729 research outputs found
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
Long spin relaxation times in wafer scale epitaxial graphene on SiC(0001)
We developed an easy, upscalable process to prepare lateral spin-valve
devices on epitaxially grown monolayer graphene on SiC(0001) and perform
nonlocal spin transport measurements. We observe the longest spin relaxation
times tau_S in monolayer graphene, while the spin diffusion coefficient D_S is
strongly reduced compared to typical results on exfoliated graphene. The
increase of tau_S is probably related to the changed substrate, while the cause
for the small value of D_S remains an open question.Comment: 16 pages, 3 figures, 1 tabl
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
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
Quantum Trajectory Approach to the Stochastic Thermodynamics of a Forced Harmonic Oscillator
I formulate a quantum stochastic thermodynamics for the quantum trajectories
of a continuously-monitored forced harmonic oscillator coupled to a thermal
reservoir. Consistent trajectory-dependent definitions are introduced for work,
heat, and entropy, through engineering the thermal reservoir from a sequence of
two-level systems. Within this formalism the connection between irreversibility
and entropy production is analyzed and confirmed by proving a detailed
fluctuation theorem for quantum trajectories. Finally, possible experimental
verifications are discussed.Comment: 16 pages, 3 figures, submitted to PRE; expanded introduction and
conclusion, corrected typos, new figure
Spin dependent quantum interference in non-local graphene spin valves
Spin dependent electron transport measurements on graphene are of high
importance to explore possible spintronic applications. Up to date all spin
transport experiments on graphene were done in a semi-classical regime,
disregarding quantum transport properties such as phase coherence and
interference. Here we show that in a quantum coherent graphene nanostructure
the non-local voltage is strongly modulated. Using non-local measurements, we
separate the signal in spin dependent and spin independent contributions. We
show that the spin dependent contribution is about two orders of magnitude
larger than the spin independent one, when corrected for the finite
polarization of the electrodes. The non-local spin signal is not only strongly
modulated but also changes polarity as a function of the applied gate voltage.
By locally tuning the carrier density in the constriction we show that the
constriction plays a major role in this effect and indicates that it can act as
a spin filter device. Our results show the potential of quantum coherent
graphene nanostructures for the use in future spintronic devices
Simultaneous minimum-uncertainty measurement of discrete-valued complementary observables
We have made the first experimental demonstration of the simultaneous minimum
uncertainty product between two complementary observables for a two-state
system (a qubit). A partially entangled two-photon state was used to perform
such measurements. Each of the photons carries (partial) information of the
initial state thus leaving a room for measurements of two complementary
observables on every member in an ensemble.Comment: 4 pages, 4 figures, REVTeX, submitted to PR
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