Frequency Upconversion and State Transduction

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Frequency upconversion is the process by which two photons of different frequencies can join together in a non-linear crystal and create a new photon whose frequency is the sum of the first two. We use this process to convert single photons from 1550-nm to the visible where they can be detected by silicon avalanche photodiodes. These detectors have much higher efficiency than the InGaAs detectors normally used for detecting IR photons, thereby allowing high efficiency detection of single IR photons. We also have demonstrated the coherence of this process by performing the upconversion process on two paths of an interferometer, and observed fringes in the upconverted light. Preserving the coherence is a key requirement for handling qubits, thereby making this process suitable for applications such as quantum cryptography and quantum networking.