141 research outputs found
Decoherence processes in a current biased dc SQUID
A current bias dc SQUID behaves as an anharmonic quantum oscillator
controlled by a bias current and an applied magnetic flux. We consider here its
two level limit consisting of the two lower energy states | 0 \right> and |
1 \right>. We have measured energy relaxation times and microwave absorption
for different bias currents and fluxes in the low microwave power limit.
Decoherence times are extracted. The low frequency flux and current noise have
been measured independently by analyzing the probability of current switching
from the superconducting to the finite voltage state, as a function of applied
flux. The high frequency part of the current noise is derived from the
electromagnetic environment of the circuit. The decoherence of this quantum
circuit can be fully accounted by these current and flux noise sources.Comment: 4 pages, 4 figure
Coherent oscillations in a superconducting multi-level quantum system
We have observed coherent time evolution of states in a multi-level quantum
system, formed by a current-biased dc SQUID. The manipulation of the quantum
states is achieved by resonant microwave pulses of flux. The number of quantum
states participating in the coherent oscillations increases with increasing
microwave power. Quantum measurement is performed by a nanosecond flux pulse
which projects the final state onto one of two different voltage states of the
dc SQUID, which can be read out
Nanosecond quantum state detection in a current biased dc SQUID
This article presents our procedure to measure the quantum state of a dc
SQUID within a few nanoseconds, using an adiabatic dc flux pulse. Detection of
the ground state is governed by standard macroscopic quantum theory (MQT), with
a small correction due to residual noise in the bias current. In the two level
limit, where the SQUID constitutes a phase qubit, an observed contrast of 0.54
indicates a significant loss in contrast compared to the MQT prediction. It is
attributed to spurious depolarization (loss of excited state occupancy) during
the leading edge of the adiabatic flux measurement pulse. We give a simple
phenomenological relaxation model which is able to predict the observed
contrast of multilevel Rabi oscillations for various microwave amplitudes.Comment: 10 pages, 8 figure
Observation of transition from escape dynamics to underdamped phase diffusion in a Josephson junction
We have investigated the dynamics of underdamped Josephson junctions. In
addition to the usual crossover between macroscopic quantum tunnelling and
thermally activated (TA) behaviour we observe in our samples with relatively
small Josephson coupling E_J, for the first time, the transition from TA
behaviour to underdamped phase diffusion. Above the crossover temperature the
threshold for switching into the finite voltage state becomes extremely sharp.
We propose a (T,E_J) phase-diagram with various regimes and show that for a
proper description of it dissipation and level quantization in a metastable
well are crucial.Comment: 4 pages, 3 figure
Measuring the distribution of current fluctuations through a Josephson junction with very short current pulses
We propose to probe the distribution of current fluctuations by means of the
escape probability histogram of a Josephson junction (JJ), obtained using very
short bias current pulses in the adiabatic regime, where the low-frequency
component of the current fluctuations plays a crucial role. We analyze the
effect of the third cumulant on the histogram in the small skewness limit, and
address two concrete examples assuming realistic parameters for the JJ. In the
first one we study the effects due to fluctuations produced by a tunnel
junction, finding that the signature of higher cumulants can be detected by
taking the derivative of the escape probability with respect to current. In
such a realistic situation, though, the determination of the whole distribution
of current fluctuations requires an amplification of the cumulants. As a second
example we consider magnetic flux fluctuations acting on a SQUID produced by a
random telegraph source of noise.Comment: 6 pages, 6 figures; final versio
Evidence of two-dimensional macroscopic quantum tunneling of a current-biased DC-SQUID
The escape probability out of the superconducting state of a hysteretic
DC-SQUID has been measured at different values of the applied magnetic flux. At
low temperature, the escape current and the width of the probability
distribution are temperature independent but they depend on flux. Experimental
results do not fit the usual one-dimensional (1D) Macroscopic Quantum Tunneling
(MQT) law but are perfectly accounted for by the two-dimensional (2D) MQT
behaviour as we propose here. Near zero flux, our data confirms the recent MQT
observation in a DC-SQUID \cite{Li02}.Comment: 4 pages, 4 figures Accepted to PR
Weak coupling Josephson junction as a current probe: effect of dissipation on escape dynamics
We have studied the temperature dependence of escape phenomena in various underdamped Josephson junctions (JJs). The junctions had different Josephson coupling energies EJ which were relatively small, but larger than the charging energy EC. Upon increasing the temperature T, we first observe the usual cross-over between macroscopic quantum tunnelling and thermally activated (TA) behaviour at temperatures kBT ~ planckÏp, where Ïp is the plasma frequency of the junction. Increasing T further, the width of the switching current distribution has, counterintuitively, a non-monotonic temperature dependence. This can be explained by the novel cross-over from TA behaviour to underdamped phase diffusion. We show that this cross-over is expected to occur at temperatures such that kBT ~ EJ(1 â 4/ÏQ)3/2, where Q is the quality factor of the junction at the plasma frequency, in agreement with experiment. Our findings can be compared with detailed model calculations which take into account dissipation and level quantization in a metastable well.
