150 research outputs found
Resonant effects in a SQUID qubit subjected to non adiabatic changes
By quickly modifying the shape of the effective potential of a double SQUID
flux qubit from a single-well to a double-well condition, we experimentally
observe an anomalous behavior, namely an alternance of resonance peaks, in the
probability to find the qubit in a given flux state. The occurrence of
Landau-Zener transitions as well as resonant tunneling between degenerate
levels in the two wells may be invoked to partially justify the experimental
results. A quantum simulation of the time evolution of the system indeed
suggests that the observed anomalous behavior can be imputable to quantum
coherence effects. The interplay among all these mechanisms has a practical
implication for quantum computing purposes, giving a direct measurement of the
limits on the sweeping rates possible for a correct manipulation of the qubit
state by means of fast flux pulses, avoiding transitions to non-computational
states.Comment: 6 pages and 6 figures. The paper, as it is, has been accepted for
publication on PRB on March 201
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
Artificial Neural Network based on SQUIDs: demonstration of network training and operation
We propose a scheme for the realization of artificial neural networks based
on Superconducting Quantum Interference Devices (SQUIDs). In order to
demonstrate the operation of this scheme we designed and successfully tested a
small network that implements a XOR gate and is trained by means of examples.
The proposed scheme can be particularly convenient as support for
superconducting applications such as detectors for astrophysics, high energy
experiments, medicine imaging and so on.Comment: 10 pages, 6 figure
Static flux bias of a flux qubit using persistent current trapping
Qubits based on the magnetic flux degree of freedom require a flux bias,
whose stability and precision strongly affect the qubit performance, up to a
point of forbidding the qubit operation. Moreover, in the perspective of
multiqubit systems, it must be possible to flux-bias each qubit independently,
hence avoiding the traditional use of externally generated magnetic fields in
favour of on-chip techniques that minimize cross-couplings. The solution
discussed in this paper exploits a persistent current, trapped in a
superconducting circuit integrated on chip that can be inductively coupled with
an individual qubit. The circuit does not make use of resistive elements that
can be detrimental for the qubit coherence. The trapping procedure allows to
control and change stepwise the amount of stored current; after that, the
circuit can be completely disconnected from the external sources. We show in a
practical case how this works and how to drive the bias circuit at the required
value.Comment: 5 figures submitted to Superconductor Science and Technolog
Coherent oscillations in a superconducting tunable flux qubit manipulated without microwaves
We experimentally demonstrate the coherent oscillations of a tunable
superconducting flux qubit by manipulating its energy potential with a
nanosecond-long pulse of magnetic flux. The occupation probabilities of two
persistent current states oscillate at a frequency ranging from 6 GHz to 21
GHz, tunable via the amplitude of the flux pulse. The demonstrated operation
mode allows to realize quantum gates which take less than 100 ps time and are
thus much faster compared to other superconducting qubits. An other advantage
of this type of qubit is its insensitivity to both thermal and magnetic field
fluctuations.Comment: 5 pages, 5 figure
An Optimal Tunable Josephson Element for Quantum Computing
We introduce a three-junction SQUID that can be effectively used as an
optimal tunable element in Josephson quantum computing applications. This
device can replace the simple dc SQUID generally used as tunable element in
this kind of applications, with a series of advantages for the coherence time
and for the tolerance to small errors. We study the device both theoretically
and experimentally at 4.2 K, obtaining a good agreement between the results.Comment: 3 pages, 4 figure
Increasing the bandwidth of resonant gravitational antennas: The case of Explorer
Resonant gravitational wave detectors with an observation bandwidth of tens
of hertz are a reality: the antenna Explorer, operated at CERN by the ROG
collaboration, has been upgraded with a new read-out. In this new
configuration, it exhibits an unprecedented useful bandwidth: in over 55 Hz
about its frequency of operation of 919 Hz the spectral sensitivity is better
than 10^{-20} /sqrt(Hz) . We describe the detector and its sensitivity and
discuss the foreseable upgrades to even larger bandwidths.Comment: 4 pages- 4 figures Acceted for publication on Physical Review Letter
Study of the coincidences between the gravitational wave detectors EXPLORER and NAUTILUS in 2001
We report the result from a search for bursts of gravitational waves using
data collected by the cryogenic resonant detectors EXPLORER and NAUTILUS during
the year 2001, for a total measuring time of 90 days. With these data we
repeated the coincidence search performed on the 1998 data (which showed a
small coincidence excess) applying data analysis algorithms based on known
physical characteristics of the detectors. With the 2001 data a new interesting
coincidence excess is found when the detectors are favorably oriented with
respect to the Galactic Disk
Search for correlation between GRB's detected by BeppoSAX and gravitational wave detectors EXPLORER and NAUTILUS
Data obtained during five months of 2001 with the gravitational wave (GW)
detectors EXPLORER and NAUTILUS were studied in correlation with the gamma ray
burst data (GRB) obtained with the BeppoSAX satellite. During this period
BeppoSAX was the only GRB satellite in operation, while EXPLORER and NAUTILUS
were the only GW detectors in operation.
No correlation between the GW data and the GRB bursts was found. The
analysis, performed over 47 GRB's, excludes the presence of signals of
amplitude h >=1.2 * 10^{-18}, with 95 % probability, if we allow a time delay
between GW bursts and GRB within +-400 s, and h >= 6.5 * 10^{-19}, if the time
delay is within +- 5 s. The result is also provided in form of scaled
likelihood for unbiased interpretation and easier use for further analysis.Comment: 14 pages, 7 figures. Latex file, compiled with cernik.cls (provided
in the package
The TES-based Cryogenic AntiCoincidence Detector (CryoAC) of ATHENA X-IFU: a large area silicon microcalorimeter for background particles detection
We are developing the Cryogenic AntiCoincidence detector (CryoAC) of the
ATHENA X-IFU spectrometer. It is a TES-based particle detector aimed to reduce
the background of the instrument. Here, we present the result obtained with the
last CryoAC single-pixel prototype. It is based on a 1 cm2 silicon absorber
sensed by a single 2mm x 1mm Ir/Au TES, featuring an on-chip heater for
calibration and diagnostic purposes. We have illuminated the sample with 55Fe
(6 keV line) and 241Am (60 keV line) radioactive sources, thus studying the
detector response and the heater calibration accuracy at low energy.
Furthermore, we have operated the sample in combination with a past-generation
CryoAC prototype. Here, by analyzing the coincident detections between the two
detectors, we have been able to characterize the background spectrum of the
laboratory environment and disentangle the primary (i.e. cosmic muons) and
secondaries (mostly secondary photons and electrons) signatures in the spectral
shape.Comment: Accepted for publication in the Journal of Low Temperature Physics
for LTD-20 special issu
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