148 research outputs found
Probing High Frequency Noise with Macroscopic Resonant Tunneling
We have developed a method for extracting the high-frequency noise spectral
density of an rf-SQUID flux qubit from macroscopic resonant tunneling (MRT)
rate measurements. The extracted noise spectral density is consistent with that
of an ohmic environment up to frequencies ~ 4 GHz. We have also derived an
expression for the MRT lineshape expected for a noise spectral density
consisting of such a broadband ohmic component and an additional strongly
peaked low-frequency component. This hybrid model provides an excellent fit to
experimental data across a range of tunneling amplitudes and temperatures
Geometrical dependence of low frequency noise in superconducting flux qubits
A general method for directly measuring the low-frequency flux noise (below
10 Hz) in compound Josephson junction superconducting flux qubits has been used
to study a series of 85 devices of varying design. The variation in flux noise
across sets of qubits with identical designs was observed to be small. However,
the levels of flux noise systematically varied between qubit designs with
strong dependence upon qubit wiring length and wiring width. Furthermore,
qubits fabricated above a superconducting ground plane yielded lower noise than
qubits without such a layer. These results support the hypothesis that
localized magnetic impurities in the vicinity of the qubit wiring are a key
source of low frequency flux noise in superconducting devices.Comment: 5 pages, 5 figure
A scalable readout system for a superconducting adiabatic quantum optimization system
We have designed, fabricated and tested an XY-addressable readout system that
is specifically tailored for the reading of superconducting flux qubits in an
integrated circuit that could enable adiabatic quantum optimization. In such a
system, the flux qubits only need to be read at the end of an adiabatic
evolution when quantum mechanical tunneling has been suppressed, thus
simplifying many aspects of the readout process. The readout architecture for
an -qubit adiabatic quantum optimization system comprises hysteretic dc
SQUIDs and rf SQUID latches controlled by bias lines. The
latching elements are coupled to the qubits and the dc SQUIDs are then coupled
to the latching elements. This readout scheme provides two key advantages:
First, the latching elements provide exceptional flux sensitivity that
significantly exceeds what may be achieved by directly coupling the flux qubits
to the dc SQUIDs using a practical mutual inductance. Second, the states of the
latching elements are robust against the influence of ac currents generated by
the switching of the hysteretic dc SQUIDs, thus allowing one to interrogate the
latching elements repeatedly so as to mitigate the effects of stochastic
switching of the dc SQUIDs. We demonstrate that it is possible to achieve
single qubit read error rates of with this readout scheme. We have
characterized the system-level performance of a 128-qubit readout system and
have measured a readout error probability of in the presence
of optimal latching element bias conditions.Comment: Updated for clarity, final versio
Deep-well ultrafast manipulation of a SQUID flux qubit
Superconducting devices based on the Josephson effect are effectively used
for the implementation of qubits and quantum gates. The manipulation of
superconducting qubits is generally performed by using microwave pulses with
frequencies from 5 to 15 GHz, obtaining a typical operating clock from 100MHz
to 1GHz. A manipulation based on simple pulses in the absence of microwaves is
also possible. In our system a magnetic flux pulse modifies the potential of a
double SQUID qubit from a symmetric double well to a single deep well
condition. By using this scheme with a Nb/AlOx/Nb system we obtained coherent
oscillations with sub-nanosecond period (tunable from 50ps to 200ps), very fast
with respect to other manipulating procedures, and with a coherence time up to
10ns, of the order of what obtained with similar devices and technologies but
using microwave manipulation. We introduce the ultrafast manipulation
presenting experimental results, new issues related to this approach (such as
the use of a feedback procedure for cancelling the effect of "slow"
fluctuations), and open perspectives, such as the possible use of RSFQ logic
for the qubit control.Comment: 9 pages, 7 figure
Dynamics of Josephson junctions and single-flux-quantum networks with superconductor-insulator-normal metal junction shunts
Within the framework of the microscopic model of tunneling, we modelled the
behavior of the Josephson junction shunted by the
Superconductor-Insulator-Normal metal (SIN) tunnel junction. We found that the
electromagnetic impedance of the SIN junction yields both the
frequency-dependent damping and dynamic reactance which leads to an increase in
the effective capacitance of the circuit. We calculated the dc I-V curves and
transient characteristics of these circuits and explained their quantitative
differences to the curves obtained within the resistively shunted junction
model. The correct operation of the basic single-flux-quanta circuits with such
SIN-shunted junctions, i.e. the Josephson transmission line and the toggle
flip-flop, have also been modelled.Comment: 8 pages incl. 7 figure
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