Professor Ian Walmsley CBE FRS FCGI

Abstract:

Detectors that can resolve photon number are needed in many quantum information technologies. In order to be useful in quantum information processing, such detectors should be simple, easy to use, and be scalable to resolve any number of photons, as the application may require great portability such as in quantum cryptography. Here we describe the construction of a time-multiplexed detector, which uses a pair of standard avalanche photodiodes operated in Geiger mode. The detection technique is analysed theoretically and tested experimentally using a pulsed source of weak coherent light.

Abstract:

The ability to create and characterize photon number states is of utmost importance in most optically-implemented quantum information schemes. In the past, progress in this area has been hindered by a lack of photodetectors that are both highly efficient and can discriminate different photon number states. We have utilized a time-multiplexed detector to characterize the properties of a heralded photon source based on parametric downconversion. We also show that the different losses in our detector can be described by a single loss parameter, greatly simplifying the data analysis.

Abstract:

The reliable generation of true single-photon wavepackets with well-defined modal structure is a crucial ingredient for the realization of linear optical quantum computing. We have implemented a highly efficient waveguided parametric downconversion source pumped by ultrashort pulses, which is characterized by the spatial decoupling of the signal and idler photon while exhibiting a conditional detection efficiency of 51%, corresponding to a preparation efficiency of 85%. We have performed various tests to confirm the single-photon character of the conditionally prepared state.