76 research outputs found
Effect of Cherenkov radiation on the jitter of solitons in the driven underdamped Frenkel-Kontorova model
The effect of complex dynamics of solitons on the output noise of the system (thermal jitter) is studied in the frame of the driven underdamped Frenkel-Kontorova model. In contrast to the continuous case, we have observed a dramatic splash of the jitter. It is demonstrated that this jitter increase is related to the joining of an initial soliton with the one generated by large amplitude oscillations of the Cherenkov radiation tail, which results in the establishment of a unified soliton structure
Observation of the second harmonic in superconducting current-phase relation of Nb/Au/(001)YBa2Cu3Ox heterojunctions
The superconducting current-phase relation (CPR) of Nb/Au/(001)YBa2Cu3Ox
heterojunctions prepared on epitaxial c-axis oriented YBa2Cu3Ox thin films has
been measured in a single-junction interferometer. For the first time, the
second harmonic of the CPR of such junctions has been observed. The appearance
of the second harmonic and the relative sign of the first and second harmonics
of the CPR can be explained assuming, that the macroscopic pairing symmetry of
our YBa2Cu3Ox thin films is of the d+s typeComment: 11 pages, 4 figure
Distinguishing quantum from classical oscillations in a driven phase qubit
Rabi oscillations are coherent transitions in a quantum two-level system
under the influence of a resonant perturbation, with a much lower frequency
dependent on the perturbation amplitude. These serve as one of the signatures
of quantum coherent evolution in mesoscopic systems. It was shown recently [N.
Gronbech-Jensen and M. Cirillo, Phys. Rev. Lett. 95, 067001 (2005)] that in
phase qubits (current-biased Josephson junctions) this effect can be mimicked
by classical oscillations arising due to the anharmonicity of the effective
potential. Nevertheless, we find qualitative differences between the classical
and quantum effect. First, while the quantum Rabi oscillations can be produced
by the subharmonics of the resonant frequency (multiphoton processes), the
classical effect also exists when the system is excited at the overtones.
Second, the shape of the resonance is, in the classical case,
characteristically asymmetric; while quantum resonances are described by
symmetric Lorentzians. Third, the anharmonicity of the potential results in the
negative shift of the resonant frequency in the classical case, in contrast to
the positive Bloch-Siegert shift in the quantum case. We show that in the
relevant range of parameters these features allow to confidently distinguish
the bona fide Rabi oscillations from their classical Doppelganger.Comment: 8 pages, 4 figures; v2: minor corrections, Fig.1 added, introduction
expande
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
Degenerate ground state in a mesoscopic YBaCuO grain boundary Josephson junction
We have measured the current-phase relationship (CPR) of symmetric 45 degree
YBaCuO grain boundary Josephson junctions. Substantial deviations of the CPR
from conventional tunnel-junction behavior have been observed: (i) The critical
current exhibits, as a function of temperature T, a local minimum at a
temperature T*. (ii) At T approximately equal to T*, the first harmonic of the
CPR changes sign. (iii) For T<T*, the second harmonic of the CPR is comparable
to the first harmonic, and (iv) the ground state of the junction becomes
degenerate. The results are in good agreement with a microscopic model of
Josephson junctions between d-wave superconductors.Comment: 4 pages, 5 figures, a reference adde
Measurement of the ground-state flux diagram of three coupled qubits as a first step towards the demonstration of adiabatic quantum computation
The ground state susceptibility of a system consisting of three flux-qubits
was measured in the complete three dimensional flux space around the common
degeneracy point of the qubits. The system's Hamiltonian could be completely
reconstructed from measurements made far away from the common degeneracy point.
The subsequent measurements made around this point show complete agreement with
the theoretical predictions which follow from this Hamiltonian. The ground
state anti-crossings of the system could be read-out directly from these
measurements. This allows one to determine the ground-state flux diagram, which
provides the solution for the non-polynomial optimization problem MAXCUT
encoded in the Hamiltonian of the three-flux-qubit system. Our results show
that adiabatic quantum computation can be demonstrated with this system
provided that the energy gap and/or the speed of the read-out is increased.Comment: accepted for publication by Europhysics Letter
Stepping closer to pulsed single microwave photon detectors for axions search
Axions detection requires the ultimate sensitivity down to the single photon
limit. In the microwave region this corresponds to energies in the yJ range.
This extreme sensitivity has to be combined with an extremely low dark count
rate, since the probability of axions conversion into microwave photons is
supposed to be very low. To face this complicated task, we followed two
promising approaches that both rely on the use of superconducting devices based
on the Josephson effect. The first one is to use a single Josephson junction
(JJ) as a switching detector (i.e. exploiting the superconducting to normal
state transition in presence of microwave photons). We designed a device
composed of a coplanar waveguide terminated on a current biased Josephson
junction. We tested its efficiency to pulsed (pulse duration 10 ns) microwave
signals, since this configuration is closer to an actual axions search
experiment. We show how our device is able to reach detection capability of the
order of 10 photons with frequency 8 GHz. The second approach is based on an
intrinsically quantum device formed by two resonators coupled only via a
superconducting qubit network (SQN). This approach relies on quantum
nondemolition measurements of the resonator photons. We show that injecting RF
power into the resonator, the frequency position of the resonant drop in the
transmission coefficient (S21) can be modulated up to 4 MHz. We anticipate
that, once optimized, both the devices have the potential to reach single
photon sensitivity
Josephson effect in d-wave superconductor junctions in a lattice model
Josephson current between two d-wave superconductors is calculated by using a
lattice model. Here we consider two types of junctions, , the parallel
junction and the mirror-type junction. The maximum Josephson current
shows a wide variety of temperature () dependence depending on the
misorientation angles and the types of junctions. When the misorientation
angles are not zero, the Josephson current shows the low-temperature anomaly
because of a zero energy state (ZES) at the interfaces. In the case of
mirror-type junctions, has a non monotonic temperature dependence. These
results are consistent with the previous results based on the quasiclassical
theory. [Y. Tanaka and S. Kashiwaya: Phys. Rev. B \textbf{56} (1997) 892.] On
the other hand, we find that the ZES disappears in several junctions because of
the Freidel oscillations of the wave function, which is peculiar to the lattice
model. In such junctions, the temperature dependence of is close to the
Ambegaokar-Baratoff relation.Comment: 17 pages, 10 figures, using jpsj2.cls and oversite.st
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