425 research outputs found
Information transmission over an amplitude damping channel with an arbitrary degree of memory
We study the performance of a partially correlated amplitude damping channel
acting on two qubits. We derive lower bounds for the single-shot classical
capacity by studying two kinds of quantum ensembles, one which allows to
maximize the Holevo quantity for the memoryless channel and the other allowing
the same task but for the full-memory channel. In these two cases, we also show
the amount of entanglement which is involved in achieving the maximum of the
Holevo quantity. For the single-shot quantum capacity we discuss both a lower
and an upper bound, achieving a good estimate for high values of the channel
transmissivity. We finally compute the entanglement-assisted classical channel
capacity.Comment: 17 pages, 7 figure
Classical and quantum capacities of a fully correlated amplitude damping channel
We study information transmission over a fully correlated amplitude damping
channel acting on two qubits. We derive the single-shot classical channel
capacity and show that entanglement is needed to achieve the channel best
performance. We discuss the degradability properties of the channel and
evaluate the quantum capacity for any value of the noise parameter. We finally
compute the entanglement-assisted classical channel capacity.Comment: 16 pages, 9 figure
Entanglement between two superconducting qubits via interaction with non-classical radiation
We propose a scheme to physically interface superconducting nano-circuits and
quantum optics. We address the transfer of quantum information between systems
having different physical natures and defined in Hilbert spaces of different
dimensions. In particular, we investigate the transfer of the entanglement
initially in a non-classical state of a continuous-variable system to a pair of
superconducting charge qubits. This set-up is able to drive an initially
separable state of the qubits into an almost pure, highly entangled state
suitable for quantum information processing.Comment: 4 pages, RevTeX; revised versio
Andreev tunneling into a one-dimensional Josephson junction array
In this letter we consider Andreev tunneling between a normal metal and a one
dimensional Josephson junction array with finite-range Coulomb energy. The
characteristics strongly deviate from the classical linear Andreev
current. We show that the non linear conductance possesses interesting scaling
behavior when the chain undergoes a T=0 superconductor-insulator transition of
Kosterlitz-Thouless-Berezinskii type. When the chain has quasi-long range
order, the low lying excitation are gapless and the curves are power-law
(the linear relation is recovered when charging energy can be disregarded).
When the chain is in the insulating phase the Andreev current is blocked at a
threshold which is proportional to the inverse correlation length in the chain
(much lower than the Coulomb gap) and which vanishes at the transition point.Comment: 8 pages LATEX, 3 figures available upon reques
Characterization of coherent impurity effects in solid state qubits
We propose a characterisation of the effects of bistable coherent impurities
in solid state qubits. We introduce an effective impurity description in terms
of a tunable spin-boson environment and solve the dynamics for the qubit
coherences. The dominant rate characterizing the asymptotic time limit is
identified and signatures of non-Gaussian behavior of the quantum impurity at
intermediate times are pointed out. An alternative perspective considering the
qubit as a measurement device for the spin-boson impurity is proposed.Comment: 4 pages, 5 figures. Replaced with published version, minor change
Correlated tunneling into a superconductor in a multiprobe hybrid structure
We consider tunneling in a hybrid system consisting of a superconductor with
two or more probe electrodes which can be either normal metals or polarized
ferromagnets. In particular we study transport at subgap voltages and
temperatures. Besides Andreev pair tunneling at each contact, in multi-probe
structures subgap transport involves additional channels, which are due to
coherent propagation of two particles (electrons or holes), each originating
from a different probe electrode. The relevant processes are electron
cotunneling through the superconductor and conversion of two electrons stemming
from different probes in a Cooper pair. These processes are non-local and decay
when the distance between the pair of involved contacts is larger than the
superconducting coherence length. The conductance matrix of a the three
terminal hybrid structure is calculated. The multi-probe processes enhance the
conductance of each contact. If the contacts are magnetically polarized the
contribution of the various conduction channels may be separately detected.Comment: 7 pages, 1 figure, accepted in Europhysics Letters, minor changes, 3
references adde
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