3,906 research outputs found
"Interaction-Free" Imaging
Using the complementary wave- and particle-like natures of photons, it is
possible to make ``interaction-free'' measurements where the presence of an
object can be determined with no photons being absorbed. We investigated
several ``interaction-free'' imaging systems, i.e. systems that allow optical
imaging of photosensitive objects with less than the classically expected
amount of light being absorbed or scattered by the object. With the most
promising system, we obtained high-resolution (10 \mu m), one-dimensional
profiles of a variety of objects (human hair, glass and metal wires, cloth
fibers), by raster scanning each object through the system. We discuss possible
applications and the present and future limits for interaction-free imaging.Comment: 10 pages, 6 encapsulated Postscript figure files, accepted for
publication in Physical Review
Ultra-bright source of polarization-entangled photons
Using the process of spontaneous parametric down conversion in a novel
two-crystal geometry, one can generate a source of polarization-entangled
photon pairs which is orders of magnitude brighter than previous sources. We
have measured a high level of entanglement between photons emitted over a
relatively large collection angle, and over a 10-nm bandwidth. As a
demonstration of the source intensity, we obtained a 242- violation of
Bell's inequalities in less than three minutes.Comment: 4 pages, 5 encapsulated Postscript figures. To appear in Physical
Review A (Rapid Communication
Hardy's paradox and violation of a state-independent Bell inequality in time
Tests such as Bell's inequality and Hardy's paradox show that joint
probabilities and correlations between distant particles in quantum mechanics
are inconsistent with local realistic theories. Here we experimentally
demonstrate these concepts in the time domain, using a photonic entangling gate
to perform nondestructive measurements on a single photon at different times.
We show that Hardy's paradox is much stronger in time and demonstrate the
violation of a temporal Bell inequality independent of the quantum state,
including for fully mixed states.Comment: Published Version, 4 pages, 3 figures. New, more boring titl
Characterizing quantum dynamics with initial system-environment correlations
We fully characterize the reduced dynamics of an open quantum system
initially correlated with its environment. Using a photonic qubit coupled to a
simulated environment we tomographically reconstruct a superchannel---a
generalised channel that treats preparation procedures as inputs---from
measurement of the system alone, despite its coupling to the environment. We
introduce novel quantitative measures for determining the strength of initial
correlations, and to allow an experiment to be optimised in regards to its
environment.Comment: 10 pages, 15 figure
Entanglement-free certification of entangling gates
Not all quantum protocols require entanglement to outperform their classical
alternatives. The nonclassical correlations that lead to this quantum advantage
are conjectured to be captured by quantum discord. Here we demonstrate that
discord can be explicitly used as a resource: certifying untrusted entangling
gates without generating entanglement at any stage. We implement our protocol
in the single-photon regime, and show its success in the presence of high
levels of noise and imperfect gate operations. Our technique offers a practical
method for benchmarking entangling gates in physical architectures in which
only highly-mixed states are available.Comment: 5 pages, 2 figure
Single-photon device requirements for operating linear optics quantum computing outside the post-selection basis
Photonics is a promising architecture for the realisation of quantum
information processing, since the two-photon interaction, or non-linearity,
necessary to build logical gates can efficiently be realised by the use of
interference with ancillary photons and detection. Although single-photon
sources and detectors are pivotal in realisations of such systems, clear
guidelines for the required performance of realistic systems are yet to be
defined. We present our detailed numerical simulation of several quantum optics
circuits including sources and detectors all represented in multi-dimensional
Fockspaces, which allows to obtain experimentally realistic performance bounds
for for these devices. In addition, the single-photon source based on switched
parametric down-conversion is studied, which in principle could reach the
required performance. Three approaches for implementing the switching hierarchy
of the photons are simulated, and their anticipated performance is obtained.
Our results define the bar for the optical devices needed to achieve the first
level of linear-optics quantum computing outside the coincidence basis.Comment: To appear in J. Mod. Optic
Probing the Masses of the PSR J0621+1002 Binary System Through Relativistic Apsidal Motion
Orbital, spin and astrometric parameters of the millisecond pulsar PSR
J0621+1002 have been determined through six years of timing observations at
three radio telescopes. The chief result is a measurement of the rate of
periastron advance, omega_dot = 0.0116 +/- 0.0008 deg/yr. Interpreted as a
general relativistic effect, this implies the sum of the pulsar mass, m_1, and
the companion mass, m_2, to be M = m_1 + m_2 = 2.81 +/- 0.30 msun. The
Keplerian parameters rule out certain combinations of m_1 and m_2, as does the
non-detection of Shapiro delay in the pulse arrival times. These constraints,
together with the assumption that the companion is a white dwarf, lead to the
68% confidence maximum likelihood values of m_1 = 1.70(+0.32 -0.29) msun and
m_2 =0.97(+0.27 - 0.15) msun and to the 95% confidence maximum likelihood
values of m_1 = 1.70(+0.59 -0.63) msun and m_2 = 0.97(+0.43 -0.24) msun. The
other major finding is that the pulsar experiences dramatic variability in its
dispersion measure (DM), with gradients as steep as 0.013 pc cm^{-3} / yr. A
structure function analysis of the DM variations uncovers spatial fluctuations
in the interstellar electron density that cannot be fit to a single power law,
unlike the Kolmogorov turbulent spectrum that has been seen in the direction of
other pulsars. Other results from the timing analysis include the first
measurements of the pulsar's proper motion, mu = 3.5 +/- 0.3 mas / yr, and of
its spin-down rate, dP/dt = 4.7 x 10^{-20}, which, when corrected for kinematic
biases and combined with the pulse period, P = 28.8 ms, gives a characteristic
age of 1.1 x 10^{10} yr and a surface magnetic field strength of 1.2 x 10^{9}
G.Comment: Accepted by ApJ, 10 pages, 5 figure
- …