876 research outputs found
Observation of macroscopic Landau-Zener transitions in a superconducting device
A two-level system traversing a level anticrossing has a small probability to
make a so-called Landau-Zener (LZ) transition between its energy bands, in
deviation from simple adiabatic evolution. This effect takes on renewed
relevance due to the observation of quantum coherence in superconducting qubits
(macroscopic "Schrodinger cat" devices). We report an observation of LZ
transitions in an Al three-junction qubit coupled to a Nb resonant tank
circuit.Comment: REVTeX4, 4pp., 4 EPS figures. v2: clarifications added; final, to
appear in EP
Development and test of resistive superconducting fault current limiter; acting time and its recovery conditions
Resistive-type of superconducting fault current limiters (RSFCL) have been developed for medium voltage class aiming to operate at 1 MVA power capacity and short time recovery (< 2 s). A RSFCL in form of superconducting modular device was designed and constructed using 50 m-length of YBCO coated conductor tapes for operation under 1 kV / 1 kA and acting time of 0.1 s. In order to increase the acting time the RSFCL was combined with an air-core reactor in parallel to increase the fault limiting time up to 1 s. The tests determined the electrical and thermal characteristics of the combined resistive/ inductive protection unit. The combined fault current limiter reached a limiting current of 583 A, corresponding to a limiting factor of 3.3 times within an acting time of up to 1 s
Proposal for an optical laser producing light at half the Josephson frequency
We describe a superconducting device capable of producing laser light in the
visible range at half of the Josephson generation frequency with the optical
phase of the light locked to the superconducting phase difference. It consists
of two single-level quantum dots embedded into a p-n semiconducting
heterostructure and surrounded by a cavity supporting a resonant optical mode.
We study decoherence and spontaneous switching in the device.Comment: 4+3 pages, 3+1 figure
The effect of charge fluctuation on a normal-superconducting-normal single-electron transistor
We theoretically investigate quantum fluctuation of charge between even and
odd states of a normal-superconducting-normal single-electron tunneling
transistor. It is shown that due to the superconducting gap, the charge
fluctuation in the Coulomb blockade regime for even state is larger than that
for odd state. We show that large energy correction in the former regime caused
by charge fluctuation can be explained by considering the charging energy
renormalization.Comment: 4 pages, 3 figures, prepared for Proccedings of International
Symposium on Superconducting Device Physics (SDP 2001
Spin Injection and Detection in Magnetic Nanostructures
We study theoretically the spin transport in a nonmagnetic metal connected to
ferromagnetic injector and detector electrodes. We derive a general expression
for the spin accumulation signal which covers from the metallic to the
tunneling regime. This enables us to discuss recent controversy on spin
injection and detection experiments. Extending the result to a superconducting
device, we find that the spin accumulation signal is strongly enhanced by
opening of the superconducting gap since a gapped superconductor is a low
carrier system for spin transport but not for charge. The enhancement is also
expected in semiconductor devices.Comment: 4 pages, 3 figure
Quantum computation in decoherence-free subspace with superconducting devices
We propose a scheme to implement quantum computation in decoherence-free
subspace with superconducting devices inside a cavity by unconventional
geometric manipulation. Universal single-qubit gates in encoded qubit can be
achieved with cavity assisted interaction. A measurement-based two-qubit
Controlled-Not gate is produced with parity measurements assisted by an
auxiliary superconducting device and followed by prescribed single-qubit gates.
The measurement of currents on two parallel devices can realize a projective
measurement, which is equivalent to the parity measurement on the involved
devices.Comment: v2: thoroughly rewritten version with title and motivation changed;
v3: published version with detail dirivation
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