Criteria for Continuous-Variable Quantum Teleportation

We derive an experimentally testable criterion for the teleportation of quantum states of continuous variables. This criterion is especially relevant to the recent experiment of Furusawa et al. [Science 282, 706-709 (1998)] where an input-output fidelity of $0.58 \pm 0.02$ was achieved for optical coherent states. Our derivation demonstrates that fidelities greater than 1/2 could not have been achieved through the use of a classical channel alone; quantum entanglement was a crucial ingredient in the experiment.Comment: 12 pages, to appear in Journal of Modern Optic

Nonlinear viscoelasticity of metastable complex fluids

Many metastable complex fluids such as colloidal glasses and gels show distinct nonlinear viscoelasticity with increasing oscillatory-strain amplitude; the storage modulus decreases monotonically as the strain amplitude increases whereas the loss modulus has a distinct peak before it decreases at larger strains. We present a qualitative argument to explain this ubiquitous behavior and use mode coupling theory (MCT) to confirm it. We compare theoretical predictions to the measured nonlinear viscoelasticity in a dense hard sphere colloidal suspensions; reasonable agreement is obtained. The argument given here can be used to obtain new information about linear viscoelasticity of metastable complex fluids from nonlinear strain measurements.Comment: 7 pages, 3 figures, accepted for publication in Europhys. Let

HTS quasiparticle injection devices with large current gain at 77 K

Recent progress on the development of planar QP-injection devices using YBCO and STO as an epitaxial injection barrier will be discussed. The main problem for HTS injection devices is to grow reliably a well defined, ultra-thin tunneling barrier suitable for QP tunneling. For this purpose, we used inverted cylindrical magnetron sputtering to first optimize the smoothness of our YBCO films by controlling tightly an relevant sputtering conditions. We are able to prepare smooth (001) YBCO films on (001) STO substrates on a routine basis with an average roughness varying between 1 and 2 nm. With these flat YBCO films both planar as well as grain boundary junctions were fabricated using epitaxial STO barriers between 2 and 8 nm thick and a 50 nm of Au counter electrode. Planar junctions with 6 nm STO barriers were in most cases fully insulating, in some cases, a current gain of up to 7.4 at 77 K was obtained. For 3 nm STO barriers, the highest current gain was 15 at 81 K. The injection results also show a scaling behavior with junction size. Based on the present materials development and device understanding, we consider a current gain of up to 20 at 77 K possibl

Anisotropic magnetic behavior of GdBa_2Cu_3O_{6+y} single crystals

Magnetic properties of high-quality Al-free nonsuperconducting GdBa_2Cu_3O_{6+y} single crystals grown by flux method have been studied. The magnetic anisotropy below the N\'eel temperature T_N~2.3K corresponds to the direction of Gd^{3+} magnetic moments along the tetragonal c-axis. At T < T_N clear indications of spin-flop transitions for H||c have been observed on magnetization curves at H_{sf}~10kOe. Magnetic phase diagrams have been obtained for H||c as well as for H||ab. A pronounced anisotropy in the magnetic susceptibility (unexpected for Gd-based compounds) has been found above T_N.Comment: 2 pages, 3 figures; LT23 (Aug. 2002; Hiroshima), accepted to Physica

Quantum probabilities as Bayesian probabilities

In the Bayesian approach to probability theory, probability quantifies a degree of belief for a single trial, without any a priori connection to limiting frequencies. In this paper we show that, despite being prescribed by a fundamental law, probabilities for individual quantum systems can be understood within the Bayesian approach. We argue that the distinction between classical and quantum probabilities lies not in their definition, but in the nature of the information they encode. In the classical world, maximal information about a physical system is complete in the sense of providing definite answers for all possible questions that can be asked of the system. In the quantum world, maximal information is not complete and cannot be completed. Using this distinction, we show that any Bayesian probability assignment in quantum mechanics must have the form of the quantum probability rule, that maximal information about a quantum system leads to a unique quantum-state assignment, and that quantum theory provides a stronger connection between probability and measured frequency than can be justified classically. Finally we give a Bayesian formulation of quantum-state tomography.Comment: 6 pages, Latex, final versio

Gauge Invariant Action for the Open Bosonic String: Tachyon Action

A gauge invariant action for the open bosonic string has been proposed in an earlier paper. We work out the consequences of this proposal for the lowest mode, viz. the tachyon. The action can be calculated for generic momenta, perturbatively, order by order in the tachyon field. For on shell tachyons we explicitly calculate the cubic action and show that it reproduces the correct equations of motion and coincides wih the $\beta$ function to the required order. The calculation is done in terms of bare fields with a finite cutoff, which is the original prescription. We also show that it is possible in some momentum regions to renormalize the theory and eliminate the cutoff dependence so that the continuum limit can be taken. After renormalization, the parameter $R\over a$ is replaced by $R\over L$ where $R$ is an IR cutoff, $a$ is the UV cutoff and $L$ is some renormalization scale. There is also some arbitrariness in the overall normalization due to the choice of regularization scheme - this does not affect on-shell quantities. We also rederive within this scheme, the action in the region of zero momentum, which gives the exact (tree level) tachyon potential. The tachyon potential is consistent with Sen's conjecture that the height of the potential is the same as the tension of the brane.Comment: 31 pages, Late

Possible new vortex matter phases in BSCCO

The vortex matter phase diagram of BSCCO crystals is analyzed by investigating vortex penetration through the surface barrier in the presence of a transport current. The strength of the effective surface barrier, its nonlinearity, and asymmetry are used to identify a possible new ordered phase above the first-order transition. This technique also allows sensitive determination of the depinning temperature. The solid phase below the first-order transition is apparently subdivided into two phases by a vertical line extending from the multicritical point.Comment: 11 pages, 3 figures, accepted for publication in PR