365 research outputs found
Quantum theory of the low-frequency linear susceptibility of interferometer-type superconducting qubits
We use the density matrix formalism to analyze the interaction of
interferometer-type superconducting qubits with a high quality tank circuit,
which frequency is well below the gap frequency of a qubit. We start with the
ground state characterization of the superconducting flux and charge qubits.
Then, by making use of a dressed state approach we describe the qubits'
spectroscopy when the qubit is irradiated by a microwave field which is tuned
to the gap frequency. The last section of the paper is devoted to continuous
monitoring of qubit states by using a DC SQUID in the inductive mode.Comment: 11 pages, 5 figures; the title and abstract are slightly changed;
several typos are corrected; in order to make our argumentation more clear we
added some comments in the introduction and other section
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
Radio-Frequency Method for Investigation of Quantum Properties of Superconducting Structures
We implement the impedance measurement technique (IMT) for characterization
of interferometer-type superconducting qubits. In the framework of this method,
the interferometer loop is inductively coupled to a high-quality tank circuit.
We show that the IMT is a powerful tool to study a response of externally
controlled two-level system to different types of excitations. Conclusive
information about qubits is obtained from the read-out of the tank properties.Comment: 10 pages, 10 figures;to be published in Fizika Nizkikh Temperatur
(Low Temperature Physics); v3: minor polishing; fina
Paramagnetic effect in YBaCuO grain boundary junctions
A detailed investigation of the magnetic response of YBaCuO grain boundary
Josephson junctions has been carried out using both radio-frequency
measurements and Scanning SQUID Microscopy. In a nominally zero-field-cooled
regime we observed a paramagnetic response at low external fields for 45 degree
asymmetric grain boundaries. We argue that the observed phenomenology results
from the d-wave order parameter symmetry and depends on Andreev bound states.Comment: To be published in Phys. Rev.
High-- RF SQUIDs with Large Inductance of Quantization Loop
Experimental data on signal and noise characteristics of high-- RF
SQUIDs with large inductance of quantization loop are presented. The SQUIDs
were produced by a thick--film HTS--technique of painting on the YBaCuO
substrate. For the first time, a steady quantum interference was observed in RF
SQUID with the inductance as large as nH, that is, 60 times higher
than the fluctuation inductance at the liquid nitrogen
temperature T=77 K. A new method is offered to evaluate the sensitivity of RF
SQUID with optimum inductance of the quantization loop.Comment: 12 pages, 4 figures, 1 tabl
Continuous Monitoring of Rabi Oscillations in a Josephson Flux Qubit
Under resonant irradiation, a quantum system can undergo coherent (Rabi)
oscillations in time. We report evidence for such oscillations in a
_continuously_ observed three-Josephson-junction flux qubit, coupled to a
high-quality tank circuit tuned to the Rabi frequency. In addition to
simplicity, this method of_Rabi spectroscopy_ enabled a long coherence time of
about 2.5 microseconds, corresponding to an effective qubit quality factor
\~7000.Comment: REVTeX4, 4pp., 4 EPS figure files. v3: changed title, fixed typos;
final, to appear in PR
Low-frequency characterization of quantum tunneling in flux qubits
We propose to investigate flux qubits by the impedance measurement technique
(IMT), currently used to determine the current--phase relation in Josephson
junctions. We analyze in detail the case of a high-quality tank circuit coupled
to a persistent-current qubit, to which IMT was successfully applied in the
classical regime. It is shown that low-frequency IMT can give considerable
information about the level anticrossing, in particular the value of the
tunneling amplitude. An interesting difference exists between applying the ac
bias directly to the tank and indirectly via the qubit. In the latter case, a
convenient way to find the degeneracy point in situ is described. Our design
only involves existing technology, and its noise tolerance is quantitatively
estimated to be realistic.Comment: 6 pages, 11 figures, to appear in Phys.Rev.
Quantum behaviour of a flux qubit coupled to a resonator
We present a detailed theoretical analysis for a system of a superconducting
flux qubit coupled to a transmission line resonator. The master equation,
accounting incoherent processes for a weakly populated resonator, is
analytically solved. An electromagnetic wave transmission coefficient through
the system, which provides a tool for probing dressed states of the qubit, is
derived. We also consider a general case for the resonator with more than one
photon population and compare the results with an experiment on the
qubit-resonator system in the intermediate coupling regime, when the coupling
energy is comparable with the qubit relaxation rate.Comment: 16 pages, 6 figure
Boundary Condition Effect On Photoacoustic Response Of Absorbing Liquid To Modulation Of Laser Intensity
The photoacoustic (PA) pressure response of the absorbing liquid (water, ethanol) with free and confined surface to laser pulse with modulated intensity is studied. Modulation amplitude of the PA signal demonstrates non-monotonous behavior during laser pulse action which can be explained as interference between thermo-acoustic and vaporization mechanisms of pressure generation. The phase behavior of modulated part of PA signal also gives important information on processes in the irradiated matter. Doppler photoacoustic monitoring (DPM) is used to determine surface movement during laser irradiation (wavelength – 2.94 um, pulse duration ~ 200-300 ns, fluence ~ 0.3-1.6 J/cm2). In the case of free surface this movement is possible into the upward and downward directions while for the constrained irradiated surface only one movement is supposed. This supposition is in agreement with the experiment. However the total behavior of PA response is more complicated due to probably vapor cavity formation between liquid and solid surfaces. In particular, despite the imposed constraint PA response behavior resembles the free surface case
Decoherence of Phase Qubit using High-Tc Superconductor
We discuss how to make use of high-Tc d-wave Josephson junctions in the
construction of a phase qubit. We especially focus on the effect of the
quasiparticle dissipation and the zero energy bound state on the macroscopic
quantum tunneling which corresponds to the final measurement process of the
d-wave phase qubit.Comment: 4 pages, 2 figures, to appear in Physica
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