5,488 research outputs found
Disagreement between correlations of quantum mechanics and stochastic electrodynamics in the damped parametric oscillator
Intracavity and external third order correlations in the damped nondegenerate
parametric oscillator are calculated for quantum mechanics and stochastic
electrodynamics (SED), a semiclassical theory. The two theories yield greatly
different results, with the correlations of quantum mechanics being cubic in
the system's nonlinear coupling constant and those of SED being linear in the
same constant. In particular, differences between the two theories are present
in at least a mesoscopic regime. They also exist when realistic damping is
included. Such differences illustrate distinctions between quantum mechanics
and a hidden variable theory for continuous variables.Comment: accepted by PR
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Facing up to the challenge of behavioural observation in infant hearing assessment
The ability to assess detection and discrimination of speech by infants has proved elusive. Dr Iain Jackson and colleagues discuss how new technologies and fresh approaches might offer valuable insight into young infants’ behavioural responses to sound
Hybrid quantum repeater based on dispersive CQED interactions between matter qubits and bright coherent light
We describe a system for long-distance distribution of quantum entanglement,
in which coherent light with large average photon number interacts dispersively
with single, far-detuned atoms or semiconductor impurities in optical cavities.
Entanglement is heralded by homodyne detection using a second bright light
pulse for phase reference. The use of bright pulses leads to a high success
probability for the generation of entanglement, at the cost of a lower initial
fidelity. This fidelity may be boosted by entanglement purification techniques,
implemented with the same physical resources. The need for more purification
steps is well compensated for by the increased probability of success when
compared to heralded entanglement schemes using single photons or weak coherent
pulses with realistic detectors. The principle cause of the lower initial
fidelity is fiber loss; however, spontaneous decay and cavity losses during the
dispersive atom/cavity interactions can also impair performance. We show that
these effects may be minimized for emitter-cavity systems in the weak-coupling
regime as long as the resonant Purcell factor is larger than one, the cavity is
over-coupled, and the optical pulses are sufficiently long. We support this
claim with numerical, semiclassical calculations using parameters for three
realistic systems: optically bright donor-bound impurities such as 19-F:ZnSe
with a moderate-Q microcavity, the optically dim 31-P:Si system with a high-Q
microcavity, and trapped ions in large but very high-Q cavities.Comment: Please consult the published version, where assorted typos are
corrected. It is freely available at http://stacks.iop.org/1367-2630/8/18
Efficient optical quantum information processing
Quantum information offers the promise of being able to perform certain
communication and computation tasks that cannot be done with conventional
information technology (IT). Optical Quantum Information Processing (QIP) holds
particular appeal, since it offers the prospect of communicating and computing
with the same type of qubit. Linear optical techniques have been shown to be
scalable, but the corresponding quantum computing circuits need many auxiliary
resources. Here we present an alternative approach to optical QIP, based on the
use of weak cross-Kerr nonlinearities and homodyne measurements. We show how
this approach provides the fundamental building blocks for highly efficient
non-absorbing single photon number resolving detectors, two qubit parity
detectors, Bell state measurements and finally near deterministic control-not
(CNOT) gates. These are essential QIP devicesComment: Accepted to the Journal of optics B special issue on optical quantum
computation; References update
Stabilizer Quantum Error Correction with Qubus Computation
In this paper we investigate stabilizer quantum error correction codes using
controlled phase rotations of strong coherent probe states. We explicitly
describe two methods to measure the Pauli operators which generate the
stabilizer group of a quantum code. First, we show how to measure a Pauli
operator acting on physical qubits using a single coherent state with large
average photon number, displacement operations, and photon detection. Second,
we show how to measure the stabilizer operators fault-tolerantly by the
deterministic preparation of coherent cat states along with one-bit
teleportations between a qubit-like encoding of coherent states and physical
qubits.Comment: 4 pages, 5 figure
The efficiencies of generating cluster states with weak non-linearities
We propose a scalable approach to building cluster states of matter qubits
using coherent states of light. Recent work on the subject relies on the use of
single photonic qubits in the measurement process. These schemes can be made
robust to detector loss, spontaneous emission and cavity mismatching but as a
