8 research outputs found
The Voltage-Current Characteristic of high T_c DC SQUID: theory, simulation, experiment
The analytical theory for the voltage-current characteristics of the large
inductance (L>100 pH) high-T_c DC SQUIDs that has been developed previously is
consistently compared with the computer simulations and the experiment. The
theoretical voltage modulation for symmetric junctions is shown to be in a good
agreement with the results of known computer simulations. It is shown that the
asymmetry of the junctions results in the increase of the voltage modulation if
the critical current is in excess of some threshold value (about 8 microAmps).
Below this value the asymmetry leads to the reduction of the voltage modulation
as compared to the symmetric case. The comparison with the experiment shows
that the asymmetry can explain a large portion of experimental values of the
voltage modulation which lie above the theoretical curve for symmetric DC
SQUID. It also explains experimental points which lie below the curve at small
critical currents. However, a significant portion of these values which lie
below the curve cannot be explained by the junction asymmetry.Comment: 22 pages, 14 figure
Signal amplification in a qubit-resonator system
We study the dynamics of a qubit-resonator system, when the resonator is driven by two signals. The interaction
of the qubit with the high-amplitude driving we consider in terms of the qubit dressed states. Interaction of
the dressed qubit with the second probing signal can essentially change the amplitude of this signal. We calculate
the transmission amplitude of the probe signal through the resonator as a function of the qubit’s energy and the
driving frequency detuning. The regions of increase and attenuation of the transmitted signal are calculated and
demonstrated graphically. We present the influence of the signal parameters on the value of the amplification,
and discuss the values of the qubit-resonator system parameters for an optimal amplification and attenuation of
the weak probe signal
Quantum behavior of a flux qubit coupled to a resonator
The detailed theory for the system of a superconducting qubit coupled to the transmission line resonator is presented. We describe the system by solving analytically and numerically the master equation for the density matrix, which includes dissipative Lindblad term. We calculate the transmission coefficient, which provides the way to probe the dressed states of the qubit. The theoretical results are related to the experiment with the intermediate coupling between the qubit and the resonator, when the coupling energy is of the same order as the qubit relaxation rate
Voltage-current and voltage-flux characteristics of asymmetric high TC DC SQUIDs
We report measurements of transfer functions and flux shifts of 20 on-chip
high T DC SQUIDs half of which were made purposely geometrically
asymmetric. All of these SQUIDs were fabricated using standard high T thin
film technology and they were single layer ones, having 140 nm thickness of
YBaCuO film deposited by laser ablation onto MgO bicrystal
substrates with 24 misorientation angle. For every SQUID the parameters of
its intrinsic asymmetry, i. e., the density of critical current and resistivity
of every junction, were measured directly and independently. We showed that the
main reason for the on-chip spreading of SQUIDs' voltage-current and
voltage-flux characteristics was the intrinsic asymmetry. We found that for
SQUIDs with a relative large inductance ( pH) both the voltage
modulation and the transfer function were not very sensitive to the junctions
asymmetry, whereas SQUIDs with smaller inductance ( pH) were
more sensitive. The results obtained in the paper are important for the
implementation in the sensitive instruments based on high T SQUID arrays
and gratings.Comment: 11 pages, 4 tables, 17 figures This version is substantially
modified. The Introduction and Section 2 are completely rewritten, while
experimental part is mainly the same as in previous versio
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
Decoherence and Relaxation of a Quantum Bit in the Presence of Rabi Oscillations
Dissipative dynamics of a quantum bit driven by a strong resonant field and
interacting with a heat bath is investigated. We derive generalized Bloch
equations and find modifications of the qubit's damping rates caused by Rabi
oscillations. Nonequilibrium decoherence of a phase qubit inductively coupled
to a LC-circuit is considered as an illustration of the general results. It is
argued that recent experimental results give a clear evidence of effective
suppression of decoherence in a strongly driven flux qubit.Comment: 14 pages; misprints correcte