151 research outputs found
Resonant tunneling through a macroscopic charge state in a superconducting SET transistor
We predict theoretically and observe in experiment that the differential
conductance of a superconducting SET transistor exhibits a peak which is a
complete analogue in a macroscopic system of a standard resonant tunneling peak
associated with tunneling through a single quantum state. In particular, in a
symmetric transistor, the peak height is universal and equal to . Away from the resonance we clearly observe the co-tunneling current
which in contrast to the normal-metal transistor varies linearly with the bias
voltage.Comment: 11 pages, 3 figures, Fig. 1 available upon request from the first
autho
Towards the observation of phase locked Bloch oscillations in arrays of small Josephson junctions
We have designed an experiment and performed extensive simulations and
preliminary measurements to identify a set of realistic circuit parameters that
should allow the observation of constant-current steps at I=2ef in short arrays
of small Josephson junctions under external AC drive of frequency f.
Observation of these steps demonstrating phase lock of the Bloch oscillations
with the external drive requires a high-impedance environment for the array,
which is provided by on-chip resistors close to the junctions. We show that the
width and shape of the steps crucially depend on the shape of the drive and the
electron temperature in the resistors
Resistively-shunted superconducting quantum point contacts
We have studied the Josephson dynamics of resistively-shunted ballistic
superconducting quantum point contacts at finite temperatures and arbitrary
number of conducting modes. Compared to the classical Josephson dynamics of
tunnel junctions, dynamics of quantum point contacts exhibits several new
features associated with temporal fluctuations of the Josephson potential
caused by fluctuations in the occupation of the current-carrying Andreev
levels.Comment: 5 pages, RevTex, 3 postscript figures include
Single-electron current sources: towards a refined definition of ampere
Controlling electrons at the level of elementary charge has been
demonstrated experimentally already in the 1980's. Ever since, producing an
electrical current , or its integer multiple, at a drive frequency has
been in a focus of research for metrological purposes. In this review we first
discuss the generic physical phenomena and technical constraints that influence
charge transport. We then present the broad variety of proposed realizations.
Some of them have already proven experimentally to nearly fulfill the demanding
needs, in terms of transfer errors and transfer rate, of quantum metrology of
electrical quantities, whereas some others are currently "just" wild ideas,
still often potentially competitive if technical constraints can be lifted. We
also discuss the important issues of read-out of single-electron events and
potential error correction schemes based on them. Finally, we give an account
of the status of single-electron current sources in the bigger framework of
electric quantum standards and of the future international SI system of units,
and briefly discuss the applications and uses of single-electron devices
outside the metrological context.Comment: 55 pages, 38 figures; (v2) fixed typos and misformatted references,
reworded the section on AC pump
Non-equilibrium current noise in mesoscopic disordered SNS junctions
Current noise in superconductor-normal metal-superconductor (SNS) junctions
is calculated within the scattering theory of multiple Andreev reflections
(MAR). It is shown that the noise exhibits subharmonic gap singularities at
, both in single-mode junctions with arbitrary
transparency and in multi-mode disordered junctions. The subharmonic
structure is superimposed with monotonic increase of the effective transferred
charge with decreasing bias voltage. Other features of the
noise include a step-like increase of in junctions with small , and a
divergence at small voltages and excess noise , where is the excess current, at large voltages, in
junctions with diffusive transport.Comment: 5 page
Методологический анализ при построении количественного социологического исследования: проблемы и решения
This article investigates the logic, main procedures and the content of the theoretical and methodological construction of the sociological research project. Key issues at every stage of project design and during the process of transition from one stage to another are considered. The author proposes several ways of improving the accuracy and scientific significance of the results of sociological research by constructing models of different levels, consequential from one to another.Статья посвящена проблемам разработки логики, процедур и содержания теоретического и методического построения социологического исследования как одному из приоритетных направлений развития его методологии. Раскрыты проблемы, с которыми сталкивается исследователь на каждом этапе разработки программы и при переходе от одного этапа к другому. Предложены пути решения данных проблем посредством построения моделей различного уровня, логически вытекающих друг из друга, что позволяет повысить достоверность и научную значимость результатов социологического исследования
Macroscopic resonant tunneling of magnetic flux
We have developed a quantitative theory of resonant tunneling of magnetic
flux between discrete macroscopically distinct quantum states in SQUID systems.
The theory is based on the standard density-matrix approach. Its new elements
include the discussion of the two different relaxation mechanisms that exist
for the double-well potential, and description of the ``photon-assisted''
tunneling driven by external rf radiation. It is shown that in the case of
coherent flux dynamics, rf radiation should lead to splitting of the peaks of
resonant flux tunneling, indicating that the resonant tunneling is a convenient
tool for studying macroscopic quantum coherence of flux.Comment: 11 pages, 8 figure
Coherent control of macroscopic quantum states in a single-Cooper-pair box
A small superconducting electrode (a single-Cooper-pair box) connected to a
reservoir via a Josephson junction constitutes an artificial two-level system,
in which two charge states that differ by 2e are coupled by tunneling of Cooper
pairs. Despite its macroscopic nature involving a large number of electrons,
the two-level system shows coherent superposition of the two charge states, and
has been suggested as a candidate for a qubit, i.e. a basic component of a
quantum computer. Here we report on time-domain observation of the coherent
quantum-state evolution in the two-level system by applying a short voltage
pulse that modifies the energies of the two levels nonadiabatically to control
the coherent evolution. The resulting state was probed by a tunneling current
through an additional probe junction. Our results demonstrate coherent
operation and measurement of a quantum state of a single two-level system, i.e.
a qubit, in a solid-state electronic device.Comment: 4 pages, 4 figures; to be published in Natur
Active suppression of dephasing in Josephson-junction qubits
Simple majority code correcting dephasing errors by encoding a qubit of
information into physical qubits is studied quantitatively. We derive an
equation for quasicontinuous evolution of the density matrix of encoded quantum
information under the error correction procedure in the presence of dephasing
noise that in general can be correlated at different qubits. Specific design of
the Josephson-junction circuit implementing this scheme is suggested.Comment: 4 pages, 1 figur
Direct Observation of Josephson Capacitance
The effective capacitance has been measured in the split Cooper pair box
(CPB) over its phase-gate bias plane. Our low-frequency reactive measurement
scheme allows to probe purely the capacitive susceptibility due to the CPB band
structure. The data are quantitatively explained using parameters determined
independently by spectroscopic means. In addition, we show in practice that the
method offers an efficient way to do non-demolition readout of the CPB quantum
state.Comment: 4 page
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