514 research outputs found
Magnetospheric convection from Cluster EDI measurements compared with the ground-based ionospheric convection model IZMEM
Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for more than seven and a half years (2001â2008) have been used to derive a statistical model of the high-latitude electric potential distribution for summer conditions. Based on potential pattern for different orientations of the interplanetary magnetic field (IMF) in the GSM y-z-plane, basic convection pattern (BCP) were derived, that represent the main characteristics of the electric potential distribution in dependence on the IMF. The BCPs comprise the IMF-independent potential distribution as well as patterns, which describe the dependence on positive and negative IMF<I>B<sub>z</sub></I> and IMF<I>B<sub>y</sub></I> variations. The full set of BCPs allows to describe the spatial and temporal variation of the high-latitude electric potential (ionospheric convection) for any solar wind IMF condition near the Earth's magnetopause within reasonable ranges. The comparison of the Cluster/EDI model with the IZMEM ionospheric convection model, which was derived from ground-based magnetometer observations, shows a good agreement of the basic patterns and its variation with the IMF. According to the statistical models, there is a two-cell antisunward convection within the polar cap for northward IMF<I>B<sub>z</sub></I>+&le;2 nT, while for increasing northward IMF<I>B<sub>z</sub></I>+ there appears a region of sunward convection within the high-latitude daytime sector, which assumes the form of two additional cells with sunward convection between them for IMF<I>B<sub>z</sub></I>+&asymp;4â5 nT. This results in a four-cell convection pattern of the high-latitude convection. In dependence of the &plusmn;IMF<I>B<sub>y</sub></I> contribution during sufficiently strong northward IMF<I>B<sub>z</sub></I> conditions, a transformation to three-cell convection patterns takes place
Practical quantum key distribution: On the security evaluation with inefficient single-photon detectors
Quantum Key Distribution with the BB84 protocol has been shown to be
unconditionally secure even using weak coherent pulses instead of single-photon
signals. The distances that can be covered by these methods are limited due to
the loss in the quantum channel (e.g. loss in the optical fiber) and in the
single-photon counters of the receivers. One can argue that the loss in the
detectors cannot be changed by an eavesdropper in order to increase the covered
distance. Here we show that the security analysis of this scenario is not as
easy as is commonly assumed, since already two-photon processes allow
eavesdropping strategies that outperform the known photon-number splitting
attack. For this reason there is, so far, no satisfactory security analysis
available in the framework of individual attacks.Comment: 11 pages, 6 figures; Abstract and introduction extended, Appendix
added, references update
Nonorthogonal Quantum States Maximize Classical Information Capacity
I demonstrate that, rather unexpectedly, there exist noisy quantum channels
for which the optimal classical information transmission rate is achieved only
by signaling alphabets consisting of nonorthogonal quantum states.Comment: 5 pages, REVTeX, mild extension of results, much improved
presentation, to appear in Physical Review Letter
Implementation of generalized quantum measurements: superadditive quantum coding, accessible information extraction, and classical capacity limit
Quantum information theory predicts that when the transmission resource is
doubled in quantum channels, the amount of information transmitted can be
increased more than twice by quantum channel coding technique, whereas the
increase is at most twice in classical information theory. This remarkable
feature, the superadditive quantum coding gain, can be implemented by
appropriate choices of code words and corresponding quantum decoding which
requires a collective quantum measurement. Recently, the first experimental
demonstration was reported [Phys. Rev. Lett. 90, 167906 (2003)]. The purpose of
this paper is to describe our experiment in detail. Particularly, a design
strategy of quantum collective decoding in physical quantum circuits is
emphasized. We also address the practical implication of the gain on
communication performance by introducing the quantum-classical hybrid coding
scheme. We show how the superadditive quantum coding gain, even in a small code
length, can boost the communication performance of conventional coding
technique.Comment: 15 pages, 14 figure
Experimental Demonstration of Post-Selection based Continuous Variable Quantum Key Distribution in the Presence of Gaussian Noise
In realistic continuous variable quantum key distribution protocols, an
eavesdropper may exploit the additional Gaussian noise generated during
transmission to mask her presence. We present a theoretical framework for a
post-selection based protocol which explicitly takes into account excess
