We present a general quantum-mechanical formalism to describe photon-pair generation via four-wave mixing in a
Silicon-on-Insulator (SOI) microresonator. We also provide design principles for efficient photon-pair generation
in an SOI microresonator through numerical simulations. Ring-cavity designs are shown to have a much wider
dispersion-compensate frequency range than straight-cavity designs. Such on-chip quantum devices are highly
promising for future integrated quantum information processing.
We present a systematic study of a correlated photon-pair source based on a periodically-poled KTiOPO4
(PPKTP) waveguide. The waveguide was fabricated on a KTiOPO4 crystal supporting type-II parametric
down-conversion. In addition, periodic poling was applied along the waveguide to quasi-phase-match the type-0
down-conversion process. The design pump wavelength is 532 nm, and the wavelengths of the down-converted,
correlated photons are around 900 nm and 1300 nm. We examine the two-photon correlation spectra and singlephoton
spectra at a variety of temperature and power settings for both type-0 and type II down-conversion
processes. Our study shows that the waveguide source has a number of advantages compared to its bulk-crystal
counterpart, including higher spectral brightness, narrower emission bandwidth and single spatial-mode output.
With greatly simplified engineering, this compact, highly efficient, low photon-loss, and cost-effective waveguide
source of correlated photon pairs is promising for future chip-scale quantum information processing applications.
We report the development of a fiber-based single-spatial-mode source of photon-pairs where the efficiency of extracting photon pairs is 14× higher than a previous implementation [16]. This critical improvement in efficiency enabled a spectrally bright and pure photon-pair source having a small second-order correlation function (0.03) and a raw spectral brightness of 44,700 pairs/(s nm mW). The source can be configured to generate entangled photon-pairs, characterized via optimal and minimal quantum state tomography, to have a fidelity of 97% and tangle of 92%, without correcting for accidentals.
We describe the generation of entangled and hyperentangled photon pairs using a microstructure-fiber Sagnac
interferometer, which is formed by a polarizing beam splitter and a highly nonlinear microstructure fiber twisted
by 90° from end to end. This interferometer allows two identical four-wave mixing processes to occur on the same
fiber principal axis, ensuring perfect spatial and temporal mode matching of the two four-wave mixing outputs
on the polarizing beam splitter to create entanglement over the entire four-wave mixing phase-matching spectral
range. With an average pump power of 220 μW, we measure a two-photon coincidence rate of 1 kHz with ▵λ =
0.9 nm. Two-photon interference visibilities exceed 91% for polarization-entangled photon pairs generated from
this source, and are > 84% for both time-bin and polarization degrees of freedom for hyperentangled photons,
all without subtracting accidental coincidences.
We present a fiber based source of entangled photon-pairs in the 1550 nm telecom band that can be integrated into the existing fiber network and is well suited for quantum information processing. With this source we have demonstrated the generation, storage, and long-distance distribution of polarization entanglement in standard optical fiber. We have also investigated the origin of the large number of accidental coincidences in the experiments, which has been proved to be Raman scattering, and discussed how to suppress the Raman scattering to improve the quality of the fiber source.
We present the design and construction of a high-speed telecom-band (1.5 μm) single-photon counting system based on an InGaAs/InP avalanche photodiode (APD) operating in the gated Geiger mode. The detector can be gated at high speeds (we examine its performance up to 25 MHz) to maximize the counting rate in long-distance, telecom-band, fiber-optic quantum communication applications. Narrow gate pulses (250 ps full width at half maximum) are used to reduce the dark-count and the after-pulse probability. In order to count the avalanche events, we employ a high-speed comparator to sample the unfiltered and unamplified avalanche photocurrent. The APD and all the associated electronics are integrated onto a printed circuit board with a computer interface. In addition, we cool the APD to -27°C to reduce the dark-count probability.
We review on-going progress in the development of fiber-based
telecom-band entanglement sources. Two different schemes (a
Sagnac-loop scheme and a counter-propagating scheme) for
generating polarization entanglement are reviewed and the pros and
cons of each are summarized. A new scheme, called the double-loop
scheme is proposed, which is theoretically shown to be capable of
combining the benefits and avoiding the pitfalls of each previous
scheme.