consequence the overhead costs grow rapidly, in particular when considering
single photon loss. In contrast, our approach uses continuous variables and
highly efficient homodyne measurements. We present a two-qubit scheme, with a
simple bucket measurement system yielding an entangling operation with success
probability 1/2. Then we extend this to a three-qubit interaction, increasing
this probability to 3/4. We discuss the important issues of the overhead cost
and the time scaling. This leads to a "no-measurement" approach to building
cluster states, making use of geometric phases in phase space.Comment: 21 pages, to appear in special issue of New J. Phys. on
"Measurement-Based Quantum Information Processing
Hybrid quantum repeater using bright coherent light
We describe a quantum repeater protocol for long-distance quantum
communication. In this scheme, entanglement is created between qubits at
intermediate stations of the channel by using a weak dispersive light-matter
interaction and distributing the outgoing bright coherent light pulses among
the stations. Noisy entangled pairs of electronic spin are then prepared with
high success probability via homodyne detection and postselection. The local
gates for entanglement purification and swapping are deterministic and
measurement-free, based upon the same coherent-light resources and weak
interactions as for the initial entanglement distribution. Finally, the
entanglement is stored in a nuclear-spin-based quantum memory. With our system,
qubit-communication rates approaching 100 Hz over 1280 km with fidelities near
99% are possible for reasonable local gate errors.Comment: title changed, final published versio
Quantum Repeaters using Coherent-State Communication
We investigate quantum repeater protocols based upon atomic
qubit-entanglement distribution through optical coherent-state communication.
Various measurement schemes for an optical mode entangled with two spatially
separated atomic qubits are considered in order to nonlocally prepare
conditional two-qubit entangled states. In particular, generalized measurements
for unambiguous state discrimination enable one to completely eliminate
spin-flip errors in the resulting qubit states, as they would occur in a
homodyne-based scheme due to the finite overlap of the optical states in phase
space. As a result, by using weaker coherent states, high initial fidelities
can still be achieved for larger repeater spacing, at the expense of lower
entanglement generation rates. In this regime, the coherent-state-based
protocols start resembling single-photon-based repeater schemes.Comment: 11 pages, 8 figure
Single photon quantum non-demolition in the presence of inhomogeneous broadening
Electromagnetically induced transparency (EIT) has been often proposed for
generating nonlinear optical effects at the single photon level; in particular,
as a means to effect a quantum non-demolition measurement of a single photon
field. Previous treatments have usually considered homogeneously broadened
samples, but realisations in any medium will have to contend with inhomogeneous
broadening. Here we reappraise an earlier scheme [Munro \textit{et al.} Phys.
Rev. A \textbf{71}, 033819 (2005)] with respect to inhomogeneities and show an
alternative mode of operation that is preferred in an inhomogeneous
environment. We further show the implications of these results on a potential
implementation in diamond containing nitrogen-vacancy colour centres. Our
modelling shows that single mode waveguide structures of length in single-crystal diamond containing a dilute ensemble of NV
of only 200 centres are sufficient for quantum non-demolition measurements
using EIT-based weak nonlinear interactions.Comment: 21 pages, 9 figures (some in colour) at low resolution for arXiv
purpose
Fast Arc-Annotated Subsequence Matching in Linear Space
An arc-annotated string is a string of characters, called bases, augmented
with a set of pairs, called arcs, each connecting two bases. Given
arc-annotated strings and the arc-preserving subsequence problem is to
determine if can be obtained from by deleting bases from . Whenever
a base is deleted any arc with an endpoint in that base is also deleted.
Arc-annotated strings where the arcs are ``nested'' are a natural model of RNA
molecules that captures both the primary and secondary structure of these. The
arc-preserving subsequence problem for nested arc-annotated strings is basic
primitive for investigating the function of RNA molecules. Gramm et al. [ACM
Trans. Algorithms 2006] gave an algorithm for this problem using time
and space, where and are the lengths of and , respectively. In
this paper we present a new algorithm using time and space,
thereby matching the previous time bound while significantly reducing the space
from a quadratic term to linear. This is essential to process large RNA
molecules where the space is likely to be a bottleneck. To obtain our result we
introduce several novel ideas which may be of independent interest for related
problems on arc-annotated strings.Comment: To appear in Algoritmic
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