888 research outputs found
Quantum Zeno effect in the Cooper-pair transport through a double-island Josephson system
Motivated by recent experiments, we analyze transport of Cooper pairs through
a double-island Josephson qubit. At low bias in a certain range of gate
voltages coherent superpositions of charge states play a crucial role. Analysis
of the evolution of the density matrix allows us to cover a wide range of
parameters, incl. situations with degenerate levels, when dissipation strongly
affects the coherent eigenstates. At high noise levels the so-called Zeno
effect can be observed, which slows down the transport. Our analysis explains
certain features of the I-V curves, in particular the visibility and shape of
resonant peaks and lines
Quantum Manipulations of Small Josephson Junctions
Low-capacitance Josephson junction arrays in the parameter range where single
charges can be controlled are suggested as possible physical realizations of
the elements which have been considered in the context of quantum computers. We
discuss single and multiple quantum bit systems. The systems are controlled by
applied gate voltages, which also allow the necessary manipulation of the
quantum states. We estimate that the phase coherence time is sufficiently long
for experimental demonstration of the principles of quantum computation.Comment: RevTex, 15 pages,4 postscript figures, uuencoded, submitted to Phys.
Rev. Lett., estimates of the experimental parameters correcte
Linear scaling between momentum and spin scattering in graphene
Spin transport in graphene carries the potential of a long spin diffusion
length at room temperature. However, extrinsic relaxation processes limit the
current experimental values to 1-2 um. We present Hanle spin precession
measurements in gated lateral spin valve devices in the low to high (up to
10^13 cm^-2) carrier density range of graphene. A linear scaling between the
spin diffusion length and the diffusion coefficient is observed. We measure
nearly identical spin- and charge diffusion coefficients indicating that
electron-electron interactions are relatively weak and transport is limited by
impurity potential scattering. When extrapolated to the maximum carrier
mobilities of 2x10^5 cm^2/Vs, our results predict that a considerable increase
in the spin diffusion length should be possible
Entropic uncertainty relations and entanglement
We discuss the relationship between entropic uncertainty relations and
entanglement. We present two methods for deriving separability criteria in
terms of entropic uncertainty relations. Especially we show how any entropic
uncertainty relation on one part of the system results in a separability
condition on the composite system. We investigate the resulting criteria using
the Tsallis entropy for two and three qubits.Comment: 8 pages, 3 figures, v2: small change
Charge Transport Processes in a Superconducting Single-Electron Transistor Coupled to a Microstrip Transmission Line
We have investigated charge transport processes in a superconducting
single-electron transistor (S-SET) fabricated in close proximity to a
two-dimensional electron gas (2DEG) in a GaAs/AlGaAs heterostructure. The
macroscopic bonding pads of the S-SET along with the 2DEG form a microstrip
transmission line. We observe a variety of current-carrying cycles in the S-SET
which we attribute to simultaneous tunneling of Cooper pairs and emission of
photons into the microstrip. We find good agreement between these experimental
results and simulations including both photon emission and photon-assisted
tunneling due to the electromagnetic environment.Comment: 4 pages, 4 figures, REVTeX
Mediated tunable coupling of flux qubits
It is sketched how a monostable rf- or dc-SQUID can mediate an inductive
coupling between two adjacent flux qubits. The nontrivial dependence of the
SQUID's susceptibility on external flux makes it possible to continuously tune
the induced coupling from antiferromagnetic (AF) to ferromagnetic (FM). In
particular, for suitable parameters, the induced FM coupling can be
sufficiently large to overcome any possible direct AF inductive coupling
between the qubits.
The main features follow from a classical analysis of the multi-qubit
potential. A fully quantum treatment yields similar results, but with a
modified expression for the SQUID susceptibility.
Since the latter is exact, it can also be used to evaluate the
susceptibility--or, equivalently, energy-level curvature--of an isolated
rf-SQUID for larger shielding and at degenerate flux bias, i.e., a (bistable)
qubit. The result is compared to the standard two-level (pseudospin) treatment
of the anticrossing, and the ensuing conclusions are verified numerically.Comment: REVTeX 4, 16 pp., 4 EPS figures. N.B.: "Alec" is my first, and
"Maassen van den Brink" my family name. v2: major expansion and rewriting,
new title and co-author; to appear in New Journal of Physics special issue
(R. Fazio, ed.
Tunable coupling of superconducting qubits
We study an LC-circuit implemented using a current-biased Josephson junction
(CBJJ) as a tunable coupler for superconducting qubits. By modulating the bias
current, the junction can be tuned in and out of resonance and entangled with
the qubits coupled to it. One can thus implement two-qubit operations by
mediating entanglement. We consider the examples of CBJJ and charge--phase
qubits. A simple recoupling scheme leads to a generalization to arbitrary qubit
designs.Comment: To appear in Phys. Rev. Lett., 3 figure
Spin transport in high quality suspended graphene devices
We measure spin transport in high mobility suspended graphene (\mu ~ 10^5
cm^2/Vs), obtaining a (spin) diffusion coefficient of 0.1 m^2/s and giving a
lower bound on the spin relaxation time (\tau_s ~ 150 ps) and spin relaxation
length (\lambda_s=4.7 \mu m) for intrinsic graphene. We develop a theoretical
model considering the different graphene regions of our devices that explains
our experimental data.Comment: 22 pages, 6 figures; Nano Letters, Article ASAP (2012)
(http://pubs.acs.org/doi/abs/10.1021/nl301050a
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