Gaussian noise. We derive a quantitative expression of the secret key rates
based on the Levitin and Holevo bounds. We experimentally demonstrate that the
post-selection based scheme is still secure against both individual and
collective Gaussian attacks in the presence of this excess noise.Comment: 4 pages, 4 figure
On 1-qubit channels
The entropy H_T(rho) of a state rho with respect to a channel T and the
Holevo capacity of the channel require the solution of difficult variational
problems. For a class of 1-qubit channels, which contains all the extremal
ones, the problem can be significantly simplified by associating an Hermitian
antilinear operator theta to every channel of the considered class. The
concurrence of the channel can be expressed by theta and turns out to be a flat
roof. This allows to write down an explicit expression for H_T. Its maximum
would give the Holevo (1-shot) capacity.Comment: 12 pages, several printing or latex errors correcte
Capacities of noiseless quantum channels for massive indistinguishable particles: Bosons vs. fermions
We consider information transmission through a noiseless quantum channel,
where the information is encoded into massive indistinguishable particles:
bosons or fermions. We study the situation in which the particles are
noninteracting. The encoding input states obey a set of physically motivated
constraints on the mean values of the energy and particle number. In such a
case, the determination of both classical and quantum capacity reduces to a
constrained maximization of entropy. In the case of noninteracting bosons,
signatures of Bose Einstein condensation can be observed in the behavior of the
capacity. A major motivation for these considerations is to compare the
information carrying capacities of channels that carry bosons with those that
carry fermions. We show analytically that fermions generally provide higher
channel capacity, i.e., they are better suited for transferring bits as well as
qubits, in comparison to bosons. This holds for a large range of power law
potentials, and for moderate to high temperatures. Numerical simulations seem
to indicate that the result holds for all temperatures. Also, we consider the
low temperature behavior for the three-dimensional box and harmonic trap, and
again we show that the fermionic capacity is higher than the bosonic one for
sufficiently low temperatures.Comment: 16 pages, 8 eps figures, RevTeX4; v2: small change in a figure; v3:
significant new additions about quantum capacity, previous results unchanged,
title changed, published versio
Quantum bath suppression in a superconducting circuit by immersion cooling
Quantum circuits interact with the environment via several
temperature-dependent degrees of freedom. Yet, multiple experiments to-date
have shown that most properties of superconducting devices appear to plateau
out at mK -- far above the refrigerator base temperature. This is
for example reflected in the thermal state population of qubits, in excess
numbers of quasiparticles, and polarisation of surface spins -- factors
contributing to reduced coherence. We demonstrate how to remove this thermal
constraint by operating a circuit immersed in liquid He. This allows to
efficiently cool the decohering environment of a superconducting resonator, and
we see a continuous change in measured physical quantities down to previously
unexplored sub-mK temperatures. The He acts as a heat sink which increases
the energy relaxation rate of the quantum bath coupled to the circuit a
thousand times, yet the suppressed bath does not introduce additional circuit
losses or noise. Such quantum bath suppression can reduce decoherence in
quantum circuits and opens a route for both thermal and coherence management in
quantum processors
Topological superfluid 3He-B in magnetic field and Ising variable
The topological superfluid 3He-B provides many examples of the interplay of
symmetry and topology. Here we consider the effect of magnetic field on
topological properties of 3He-B. Magnetic field violates the time reversal
symmetry. As a result, the topological invariant supported by this symmetry
ceases to exist; and thus the gapless fermions on the surface of 3He-B are not
protected any more by topology: they become fully gapped. Nevertheless, if
perturbation of symmetry is small, the surface fermions remain relativistic
with mass proportional to symmetry violating perturbation -- magnetic field.
The 3He-B symmetry gives rise to the Ising variable I=+/- 1, which emerges in
magnetic field and which characterizes the states of the surface of 3He-B. This
variable also determines the sign of the mass term of surface fermions and the
topological invariant describing their effective Hamiltonian. The line on the
surface, which separates the surface domains with different I, contains 1+1
gapless fermions, which are protected by combined action of symmetry and
topology.Comment: 5 pages, JETP Letters style, no figures, version submitted to JETP
Letter
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