We present a quantum theory for generating two-photon states by means of four-wave mixing in optical fiber. We start with an interaction Hamiltonian that can correctly describe all nonlinear interactions among the four waves present in the fiber, namely, the frequency non-degenerate pumps, signal, and idler, including the terms responsible for self-phase modulation (SPM), cross-phase modulation (XPM), and four-photon scattering (FPS). The exact form of this Hamiltonian is obtained through comparison between the classical and quantum equations of motion. The two-photon state is then calculated by means of first-order perturbation theory. It turns out that only the FPM and the pump SPM terms contribute to the formation of the two-photon state. The entangled nature of this state is verified in a coincidence counting experiment. The results of the theoretical calculation agree well with experimental data.
SIN and SIS quasiparticle tunnel junctions on high TC superconductors (HTS) offer the possibility of low-noise, heterodyne, photon detection in the THz regime. We report progress on the development of such junctions using mechanical point contacts. In general, these contacts display the optimum characteristics that can be obtained from HTS native-surface tunnel barriers. The bismuthate, Ba1-xKxBiO3, (TC equals 25 K) displays ideal, BCS, quasiparticle characteristics at T equals 4.2 K however, at temperatures T approximately equals TC/2 there is evidence of strong quasiparticle damping which may inhibit device performance. The cuprates typically display non-ideal quasiparticle characteristics including large sub-gap conductances. Recent data for the new Hg-based cuprates (TC equals 96 K) are promising in that they exhibit very low and flat sub-gap conductances as expected from a BCS density of states. Proximity effect tunnel junctions on Bi2Sr2CaCu2O8/Au bilayers have been studied using an In tip. An induced energy gap has been consistently observed in the Au layer and the data can be understood using the McMillan model. A few junctions show much improved sub- gap characteristics compared to ones made directly on the BSCCO surface and indicate that this approach may be suitable for mixer development.
Superconducting tunnel junctions have a potential, statistically limited, energy resolution on the order of eV's. The best results to date, however, have been an order of magnitude worse than this and required operating temperatures on the order of 0.1 K. Niobium based junctions operating at approximately 1 K have shown X-ray detection capabilities, but have only achieved energy resolutions on the order of 100 eV's at best. Several mechanisms, including quasiparticle self trapping, loss of `hot' excitations, quasiparticle recombination, and loss of `cold' excitations, have been proposed to explain the degradation of energy resolution in these devices. We will present a design concept for an X-ray detector, along with recent experimental and computer modeling results, based on a 1D superlattice of superconducting tunnel junctions. This multilayered superconducting tunnel junction design has the potential for alleviating many of the potential resolution degrading mechanisms while operating in the 1 K temperature range. In addition, the possibility of engineering the device to improve the signal to noise ratio of the output and to control the transport of phonons in the structure will be discussed.
While much progress has been made towards improved energy reso1utvn in superconducting tunnel junction (STJ) detectors recently, results are still more than an order of magnitude worse than the theoretical limit. Several factors have been identified as contributing to degradation of energy resolution in STJ devices: recombination losses, parasitic quasiparticle trapping and quasiparticle diffusion into current leads. In addition, STJ detectors tend to have poor photon capture efficiency. Semiconducting detectors achieve their near theoretical energy resolutions and high efficiencies via doping and/or applying an external field to a pure substance. These methods are ineffective for STJ detectors, therefore such alternatives as engineered materials, consisting of multiple materials artificially patterned on the microscopic level, should be considered. The most common engineered structures in use are quasiparticle trapping configurations, which alleviate lead diffusion and detection efficiency problems. We have previously proposed a multilayered approach which addresses parasitic trapping, along with diffusion and efficiency. We now propose the possibility of an engineered structure which will alleviate quasiparticle recombination losses via the existence of a phononic band gap that overlaps the 2i energy of phonons produced during recombination of quasiparticles. We will present a 1D Kronig-Penny model for phonons normally incident to the layers of a multilayered superconducting tunnel junction as an idealized example
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