Exploration of Higher-Order Quantum Interference Landscapes

Abstract

Earth, Moon and Sun unite when they star together in the three-body problem, whose intricate plot still baffles us today. For some reason, the factorization of the two-body problem into two one-body problems does not, in general, cross the N=2 border. Is computational irreducibility responsible for this emergence of complexity, as Stephen Wolfram likes to think? We don't know. The introduction to this thesis in Chapter 1, however, makes it clear that the history of science is marked by intermittent encounters of sudden complexities when the number 2 is left behind. In Chapter 2, I present an experiment that is quite similar in spirit, for my colleagues and I observe three-photon interference without two-photon and single-photon interference. We had to overcome significant experimental challenges that are typical for most quantum interference experiments involving more than two photons. Next in line is the three-slit interference experiment. Again a deceptively simple extension of the famous double-slit experiment, we are faced with questions that are difficult to access experimentally: the existence of genuine three-slit interference was first denied and then affirmed, though no experiment has decided yet. My contribution to the study of this problem is outlined in Chapter 3, where I use symmetry of measurement settings in such interference experiments to theoretically derive higher-order interference terms. In Chapter 4, I take a step back in one sense, for we study a two-photon phenomenon, but we also leap forward and discover entirely new interference landscapes. Theoretically and experimentally, I demonstrate how to use a polarization-modulated lasers to go beyond the standard Hong-Ou-Mandel (HOM) dip, and generate both triangular and square wave HOM interference patterns. Two-photon interference is also subject of Chapter 5, but with an interesting twist. While laser HOM interference relies on two independent photons, here we endow the pair with the strongest known correlations, namely entanglement. More specifically, we entangle a polarization and a time-bin qubit and use this hybrid to assess the viability of a rather special interferometer for quantum communication purposes.