Unidirectionality from Polarization Singularities in Photonic Crystal Waveguides
Speaker: Ruth Oulton, School of Physics, University of Bristol
Title: Unidirectionality from Polarization Singularities in Photonic Crystal Waveguides
Abstract: Quantum dots are semiconductor artificial atoms. They are nanoscale structures that trap single electrons and holes, and their quantized energy level structure results in atomic-like transitions and single photon emission. These quantum dots act as a solid-state interface that is useful for quantum information applications, and for the past decade, semiconductor physicists have been attempting to replicate atomic cavity quantum electrodynamics in a practical semiconductor form. One can embed quantum dot into micron-sized photonic structures to capture and control the light emission, in order to use the single photon emission in quantum communication and quantum circuits.
One of the most exciting applications of quantum dots is to use their electron spins as a quantum memory. This involves transferring spin information from an electron spin to the polarization of a photon. However, as I shall explain, the definition of “polarization” for nanophotonic structures is far more complex than for a beam of light. In fact, we find that point-like “spin” emitters couple to a photonic structure in surprising ways: unlike any phenomenon observed in bulk material, simply changing the position of an emitter or the spin direction controls completely in which direction photons propagate. This unidirectional emission appears to violate the reciprocity theorem! Suddenly, a rich variety of behaviour has arisen in the semiconductor/photonic domain which has no equivalent in atomic cavity quantum optics, including a fundamental difference between how a classical dipole and a quantum dipole emitter interfere with incoming light. I will explore the properties of these waveguides using the theory of polarization singularities, then look at how directionality is linked to local spin by slowing the light down and observing how the directionality disappears. I will then finish by exploring some very unusual quantum optical spin effects in these structures that cannot be described using standard quantum optics theory.