8,197 research outputs found
Quantum copying can increase the practically available information
While it is known that copying a quantum system does not increase the amount
of information obtainable about the originals, it may increase the amount
available in practice, when one is restricted to imperfect measurements. We
present a detection scheme which using imperfect detectors, and possibly noisy
quantum copying machines (that entangle the copies), allows one to extract more
information from an incoming signal, than with the imperfect detectors alone.
The case of single-photon detection with noisy, inefficient detectors and
copiers (single controlled-NOT gates in this case) is investigated in detail.
The improvement in distinguishability between a photon and vacuum is found to
occur for a wide range of parameters, and to be quite robust to random noise.
The properties that a quantum copying device must have to be useful in this
scheme are investigated.Comment: 10 pages, 6 figures, accepted PR
Practical limitations in optical entanglement purification
Entanglement purification protocols play an important role in the
distribution of entangled systems, which is necessary for various quantum
information processing applications. We consider the effects of photo-detector
efficiency and bandwidth, channel loss and mode-mismatch on the operation of an
optical entanglement purification protocol. We derive necessary detector and
mode-matching requirements to facilitate practical operation of such a scheme,
without having to resort to destructive coincidence type demonstrations.Comment: 4 pages, 4 figure
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
Teleportation using coupled oscillator states
We analyse the fidelity of teleportation protocols, as a function of resource
entanglement, for three kinds of two mode oscillator states: states with fixed
total photon number, number states entangled at a beam splitter, and the
two-mode squeezed vacuum state. We define corresponding teleportation protocols
for each case including phase noise to model degraded entanglement of each
resource.Comment: 21 pages REVTeX, manuscript format, 7 figures postscript, many
changes to pape
Guest Edited Collection: Quantitative and computational techniques in optical coherence tomography
Optical coherence tomography (OCT) is a three-dimensional optical imaging technique, frequently (but not exclusively) used for retinal imaging, that was first reported in the early 1990s. Since this time the technological development of OCT has been strongly influenced by its potential as a medical imaging technique. The first clinical prototype for use in ophthalmology was completed in 1994, paving the way for the first commercially available ophthalmic OCT system to be released to the market in 1996. Since then, OCT has become a mainstay of ophthalmology. OCT is also widely used in research, in an array of biomedical applications, and increasingly in industrial settings. Although there is still much activity in advancing OCT technology, there has been an increased emphasis in applying OCT to translational research. One direction of this research is in the development of quantitative and computational techniques to aid in the retrieval of clinically useful information from OCT images. This Collection brings together original research articles, which exploit realistic mathematical models of OCT image formation and machine learning approaches to obtain insight not otherwise available from raw OCT images. This includes research for measuring clinically relevant parameters such as retinal nerve fibre layer thickness, fractional flow reserve, and corneal biomechanics, and for performing feature identification and image process tasks
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
Generalized parity measurements
Measurements play an important role in quantum computing (QC), by either
providing the nonlinearity required for two-qubit gates (linear optics QC), or
by implementing a quantum algorithm using single-qubit measurements on a highly
entangled initial state (cluster state QC). Parity measurements can be used as
building blocks for preparing arbitrary stabilizer states, and, together with
1-qubit gates are universal for quantum computing. Here we generalize parity
gates by using a higher dimensional (qudit) ancilla. This enables us to go
beyond the stabilizer/graph state formalism and prepare other types of
multi-particle entangled states. The generalized parity module introduced here
can prepare in one-shot, heralded by the outcome of the ancilla, a large class
of entangled states, including GHZ_n, W_n, Dicke states D_{n,k}, and, more
generally, certain sums of Dicke states, like G_n states used in secret
sharing. For W_n states it provides an exponential gain compared to linear
optics based methods.Comment: 7 pages, 1 fig; updated to the published versio
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