Alfred B. U’Ren

Experimental proposal for the generation of entangled photon triplets by third-order spontaneous parametric downconversion in optical fibers

We present an experimental proposal for the generation of photon triplets based on third-order spontaneous parametric downconversion in thin optical fibers. Our analysis includes expressions for the quantum state, which describes the photon triplets and for the generation rate in terms of all experimental parameters. We also present, for a specific source design, numerically calculated generation rates.

Effects of crystal length on the angular spectrum of spontaneous parametric downconversion photon pairs

We present a theoretical and experimental analysis of the joint effects of the transverse electric field distribution and of the nonlinear crystal characteristics on the properties of photon pairs generated by spontaneous parametric downconversion (SPDC). While it is known that for a sufficiently short crystal the pump electric field distribution fully determines the joint signal–idler properties, for longer crystals the nonlinear crystal properties also play an important role. In this paper we present experimental measurements of the angular spectrum (AS) and of the conditional angular spectrum (CAS) of photon pairs produced by SPDC, carried out through spatially resolved photon counting. In our experiment we control whether or not the source operates in the short-crystal regime through the degree of pump focusing, and explicitly show how the AS and CAS measurements differ in these two regimes. Our theory provides an understanding of the boundary between these two regimes and also predicts the corresponding differing behaviors.

Observation of non-diffracting behavior at the single-photon level

We demonstrate the generation of non-diffracting heralded single photons, i.e. which are characterized by a single-photon transverse intensity distribution which remains essentially unchanged over a significant propagation distance. For this purpose we have relied on the process of spontaneous parametric downconversion (SPDC) for the generation of signal and idler photon pairs, where our SPDC crystal is pumped by a Bessel-Gauss (BG) beam. Our experiment shows that the well-understood non-diffracting behavior of a BG beam may be directly mapped to the signal-mode, single photons heralded by the detection of a single idler photon. In our experiment, the heralded single photon is thus arranged to be non-diffracting without the need for projecting its single-photon transverse amplitude, post-generation, in any manner.

Theory of cavity-enhanced spontaneous four wave mixing

In this paper we study the generation of photon pairs through the process of spontaneous four wave mixing (SFWM) in a χ(3) cavity. Our key interest is the generation of photon pairs in a guided-wave configuration—fiber or waveguide—where at least one of the photons in a given pair is matched in frequency and bandwidth to a particular atomic transition, as required for the implementation of photon–atom interfaces. We present expressions, along with plots, for the two-photon joint intensity both in the spectral and temporal domains. We also present expressions for the absolute brightness, along with numerical simulations, and show that the presence of the cavity can result in a flux enhancement relative to an equivalent source without a cavity.

Engineering an ideal indistinguishable photon-pair source for optical quantum information processing

The eventual success of optical quantum information (QI) processing depends critically on the available technologies. Photons produced via the nonlinear process of spontaneous parametric downconversion (SPDC) have now been used in a vast array of experiments in optical quantum computing and long-distance quantum communication. However, almost no source to date has been fully optimized. Here, we describe our progress in engineering an ideal photon-pair source for optical quantum information processing – polarization-entangled photons made indistinguishable by engineering spatio-spectral unentanglement at the source. We present solutions for several key challenges encountered in the development of the indistinguishable source. We expect such a source to directly translate into significantly enhanced QI protocols with higher rates, fidelities and efficiencies.

Configurable spatiotemporal properties in a photon-pair source based on spontaneous four-wave mixing with multiple transverse modes

We present an experimental and theoretical study of photon pairs generated by spontaneous four-wave mixing (SFWM), based on birefringent phasematching, in a fiber that supports more than one transverse mode. We present SFWM spectra, obtained through single-channel and coincidence photon counting, which exhibit multiple peaks shown here to be the result of multiple SFWM processes associated with different combinations of transverse modes for the pump, signal, and idler waves.

Spontaneous Parametric Processes in Modern Optics

Spontaneous parametric processes have proved to be integral to the advancement of quantum information science. Their incredible versatility, in terms of the emission properties of entangled photons, allows for nearly unimaginable applications in the transmission and processing of quantum information.

Polarization dependence on downconversion emission angle: Investigation of the 'Migdall effect'

Photonic sources based on spontaneous parametric downconversion (SPDC) are now pervasively employed in optical quantum information processing protocols. The recent focus on engineering efficient photonic sources requires a thorough understanding of several of the subtler characteristics of SPDC, which had previously been negligible. Here, we experimentally investigate one such phenomenon – the dependence of the downconversion polarization on emission angle. Additionally, we discuss the significance of this effect from the perspective of better engineering SPDC sources. We also propose techniques, specifically in the context of a two-crystal scheme for generating polarization-entangled photons, to exploit the effect and drastically improve source quality in certain limiting cases.

Ultrabroadband photon pair preparation by spontaneous four-wave mixing in a dispersion-engineered optical fiber

We present a study of the spectral properties of photon pairs generated through the process of spontaneous four wave mixing (SFWM) in single mode fiber. Our analysis assumes narrowband pumps, which are allowed to be frequency-degenerate or non-degenerate. Based on this analysis, we derive conditions on the pump frequencies and on the fiber dispersion parameters which guarantee the generation of ultra-broadband photon pairs. Such photon pairs are characterized by: i) a very large degree of entanglement, and ii) a very high degree of temporal synchronization between the signal and idler photons. Through a numerical exercise, we find that the use of photonic crystal fiber (PCF) facilitates the fulfilment of the conditions for ultra-broadband photon pair generation; in particular, the spectral region in which emission occurs can be adjusted to particular needs through an appropriate choice of the PCF parameters. In addition, we present a novel quantum interference effect, resulting from indistinguishable pathways to the same outcome, which can occur when pumping a SFWM source with multiple spectral lines.

Classical to quantum transfer of optical vortices

We show that an optical vortex beam, implemented classically, can be transferred to the transverse amplitude of a heralded single photon. For this purpose we have relied on the process of spontaneous parametric downconversion (SPDC) for the generation of signal and idler photon pairs, using a pump in the form of a Bessel-Gauss (BG) beam with orbital angular momentum (specifically, with topological charge l = 1 and l = 2). We have designed our source so that it operates within the short SPDC crystal regime for which, the amplitude and phase of the pump may be transferred to a heralded single photon. In order to verify the vortex nature of our heralded single photon, we have shown that the conditional angular spectrum and the transverse intensity at the single-photon level match similar measurements carried out for the pump. In addition, we have shown that when our heralded single photon is diffracted through a triangular aperture, the far-field single-photon transverse intensity exhibits the expected triangular arrangement of intensity lobes associated with the presence of orbital angular momentum.