Towards ultrafast quantum processing: generation and storage of ultrafast photonic quantum states

Abstract

This thesis is divided into five parts, each with an accompanying publication. In Chapter 1, I motivate the broad scientific pursuit that underlies this thesis: the quest to build universal, fault-tolerant technology that operates on the principles of quantum mechanics and quantum information theory. I motivate the use of single photons as the quantum particles of choice for this task, and I describe the open challenges facing the field of researchers embarking on this quest. This motivation pervades each subsequent chapter of this thesis, which focus in turn on overcoming four distinct technological and scientific hurdles related to these open challenges. The first of these open challenges is to demonstrate experimentally a useful single-photon source. This source of single photons ought to release just one photon at a time, in an on-demand fashion, with high-rate, low-latency, and high quantum-state purity and indistinguishability, in a single spatial mode, at the right wavelengths for a given application, and ought to satisfy certain practical constraints on size, weight, power, and sensitivity to environmental noise and perturbation. This is a long list of performance metrics, some of which are orthogonal, some not, and non-trivial tradeoffs often exist between metrics. While in this thesis we do not present a solution to this general open problem, we modestly expand scientific knowledge of one of the fundamental mechanisms behind single-photon generation. In particular, we present a new formalism for describing the quantum properties of spontaneous Stokes scattering in the Raman interaction, and we probe its effects on the quantum state purity of resulting Stokes photons both experimentally and theoretically. This is the focus of Chapter 2. Another open challenge to universal, fault-tolerant photonic quantum processing is the experimental demonstration of a useful photonic quantum memory—a device that is capable of storing and releasing single photons in a quantum-information-conserving manner, and which possesses sufficiently high performance across, again, a large set of non-orthogonal metrics. On this challenge, we have made several substantial contributions. Chapter 3 describes a novel approach to optimizing the efficiency of photonic quantum memories in atomic ensembles possessing a three-level Lambda-type energy level structure, in an experimentally friendly fashion. Chapter 4 discusses the practical side of implementing robust quantum memory in these atomic systems, where we carefully consider the theoretical sensitivity of these systems to experimental noise and perturbation. These two Chapters lead up to the experimental work described in Chapter 5, where we experimentally demonstrate a state-of-the-art and best-in-class photonic quantum memory in neutral barium vapor that possesses simultaneously high efficiency, broad bandwidth, and low noise. Our experimental demonstration features >95% storage efficiency, at roughly 3 orders of magnitude larger bandwidth (higher speed) than the previous state of the art, as well as record-low noise for Lambda-type atomic memories. The noise is so low, and the bandwidth so large, that we are able to perform a novel characterization experiment to extract the temporal amplitude and phase of the photons retrieved from our memory, based on spectral interferometry with a known reference. This is the most demanding possible characterization of one of these memories, and has direct implications for using this type of memory in quantum processing tasks that require temporally indistinguishable photons. In Chapter 6 I do the requisite summing up of the thesis, and discuss ongoing and future work on the memory project. In the various and sundry appendices I include information that will chiefly be useful for my successors in the lab, but may be of passing interest to other readers.