351 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
Microwave-induced thermal escape in Josephson junctions
We investigate, by experiments and numerical simulations, thermal activation
processes of Josephson tunnel junctions in the presence of microwave radiation.
When the applied signal resonates with the Josephson plasma frequency
oscillations, the switching current may become multi-valued in a temperature
range far exceeding the classical to quantum crossover temperature. Plots of
the switching currents traced as a function of the applied signal frequency
show very good agreement with the functional forms expected from Josephson
plasma frequency dependencies on the bias current. Throughout, numerical
simulations of the corresponding thermally driven classical Josephson junction
model show very good agreement with the experimental data.Comment: 10 pages and 4 figure
Superconducting tunable flux qubit with direct readout scheme
We describe a simple and efficient scheme for the readout of a tunable flux
qubit, and present preliminary experimental tests for the preparation,
manipulation and final readout of the qubit state, performed in incoherent
regime at liquid Helium temperature. The tunable flux qubit is realized by a
double SQUID with an extra Josephson junction inserted in the large
superconducting loop, and the readout is performed by applying a current ramp
to the junction and recording the value for which there is a voltage response,
depending on the qubit state. This preliminary work indicates the feasibility
and efficiency of the scheme.Comment: 10 pages, 5 figure
Study of the Fabrication Process for a Dual Mass Tuning Fork Gyro
AbstractThe fabrication process of a dual mass tuning for gyroscope presents many different challenges: the aspect ratio of the sidewalls, the Aspect Ratio Dependent Etch (ARDE) which causes different gaps to be etched in different etching time [1], the stiction during the release of the free structures, the notching effect that occurs with a dielectric etch stop layer [2], the thermal contact during the etch process. In this paper are presented different processes and studies of the etching characteristics in order to avoid or minimize these problems
Return current in hysteretic Josephson junctions: Experimental distribution in the thermal activation regime
We present an experimental study on the retrapping process of a hysteretic, high-quality Josephson junction; namely, we have measured the distribution of the values at which the junction switches back from the voltage state to the zero-voltage state, as a function of the applied magnetic field. While the opposite process (escape from the zero-voltage state) has been extensively studied in the past, both from the theoretical and the experimental point of view, little is found in the literature on the retrapping process. In terms of the tilted washboard potential, the process corresponds to the retrapping from the running state to a locked state in a potential well. The interest of the measurements is in the fact that the value of the return current can be directly related to the dissipation in the junction. While the deterministic behavior, experimentally measured through the I–V curve, appears to be in agreement with the theoretical predictions, even in minor details, the statistical behavior is strongly different from what is expected. The disagreement is found even in zero-applied magnetic field and it cannot be attributed to external noise in the system. From the experimental statistical properties, we find values for the effective dissipation much lower than those obtained from the deterministic curves, a result which could be of interest in experiments on the observation of macroscopic quantum phenomena
The effective dissipation in Nb/AlOx/Nb Josephson tunnel junctions by return current measurements
Measurements of temperature dependence of the return current in high quality Nb/AlOx/Nb Josephson junctions are presented. From the experimental data, we obtain the effective resistance, i.e., the effective dissipation, for the retrapping process, according to the generalized resistively shunted junction model proposed by Chen, Fisher, and Leggett. We present a careful analysis, based on a comparison between the measured temperature dependencies of both the return and the quasiparticle tunneling current. We find that the junction subgap conductance, which includes the quasiparticle and the quasiparticle-pair interference terms, is responsible for the return process. The measurements have been performed on various samples, in a wide range of critical current densities from 50 to 2250 A/cm2, covering different damping regimes and spanning over the high and low temperature limits. Junctions with low critical current density show ideal dissipation which makes the return current scale with temperature according to the BCS exponential behavior without flattening out effects. This result may be relevant for the possible use of Nb/AlOx/Nb junctions in macroscopic quantum coherence experiments, which strongly require a very low dissipation
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
Frequency-modulated combs via on-chip field enhancement
Frequency-modulated (FM) combs feature flat intensity spectra with a linear
frequency chirp, useful for metrology and sensing applications. Generating FM
combs in semiconductor lasers generally requires a fast saturable gain, usually
limited by the intrinsic gain medium properties. Here, we show how a spatial
modulation of the laser gain medium can enhance the gain saturation dynamics
and nonlinearities to generate self-starting FM combs. We demonstrate this with
tapered planarized THz quantum cascade lasers (QCLs). While simple ridge THz
QCLs typically generate combs which are a mixture of amplitude and frequency
modulation, the on-chip field enhancement resulting from extreme spatial
confinement leads to an ultrafast saturable gain regime, generating a pure FM
comb with a flatter intensity spectrum, a clear linear frequency chirp and very
intense beatnotes up to -30 dBm. The observed linear frequency chirp is
reproduced using a spatially inhomogeneous mean-field theory model which
confirms the crucial role of field enhancement. In addition, the modified
spatial temperature distribution within the waveguide results in an improved
hightemperature comb operation, up to a heat sink temperature of 115 K, with
comb bandwidths of 600 GHz at 90 K. The spatial inhomogeneity also leads to
dynamic switching between various harmonic states in the same device.Comment: 9 pages, 6 figure
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