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Bulletin of the American Physical Society

Four-fold coincidence logic for photon counting with universal control

David A. Branning, Sarthak Khanal, Young Ho Shin, M. Beck

Published May 28, 2010• 0 citations
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In this paper, we demonstrate a coincidence counting system with 48 input channels which is aimed to count all coincidence events, up to 531 441 kinds, in a multiphoton experiment. Using the dynamic delay adjusting inside the Field Programmable Gate Array, the alignment of photon signals of 48 channels is achieved. After the alignment, clock phase shifting is used to sample signal pulses. Logic constraints are used to stabilize the pulse width. The coincidence counting data stored in a 1G bit external random access memory will be sent to the computer to analyze the amount of 2-, 3-, 4-, 5-, and 6-fold coincidence events. This system is designed for multiphoton entanglement experiments with multiple degrees of freedom of photons.
A flexible and low-cost Coincidence Counting Unit (CCU) with customizable features that are software selectable is designed and developed for the use in various experiments of quantum information and quantum metrology. A digital logic-based implementation approach using Field Programmable Gate Array is followed with specific emphasis on reducing the time for coincidence events of photons and making it easily scalable. In this paper, an 8-channel CCU with software configurable features such as coincidence window selection (in increment of 0.54 ns) and 8-fold coincidences (up to 4 sets) at a time has been implemented.
We demonstrate greatly improved results for the production of correlated photon-pairs using the four-photon scattering process in silica fiber. We achieve a true-coincidence-count to accidental-coincidence-count ratio greater than 10, when the photon-pair production rate is about 0.04 /pulse. This represents a four-fold improvement over our previous results. The contribution of spontaneous Raman scattering, the primary cause of uncorrelated photons that degrades the fidelity of this source, is reduced by decreasing the wavelength detuning between the correlated photons and the pump photons and by using polarizers to remove the cross-polarized Raman-scattered photons. Excess Raman scattering could be further suppressed by cooling the silica fiber. Even without cooling the fiber, the achieved 10 to 1 ratio of true-coincidence to accidental-coincidence makes the fiber source of correlated photon-pairs a useful tool for realizing various quantum-communication protocols.