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

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

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

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