782 research outputs found
The Inductive Single-Electron Transistor (L-SET)
We demonstrate a sensitive method of charge detection based on
radio-frequency readout of the Josephson inductance of a superconducting
single-electron transistor. Charge sensitivity ,
limited by preamplifier, is achieved in an operation mode which takes advantage
of the nonlinearity of the Josephson potential. Owing to reactive readout, our
setup has more than two orders of magnitude lower dissipation than the existing
method of radio-frequency electrometry. With an optimized sample, we expect
uncoupled energy sensitivity below in the same experimental scheme.Comment: 10 page
Quantum capacitive phase detector
We discuss how a single Cooper-pair transistor may be used to detect the
superconducting phase difference by using the phase dependence of the input
capacitance from gate to the ground. The proposed device has a low power
dissipation because its operation is in principle free from quasiparticle
generation. According to the sensitivity estimates the device may be used for
efficient qubit readout in a galvanically isolated and symmetrized circuit.Comment: 5 pages, published for
Dephasing of solid-state qubits at optimal points
Motivated by recent experiments with Josephson-junction circuits, we analyze
the influence of various noise sources on the dynamics of two-level systems at
optimal operation points where the linear coupling to low-frequency
fluctuations is suppressed. We study the decoherence due to nonlinear
(quadratic) coupling, focusing on the experimentally relevant 1/f and Ohmic
noise power spectra. For 1/f noise strong higher-order effects influence the
evolution.Comment: minor corrections and clarification
Superconducting proximity effect in a diffusive ferromagnet with spin-active interfaces
We reconsider the problem of the superconducting proximity effect in a
diffusive ferromagnet bounded by tunneling interfaces, using spin-dependent
boundary conditions. This introduces for each interface a phase-shifting
conductance Gphi which results from the spin dependence of the phase shifts
acquired by the electrons upon scattering on the interface. We show that Gphi
strongly affects the density of states and supercurrents predicted for
superconducting/ferromagnetic hybrid circuits. We show the relevance of this
effect by identifying clear signatures of Gphi in the data of T. Kontos et al
[Phys. Rev. Lett. 86, 304 (2001), ibid. 89, 137007 (2002)].Comment: submitted to Phys. Rev. Let
Effect of Measurement on the Periodicity of the Coulomb Staircase of a Superconducting Box
We report on the effect of the back-action of a Single Cooper Pair Transistor
electrometer (E) on the measurement of charge on the island of a
superconducting box (B). The charge is e-periodic in the gate bias of B when E
is operated near voltages 2Delta/e or 4Delta/e. We show that this is due to
quasiparticle poisoning of B at a rate proportional to the number of
quasiparticle tunneling events in E per second. We are able to eliminate this
back action and recover 2e charge periodicity using a new measurement method
based on switching current modulation of E.Comment: 4 pages, 4 figures, revised versio
Spectroscopy and critical temperature of diffusive superconducting/ferromagnetic hybrid structures with spin-active interfaces
The description of the proximity effect in superconducting/ferromagnetic
heterostructures requires to use spin-dependent boundary conditions. Such
boundary conditions must take into account the spin dependence of the phase
shifts acquired by electrons upon scattering on the boundaries of ferromagnets.
The present article shows that this property can strongly affect the critical
temperature and the energy dependence of the density of states of diffusive
heterostructures. These effects should allow a better caracterisation of
diffusive superconductor/ferromagnet interfaces.Comment: 12 pages, 6 figures, to be published in Phys. Rev.
Positive cross-correlations in a three-terminal quantum dot with ferromagnetic contacts
We study current fluctuations in an interacting three-terminal quantum dot
with ferromagnetic leads. For appropriately polarized contacts, the transport
through the dot is governed by a novel dynamical spin blockade, i.e., a
spin-dependent bunching of tunneling events not present in the paramagnetic
case. This leads for instance to positive zero-frequency cross-correlations of
the currents in the output leads even in the absence of spin accumulation on
the dot. We include the influence of spin-flip scattering and identify
favorable conditions for the experimental observation of this effect with
respect to polarization of the contacts and tunneling rates.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Manipulating the Quantum State of an Electrical Circuit
We have designed and operated a superconducting tunnel junction circuit that
behaves as a two-level atom: the ``quantronium''. An arbitrary evolution of its
quantum state can be programmed with a series of microwave pulses, and a
projective measurement of the state can be performed by a pulsed readout
sub-circuit. The measured quality factor of quantum coherence Qphi=25000 is
sufficiently high that a solid-state quantum processor based on this type of
circuit can be envisioned.Comment: 4 figures include
Effect of the attachment of ferromagnetic contacts on the conductivity and giant magnetoresistance of graphene nanoribbons
Carbon-based nanostructures and graphene, in particular, evoke a lot of
interest as new promising materials for nanoelectronics and spintronics. One of
the most important issue in this context is the impact of external electrodes
on electronic properties of graphene nanoribbons (GNR). The present theoretical
method is based on the tight-binding model and a modified recursive procedure
for Green's functions. The results show that within the ballistic transport
regime, the so called end-contacted geometry (of minimal GNR/electrode
interface area), is usually more advantageous for practical applications than
its side-contacted counterpart (with a larger coverage area), as far as the
electrical conductivity is concerned. As regards the giant magnetoresistance
coefficient, however, the situation is exactly opposite, since spin- splitting
effects are more pronounced in the lower conductive side-contacted setups.Comment: 8 pages, 4 figure
Photon mediated interaction between distant quantum dot circuits
Engineering the interaction between light and matter is an important goal in
the emerging field of quantum opto-electronics. Thanks to the use of cavity
quantum electrodynamics architectures, one can envision a fully hybrid
multiplexing of quantum conductors. Here, we use such an architecture to couple
two quantum dot circuits . Our quantum dots are separated by 200 times their
own size, with no direct tunnel and electrostatic couplings between them. We
demonstrate their interaction, mediated by the cavity photons. This could be
used to scale up quantum bit architectures based on quantum dot circuits or
simulate on-chip phonon-mediated interactions between strongly correlated
electrons
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