Particular attention is paid to the sample with the smallest EJ, which shows extensive phase diffusion even at the lowest temperatures. This sample consists of a dc-SQUID and a single JJ close to each other, such that the SQUID acts as a tunable inductive protection for the single junction from fluctuations of a dissipative environment. By varying the flux through the dc-SQUID, we present, for the first time, experimental evidence of the escape of a JJ from the phase diffusion regime to the free running state in a tunable environment. We also show that in the zero voltage state the losses mainly occur at frequencies near the plasmaPeer reviewe
Integrin Activation Promotes Axon Growth on Inhibitory Chondroitin Sulfate Proteoglycans by Enhancing Integrin Signaling
We report on the enhancement of the spontaneous emission in the visible red
spectral range from site-controlled InP/GaInP quantum dots by resonant coupling
to Tammplasmon modes confined beneath gold disks in a hybrid
metal/semiconductor structure. The enhancement of the emission intensity is
confirmed by spatially resolved microphotoluminescence area scans and
temperature dependent measurements. Single photon emission from our coupled
system is verified via second order autocorrelation measurements. We observe
bright single quantum dot emission of up to ~173000 detected photons per second
at a repetition rate of the excitation source of 82 MHz, and calculate an
extraction efficiency of our device as high as 7%
Strain-gradient position mapping of semiconductor quantum dots
COORDENAĂĂO DE APERFEIĂOAMENTO DE PESSOAL DE NĂVEL SUPERIOR - CAPESWe introduce a nondestructive method to determine the position of randomly distributed semiconductor quantum dots (QDs) integrated in a solid photonic structure. By setting the structure in an oscillating motion, we generate a large stress gradient across the QDs plane. We then exploit the fact that the QDs emission frequency is highly sensitive to the local material stress to map the position of QDs deeply embedded in a photonic wire antenna with an accuracy ranging from +/- 35 nm down to +/- 1 nm. In the context of fast developing quantum technologies, this technique can be generalized to different photonic nanostructures embedding any stress-sensitive quantum emitters.1181116COORDENAĂĂO DE APERFEIĂOAMENTO DE PESSOAL DE NĂVEL SUPERIOR - CAPESCOORDENAĂĂO DE APERFEIĂOAMENTO DE PESSOAL DE NĂVEL SUPERIOR - CAPES88887.059630/2014-00The authors wish to thank E. Gautier for the FIB cut and images. Sample fabrication was carried out in the Upstream Nanofabrication Facility (PTA) and CEA LETI MINATEC/DOPT clean rooms. P.-L. de Assis was financially supported by Agence Nationale de la Recherche (Project No. ANR-11-BS10-011) and CAPES Young Talents Fellowship Grant No. 88887.059630/2014-00, and D. Tumanov by a doctoral scholarship from the RhĂŽne-Alpes Region
The nowcasting SAF products and services: recent improvements in the new SW packages PPS v2018 and GEO v2018 and future plans
PresentaciĂłn realizada en la 3rd European Nowcasting Conference, celebrada en la sede central de AEMET en Madrid del 24 al 26 de abril de 2019
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