248 research outputs found
Quantum relays and noise suppression using linear optics
Probabilistic quantum non-demolition (QND) measurements can be performed
using linear optics and post-selection. Here we show how QND devices of this
kind can be used in a straightforward way to implement a quantum relay, which
is capable of extending the range of a quantum cryptography system by
suppressing the effects of detector noise. Unlike a quantum repeater, a quantum
relay system does not require entanglement purification or the ability to store
photons.Comment: minor changes; references adde
Non-realism : deep thought or a soft option ?
The claim that the observation of a violation of a Bell inequality leads to
an alleged alternative between nonlocality and non-realism is annoying because
of the vagueness of the second term.Comment: 5 page
Bounds on the Probability of Success of Postselected Non-linear Sign Shifts Implemented with Linear Optics
The fundamental gates of linear optics quantum computation are realized by
using single photons sources, linear optics and photon counters. Success of
these gates is conditioned on the pattern of photons detected without using
feedback. Here it is shown that the maximum probability of success of these
gates is typically strictly less than 1. For the one-mode non-linear sign
shift, the probability of success is bounded by 1/2. For the conditional sign
shift of two modes, this probability is bounded by 3/4. These bounds are still
substantially larger than the highest probabilities shown to be achievable so
far, which are 1/4 and 2/27, respectively.Comment: 6 page
All-Optical Switching Demonstration using Two-Photon Absorption and the Classical Zeno Effect
Low-contrast all-optical Zeno switching has been demonstrated in a silicon
nitride microdisk resonator coupled to a hot atomic vapor. The device is based
on the suppression of the field build-up within a microcavity due to
non-degenerate two-photon absorption. This experiment used one beam in a
resonator and one in free-space due to limitations related to device physics.
These results suggest that a similar scheme with both beams resonant in the
cavity would correspond to input power levels near 20 nW.Comment: 4 pages, 5 figure
Interference in dielectrics and pseudo-measurements
Inserting a lossy dielectric into one arm of an interference experiment acts
in many ways like a measurement. If two entangled photons are passed through
the interferometer, a certain amount of information is gained about which path
they took, and the interference pattern in a coincidence count measurement is
suppressed. However, by inserting a second dielectric into the other arm of the
interferometer, one can restore the interference pattern. Two of these
pseudo-measurements can thus cancel each other out. This is somewhat analogous
to the proposed quantum eraser experiments.Comment: 7 pages RevTeX 3.0 + 2 figures (postscript). Submitted to Phys. Rev.
Full Quantum Analysis of Two-Photon Absorption Using Two-Photon Wavefunction: Comparison with One-Photon Absorption
For dissipation-free photon-photon interaction at the single photon level, we
analyze one-photon transition and two-photon transition induced by photon pairs
in three-level atoms using two-photon wavefunctions. We show that the
two-photon absorption can be substantially enhanced by adjusting the time
correlation of photon pairs. We study two typical cases: Gaussian wavefunction
and rectangular wavefunction. In the latter, we find that under special
conditions one-photon transition is completely suppressed while the high
probability of two-photon transition is maintained.Comment: 6 pages, 4 figure
Clock synchronization with dispersion cancellation
The dispersion cancellation feature of pulses which are entangled in
frequency is employed to synchronize clocks of distant parties. The proposed
protocol is insensitive to the pulse distortion caused by transit through a
dispersive medium. Since there is cancellation to all orders, also the effects
of slowly fluctuating dispersive media are compensated. The experimental setup
can be realized with currently available technology, at least for a proof of
principle.Comment: 4 pages, 3 figure
An Algebraic Approach to Linear-Optical Schemes for Deterministic Quantum Computing
Linear-Optical Passive (LOP) devices and photon counters are sufficient to
implement universal quantum computation with single photons, and particular
schemes have already been proposed. In this paper we discuss the link between
the algebraic structure of LOP transformations and quantum computing. We first
show how to decompose the Fock space of N optical modes in finite-dimensional
subspaces that are suitable for encoding strings of qubits and invariant under
LOP transformations (these subspaces are related to the spaces of irreducible
unitary representations of U(N)). Next we show how to design in algorithmic
fashion
LOP circuits which implement any quantum circuit deterministically. We also
present some simple examples, such as the circuits implementing a CNOT gate and
a Bell-State Generator/Analyzer.Comment: new version with minor modification
Entanglement from longitudinal and scalar photons
The covariant quantization of the electromagnetic field in the Lorentz gauge
gives rise to longitudinal and scalar photons in addition to the usual
transverse photons. It is shown here that the exchange of longitudinal and
scalar photons can produce entanglement between two distant atoms or harmonic
oscillators. The form of the entangled states produced in this way is very
different from that obtained in the Coulomb gauge, where the longitudinal and
scalar photons do not exist. A generalized gauge transformation is used to show
that all physically observable effects are the same in the two gauges, despite
the differences in the form of the entangled states. An approach of this kind
may be useful for a covariant description of the dynamics of quantum
information processing.Comment: 12 pages, 1 figur
Simple criteria for projective measurements with linear optics
We derive a set of criteria to decide whether a given projection measurement
can be, in principle, exactly implemented solely by means of linear optics. The
derivation can be adapted to various detection methods, including photon
counting and homodyne detection. These criteria enable one to obtain easily
No-Go theorems for the exact distinguishability of orthogonal quantum states
with linear optics including the use of auxiliary photons and conditional
dynamics.Comment: final published versio
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