Quantum Optics Seminar by Rob Thew

Single photons, what are they good for? There are many ways to entangle photonic systems, polarisation and time bin entanglement being two of the more popular. In this talk I will focus on single-photon path entanglement. Single-photon entanglement is not only one of the simplest forms of entanglement to generate; it is both fundamentally fascinating and potentially practical. At times its mere existence was debated, however, today it lies at the heart of key quantum information protocols, such as quantum repeaters. Path entanglement is generated when a single photon is delocalized over several modes, or paths, e.g. via a 50/50 beam splitter, where it produces a state of the form |10> + |01>.

I will discuss how we generate this entanglement using state of the art heralded single photon sources based on spontaneous parametric down-conversion as well as our recent efforts exploiting “displacement-based detection” to measure these states. Displacement-based detection combines aspects of homodyne detection, specifically a (weak) local oscillator, with discrete variable photon counting techniques. We have used this approach to develop and test an entanglement witness that scales well for multipartite, 2D, networks and more recently performed a detection-loophole-free EPR steering experiment.