135 research outputs found
Magnetic flux noise in the three Josephson junctions superconducting ring
We analyze the influence of noise on magnetic properties of a su-
perconducting loop which contains three Josephson junctions. This circuit is a
classical analog of a persistent current (flux) qubit. A loop supercurrent
induced by external magnetic field in the presence of thermal fluctuations is
calculated. In order to get connection with experiment we calculate the
impedance of the low-frequency tank cir- cuit which is inductively coupled with
a loop of interest. We compare obtained results with the results in quantum
mode - when the three junction loop exhibits quantum tunneling of the magnetic
flux. We demonstrate that the tank-loop impedance in the classical and quan-
tum modes have different temperature dependence and can be easily distinguished
experimentally.Comment: 19 pages 9 figure
Inductance mode characteristics of a ceramic YBa2Cu3O7-x radio-frequency superconducting quantum interference device at 77 K
Anomalous Periodicity of the Current-Phase Relationship of Grain-Boundary Josephson Junctions in High-Tc Superconductors
The current-phase relation (CPR) for asymmetric 45 degree Josephson junctions
between two d-wave superconductors has been predicted to exhibit an anomalous
periodicity. We have used the single-junction interferometer to investigate the
CPR for this kind of junctions in YBCO thin films. Half-fluxon periodicity has
been experimentally found, providing a novel source of evidence for the d-wave
symmetry of the pairing state of the cuprates.Comment: 4 pages, 5 figure
Low-frequency measurement of the tunneling amplitude in a flux qubit
We have observed signatures of resonant tunneling in an Al three-junction
qubit, inductively coupled to a Nb LC tank circuit. The resonant properties of
the tank oscillator are sensitive to the effective susceptibility (or
inductance) of the qubit, which changes drastically as its flux states pass
through degeneracy. The tunneling amplitude is estimated from the data. We find
good agreement with the theoretical predictions in the regime of their
validity.Comment: REVTeX4, 3pp., 3 EPS figures. v2: new sample, textual clarifications.
v3: minor polishing; final, to appear in PRB Rapid
Multiphoton transitions in Josephson-junction qubits (Review Article)
Two basic physical models, a two-level system and a harmonic oscillator, are
realized on the mesoscopic scale as coupled qubit and resonator. The realistic
system includes moreover the electronics for controlling the distance between
the qubit energy levels and their populations and to read out the resonator's
state, as well as the unavoidable dissipative environment. Such rich system is
interesting both for the study of fundamental quantum phenomena on the
mesoscopic scale and as a promising system for future electronic devices. We
present recent results for the driven superconducting qubit-resonator system,
where the resonator can be realized as an LC circuit or a nanomechanical
resonator. Most of the results can be described by the semiclassical theory,
where a qubit is treated as a quantum two-level system coupled to the classical
driving field and the classical resonator. Application of this theory allows to
describe many phenomena for the single and two coupled superconducting qubits,
among which are the following: the equilibrium-state and weak-driving
spectroscopy, Sisyphus damping and amplification, Landau-Zener-St\"uckelberg
interferometry, the multiphoton transitions of both direct and ladder- type
character, and creation of the inverse population for lasing.Comment: 20 pages, 15 figure
Dressed-state amplification by a superconducting qubit
We demonstrate amplification of a microwave signal by a strongly driven
two-level system in a coplanar waveguide resonator. The effect known from
optics as dressed-state lasing is observed with a single quantum system formed
by a persistent current (flux) qubit. The transmission through the resonator is
enhanced when the Rabi frequency of the driven qubit is tuned into resonance
with one of the resonator modes. Amplification as well as linewidth narrowing
of a weak probe signal has been observed. The laser emission at the resonator's
fundamental mode has been studied by measuring the emission spectrum. We
analyzed our system and found an excellent agreement between the experimental
results and the theoretical predictions obtained in the dressed-state model.Comment: 5 pages, 4 figure
Induced paramagnetic states by localized -loops in grain boundaries
Recent experiments on high-temperature superconductors show paramagnetic
behavior localized at grain boundaries (GB). This paramagnetism can be
attributed to the presence unconventional d-wave induced -junctions. By
modeling the GB as an array of and conventional Josephson junction we
determine the conditions of the occurrence of the paramagnetic behavior.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Mobile kinks and half-integer zero-field-like steps in highly discrete alternating Josephson junction arrays
The dynamics of a one-dimensional, highly discrete, linear array of
alternating and Josephson junctions is studied numerically, under
constant bias current at zero magnetic field. The calculated current - voltage
characteristics exhibit half-integer and integer zero-field-like steps for even
and odd total number of junctions, respectively. Inspection of the
instantaneous phases reveals that, in the former case, single kink
excitations (discrete semi-fluxons) are supported, whose propagation in the
array gives rise to the step, while in the latter case, a pair of
kink -- antikink appears, whose propagation gives rise to the
step. When additional kinks are inserted in the array, they are
subjected to fractionalization, transforming themselves into two closely spaced
kinks. As they propagate in the array along with the single kink or
the kink - antikink pair, they give rise to higher half-integer or
integer zero-field-like steps, respectively.Comment: 7 pages, 8 figures, submitted to Supercond. Sci. Techno
Strong interconversion of non-polar phonons and Josephson plasma oscillations induced by equilibrium Josephson currents in high T_c superconductors
We analyze consequences of dynamical modulations of Josephson current by
non-polar lattice mode in the Josephson junction barrier. In the high
junctions, the effect of such modulations can be anomalously strong due to the
proximity of the insulating barrier to the superconducting state. Accordingly,
the interconversion of sound (as well as other non-polar phonons) and the
Josephson plasma oscillations mediated by stationary Josephson currents, which
may be present in the junction due to various reasons, becomes possible. We
suggest that this effect can be employed for imaging of the stationary
Josephson currents. Estimates of the effect are given.Comment: 11 RevTeX pages, no figure
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