48 research outputs found
Single Photon Source with Individualized Single Photon Certifications
As currently implemented, single-photon sources cannot be made to produce
single photons with high probability, while simultaneously suppressing the
probability of yielding two or more photons. Because of this, single photon
sources cannot really produce single photons on demand. We describe a
multiplexed system that allows the probabilities of producing one and more
photons to be adjusted independently, enabling a much better approximation of a
source of single photons on demand. The scheme uses a heralded photon source
based on parametric downconversion, but by effectively breaking the trigger
detector area into multiple regions, we are able to extract more information
about a heralded photon than is possible with a conventional arrangement. This
scheme allows photons to be produced along with a quantitative
``certification'' that they are single photons. Some of the single-photon
certifications can be significantly better than what is possible with
conventional downconversion sources (using a unified trigger detector region),
as well as being better than faint laser sources. With such a source of more
tightly certified single photons, it should be possible to improve the maximum
secure bit rate possible over a quantum cryptographic link. We present an
analysis of the relative merits of this method over the conventional
arrangement.Comment: 11 pages, 5 figures, SPIE Free-Space Laser Communication and Laser
Imaging II. To appear in the proceeding of SPIE Free-Space Laser
Communication and Laser Imaging II, vol 482
Novel Cascaded Ultra Bright Pulsed Source of Polarization Entangled Photons
A new ultra bright pulsed source of polarization entangled photons has been
realized using type-II phase matching in spontaneous parametric down conversion
process in two cascaded crystals. The optical axes of the crystals are aligned
in such a way that the extraordinarily (ordinarily) polarized cone from one
crystal overlaps with the ordinarily (extraordinarily) polarized cone from the
second crystal. This spatial overlapping removes the association between the
polarization and the output angle of the photons that exist in a single type-II
down conversion process. Hence, entanglement of photons originating from any
point on the output cones is possible if a suitable optical delay line is used.
This delay line is particularly simple and easy to implement.Comment: 8 pages 8 figure
Ancilla-assisted quantum process tomography
Complete and precise characterization of a quantum dynamical process can be
achieved via the method of quantum process tomography. Using a source of
correlated photons, we have implemented several methods investigating a wide
range of processes, e.g., unitary, decohering, and polarizing. One of these
methods, ancilla-assisted process tomography (AAPT), makes use of an additional
``ancilla system,'' and we have theoretically determined the conditions when
AAPT is possible. All prior schemes for AAPT make use of entangled states. Our
results show that, surprisingly, entanglement is not required for AAPT, and we
present process tomography data obtained using an input state that has no
entanglement. However, the use of entanglement yields superior results.Comment: To appear in Physical Review Letter
Time-dependent distinguishability: Choosing to be a wave or a particle
Interference experiments with connected parametric down-converters have demonstrated that the possibility, in principle, of identifying the photon path through the interferometer is sufficient to wipe out all interference, irrespective of whether the identification is actually made. The distinguishability of the photon path can be controlled by a time-dependent shutter, which leaves the choice whether the photon behaves as a wave or as a particle in the experimenter's hands. By contrast, in some more recent experiments involving the addition of a low-Q cavity, each idler photon makes the choice whether the associated signal photon behaves like a wave and exhibits interference, or like a particle
Generating Entangled Two-Photon States with Coincident Frequencies
It is shown that parametric downconversion, with a short-duration pump pulse
and a long nonlinear crystal that is appropriately phase matched, can produce a
frequency-entangled biphoton state whose individual photons are coincident in
frequency. Quantum interference experiments which distinguish this state from
the familiar time-coincident biphoton state are described.Comment: Revised version (a typo was corrected) as published on PR
An entangled two photon source using biexciton emission of an asymmetric quantum dot in a cavity
A semiconductor based scheme has been proposed for generating entangled
photon pairs from the radiative decay of an electrically-pumped biexciton in a
quantum dot. Symmetric dots produce polarisation entanglement, but
experimentally-realised asymmetric dots produce photons entangled in both
polarisation and frequency. In this work, we investigate the possibility of
erasing the `which-path' information contained in the frequencies of the
photons produced by asymmetric quantum dots to recover polarisation-entangled
photons. We consider a biexciton with non-degenerate intermediate excitonic
states in a leaky optical cavity with pairs of degenerate cavity modes close to
the non-degenerate exciton transition frequencies. An open quantum system
approach is used to compute the polarisation entanglement of the two-photon
state after it escapes from the cavity, measured by the visibility of
two-photon interference fringes. We explicitly relate the two-photon visibility
to the degree of Bell-inequality violation, deriving a threshold at which
Bell-inequality violations will be observed. Our results show that an ideal
cavity will produce maximally polarisation-entangled photon pairs, and even a
non-ideal cavity will produce partially entangled photon pairs capable of
violating a Bell-inequality.Comment: 16 pages, 10 figures, submitted to PR
Tailoring Single and Multiphoton Probabilities of a Single Photon On-Demand Source
As typically implemented, single photon sources cannot be made to produce
single photons with high probability, while simultaneously suppressing the
probability of yielding two or more photons. Because of this, single photon
sources cannot really produce single photons on demand. We describe a
multiplexed system that allows the probabilities of producing one and more
photons to be adjusted independently, enabling a much better approximation of a
source of single photons on demand.Comment: 4 pages, LaTex, 2 figures, twocolumn and RevTex Style for PR
Entangled-Photon Generation from Parametric Down-Conversion in Media with Inhomogeneous Nonlinearity
We develop and experimentally verify a theory of Type-II spontaneous
parametric down-conversion (SPDC) in media with inhomogeneous distributions of
second-order nonlinearity. As a special case, we explore interference effects
from SPDC generated in a cascade of two bulk crystals separated by an air gap.
The polarization quantum-interference pattern is found to vary strongly with
the spacing between the two crystals. This is found to be a cooperative effect
due to two mechanisms: the chromatic dispersion of the medium separating the
crystals and spatiotemporal effects which arise from the inclusion of
transverse wave vectors. These effects provide two concomitant avenues for
controlling the quantum state generated in SPDC. We expect these results to be
of interest for the development of quantum technologies and the generation of
SPDC in periodically varying nonlinear materials.Comment: submitted to Physical Review
Interferometric Bell-state preparation using femtosecond-pulse-pumped Spontaneous Parametric Down-Conversion
We present theoretical and experimental study of preparing maximally
entangled two-photon polarization states, or Bell states, using femtosecond
pulse pumped spontaneous parametric down-conversion (SPDC). First, we show how
the inherent distinguishability in femtosecond pulse pumped type-II SPDC can be
removed by using an interferometric technique without spectral and amplitude
post-selection. We then analyze the recently introduced Bell state preparation
scheme using type-I SPDC. Theoretically, both methods offer the same results,
however, type-I SPDC provides experimentally superior methods of preparing Bell
states in femtosecond pulse pumped SPDC. Such a pulsed source of highly
entangled photon pairs is useful in quantum communications, quantum
cryptography, quantum teleportation, etc.Comment: 11 pages, two-column format, to appear in PR