10,350 research outputs found
Superdense coding of quantum states
We describe a method to non-obliviously communicate a 2l-qubit quantum state
by physically transmitting l+o(l) qubits of communication, and by consuming l
ebits of entanglement and some shared random bits. In the non-oblivious
scenario, the sender has a classical description of the state to be
communicated. Our method can be used to communicate states that are pure or
entangled with the sender's system; l+o(l) and 3l+o(l) shared random bits are
sufficient respectively.Comment: 5 pages, revtex
Random subspaces for encryption based on a private shared Cartesian frame
A private shared Cartesian frame is a novel form of private shared
correlation that allows for both private classical and quantum communication.
Cryptography using a private shared Cartesian frame has the remarkable property
that asymptotically, if perfect privacy is demanded, the private classical
capacity is three times the private quantum capacity. We demonstrate that if
the requirement for perfect privacy is relaxed, then it is possible to use the
properties of random subspaces to nearly triple the private quantum capacity,
almost closing the gap between the private classical and quantum capacities.Comment: 9 pages, published versio
Random quantum codes from Gaussian ensembles and an uncertainty relation
Using random Gaussian vectors and an information-uncertainty relation, we
give a proof that the coherent information is an achievable rate for
entanglement transmission through a noisy quantum channel. The codes are random
subspaces selected according to the Haar measure, but distorted as a function
of the sender's input density operator. Using large deviations techniques, we
show that classical data transmitted in either of two Fourier-conjugate bases
for the coding subspace can be decoded with low probability of error. A
recently discovered information-uncertainty relation then implies that the
quantum mutual information for entanglement encoded into the subspace and
transmitted through the channel will be high. The monogamy of quantum
correlations finally implies that the environment of the channel cannot be
significantly coupled to the entanglement, and concluding, which ensures the
existence of a decoding by the receiver.Comment: 9 pages, two-column style. This paper is a companion to
quant-ph/0702005 and quant-ph/070200
High-Frequency Spin Waves in YBa2Cu3O6.15
Pulsed neutron spectroscopy is used to make absolute measurements of the
dynamic magnetic susceptibility of insulating YBa2Cu3O6.15. Acoustic and
optical modes, derived from in- and out-of-phase oscillation of spins in
adjacent CuO2 planes, dominate the spectra and are observed up to 250 meV. The
optical modes appear first at 74 meV. Linear-spin-wave theory gives an
excellent description of the data and yields intra- and inter-layer exchange
constants of J_parallel =125 meV and J_perp = 11 meV respectively and a
spin-wave intensity renormalization Z_chi = 0.4.Comment: postscript, 11 pages, 4 figures, Fig.2 fixe
Orbital transfer vehicle oxygen turbopump technology. Volume 1: Design, fabrication, and hydrostatic bearing testing
The design, fabrication, and initial testing of a rocket engine turbopump (TPA) for the delivery of high pressure liquid oxygen using hot oxygen for the turbine drive fluid are described. This TPA is basic to the dual expander engine which uses both oxygen and hydrogen as working fluids. Separate tasks addressed the key issue of materials for this TPA. All materials selections emphasized compatibility with hot oxygen. The OX TPA design uses a two-stage centrifugal pump driven by a single-stage axial turbine on a common shaft. The design includes ports for three shaft displacement/speed sensors, various temperature measurements, and accelerometers
A Comparison of the High-Frequency Magnetic Fluctuations in Insulating and Superconducting La2-xSrxCuO4
Inelastic neutron scattering performed at a spallation source is used to make
absolute measurements of the dynamic susceptibility of insulating La2CuO4 and
superconducting La2-xSrxCuO4 over the energy range 15<EN<350 meV. The effect of
Sr doping on the magnetic excitations is to cause a large broadening in
wavevector and a substantial change in the spectrum of the local spin
fluctuations. Comparison of the two compositions reveals a new energy scale of
22 meV in La1.86Sr0.14CuO4.Comment: RevTex, 7 Pages, 4 postscript figure
Entanglement-assisted local operations and classical communications conversion in the quantum critical systems
Conversions between the ground states in quantum critical systems via
entanglement-assisted local operations and classical communications (eLOCC) are
studied. We propose a new method to reveal the different convertibility by
local operations when a quantum phase transition occurs. We have studied the
ground state local convertibility in the one dimensional transverse field Ising
model, XY model and XXZ model. It is found that the eLOCC convertibility sudden
changes at the phase transition points. In transverse field Ising model the
eLOCC convertibility between the first excited state and the ground state are
also distinct for different phases. The relation between the order of quantum
phase transitions and the local convertibility is discussed.Comment: 7 pages, 5 figures, 5 table
Notes on multiplicativity of maximal output purity for completely positive qubit maps
A problem in quantum information theory that has received considerable
attention in recent years is the question of multiplicativity of the so-called
maximal output purity (MOP) of a quantum channel. This quantity is defined as
the maximum value of the purity one can get at the output of a channel by
varying over all physical input states, when purity is measured by the Schatten
-norm, and is denoted by . The multiplicativity problem is the
question whether two channels used in parallel have a combined that is
the product of the of the two channels. A positive answer would imply a
number of other additivity results in QIT.
Very recently, P. Hayden has found counterexamples for every value of .
Nevertheless, these counterexamples require that the dimension of these
channels increases with and therefore do not rule out multiplicativity
for in intervals with depending on the channel dimension. I
argue that this would be enough to prove additivity of entanglement of
formation and of the classical capacity of quantum channels.
More importantly, no counterexamples have as yet been found in the important
special case where one of the channels is a qubit-channel, i.e. its input
states are 2-dimensional. In this paper I focus attention to this qubit case
and I rephrase the multiplicativity conjecture in the language of block
matrices and prove the conjecture in a number of special cases.Comment: Manuscript for a talk presented at the SSPCM07 conference in
Myczkowce, Poland, 10/09/2007. 12 page
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