Indistinguishable single photons in pure quantum states are a cornerstone of theoretical quantum optics. However, experimentally, it is not trivial to prepare them. For example, spontaneous parametric downconversion (SPDC) is commonly used to create heralded single photons but generally emits photon pairs into multiple correlated spatio-temporal modes, which introduces frequency and time jitter. I will explain how by careful design of the space–time modes available to the photons generated by SPDC in a nonlinear crystal, we have prepared nearly indistinguishable heralded photons with a purity of at least 95%, the highest yet reported. Since the technique also raises the photon generation rate and heralding efficiency, and since purity is the current limiting factor for logic gate fidelity, this source should be very useful for photonic quantum information processing. Moreover, the design principle behind it can be widely applied. I will report on our theoretical and experimental progress in extending it to photon generation via four-wave mixing in photonic crystal fibre. For reasons I will explain, these sources should also prove uniquely useful for generating squeezed light.

As well as generating quantum light, we are also active in developing new ways to detect it. To accurately characterize or reconstruct a quantum state one must first have a detector that is also well characterized. I will describe the first experimental detector tomography and present the measured POVMs of a standard photon detector and our home built photon-number detector. The latter is revealed to be a better single-photon detector than the standard photon detector in some respects.


Seminar, 11^th of April, Conference Room

Hosted by Prof. Morgan Mitchell