Quantum optics

We examine how best to associate quantum states of a single particle to modes of a narrowly collimated beam of classical radiation modeled in the paraxial approximation, both for scalar particles and for photons. Our analysis stresses the importance of the relationship between the inner product used to define orthogonal modes of the paraxial beam, on the one hand, and the inner product underlying the statistical interpretation of the quantum theory, on the other. While several candidates for such an association have been proposed in the literature, we argue that one of them is uniquely well suited to the task. Specifically, the mapping from beam modes to ”henochromatic” fields on spacetime is unique within a large class of similar mappings in that it is unitary in a mathematically precise sense. We also show that the single-particle quantum states associated to the orthogonal modes of a classical beam in the henochromatic approach are not only orthogonal, but also complete in the quantum Hilbert space.

In this effort, we present progress toward demonstrating a Decoy-State Quantum Key Distribution (QKD) source based on a polarization-modulator and a wavelength-stable attenuated pulsed laser. A three-state QKD protocol is achieved by preparing particular quantum polarization states. The polarization-modulator-based QKD source improves security by removing several sources of side-channel attacks that exist when multiple sources are used to generate different QKD states. Here we present a QKD source design and an evaluation of critical subsystems characterized by the Quantum Bit Error Rate, Quantum State tomography, and achievable Key Rates. The QKD source is intended to operate within compact Size, Weight, and Power constraints. The Polarization-Modulator QKD source has applications in future mobile quantum networks such as Unmanned-Aerial Vehicles (UAV) and autonomous vehicles, as well as fixed fiber-based quantum networks.
Quantum mechanics can produce correlations that are stronger than classically allowed. This stronger-than-the-classical correlation is the “fuel” for quantum computing. In 1991 Schumacher forwarded a beautiful geometric approach, analogous to the well-known result of Bell, to capture the non-classicality of this correlation for a singlet state. He used well-established information distance defined on an ensemble of identically–prepared states. He calculated that for certain detector settings used to measure the entangled state, the resulting geometry violated a triangle inequality —a violation that is not possible classically. This provided novel information–based on geometric Bell inequality in terms of a “covariance distance.” Here we experimentally reproduce his construction and demonstrate a definitive violation for a Bell state of two photons based on the usual spontaneous parametric down-conversion in a paired BBO crystal. The state we produced had visibility of Vad = 0.970}0.012. We discuss generalizations to higher dimensional multipartite quantum states.

As a subset of quantum optics, single photons are one of the competing physical resources for quantum information processing. When used as carriers of quantum information, they have no equal. For the processing of quantum information, single photons have proven difficult to scale beyond the order of ⇠ 10 photons. The lack of single-photon-level interaction has led to creative approaches which rely on postselection to filter the possible measured outcomes to those which appear as though
interaction occurred. This approach of post-selection leans heavily on the ability to
not only generate but also detect scores of single photons simultaneously and with
near perfect efficiency. Our work relaxes the emphasis which has been placed on single
photons for quantum information processing to that of states with, in principle, an
arbitrary number of photons. Central moment expectations on two-mode squeezed
states are shown to exhibit post-selection behavior which reflects the single-photon
counterpart. These measures are proven to be robust to loss and return entangled
state statistics on average. With naive estimation of the central moment, states
with ~ 20 modes are within reach with current technology, closing the gap between quantum states which can and cannot be classically simulated.