In the original idea the evanescent wawe is coupled to the photocathode with a displacement of the reflected beam in the direction of the interface. Since is much larger than the photocatode thickness, a relevant enhancement of the photon absorption cross section is expected.

This effect was experimentally verified by Partner 1 with different photocathodes and a value of qe 20% was measured with an enhancement = 2. The capability of this device to discriminate different photon numbers was tested by a standard Ou-Mandel interferometer operating with twin photons generated at 532 nm. A high value of visibility = 100% was measured but some signals due to the overlapping effect of the one and two photon distributions were discarded. As a result the effective value of qe was lowered. In order to improve the detector photon discrimination several possible solutions have been tested. A new detector was adopted to achieve a better discrimination between Fock states. It was given by a hybrid photodiode that ensures a higher intrinsic photon number resolution due to the peculiar single step internal amplification.

The intrinsic amplification of the hybrid photodiode (HPD)corresponded to 3500 electrons / photoelectrons. Lighting the chatode with a strongly attenuated pulsed laser, the photon-number discrimination of the tube was tested by identifying the nphotoelectron output signal and analysing the respective amplitude spectra. In spite of the fact that HPD response curves present narrow peaks of the n-photoelelectron, photon number resolution was spoiled for an increasing number of incident photons by a growing-up of a broad background between different peaks. This phenomenon was attributed to the backscattering of the accelerated photoelectrons when inpinging on the diode and provides a partial deposition of their energy in the active diode region. Since the attenuation of the backscattering effect is highly critical to achieve a better photon-number resolution, strategies to reduce its influence should be studied and pursued.

Reported by

16152 GENOVA
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