683 research outputs found
Photon emission statistics of a driven microwave cavity
Recent experimental advances have made it possible to detect individual
quantum jumps in open quantum systems, such as the tunneling of single
electrons in nanoscale conductors or the emission of photons from non-classical
light sources. Here, we investigate theoretically the statistics of photons
emitted from a microwave cavity that is driven resonantly by an external field.
We focus on the differences between a parametric and a coherent drive, which
either squeezes or displaces the cavity field. We employ a Lindblad master
equation dressed with counting fields to obtain the generating function of the
photon emission statistics using a theoretical framework based on Gaussian
states. We then compare the distribution of photon waiting times for the two
drives as well as the -functions of the outgoing light, and we
identify important differences between these observables. In the long-time
limit, we analyze the factorial cumulants of the photon emission statistics and
the large-deviation statistics of the emission currents, which are markedly
different for the two drives. Our theoretical framework can readily be extended
to more complicated systems, for instance, with several coupled microwave
cavities, and our predictions may be tested in future experiments.Comment: 12 pages, 6 figure
Microwave quantum optics and electron transport through a metallic dot strongly coupled to a transmission line cavity
We investigate theoretically the properties of the photon state and the
electronic transport in a system consisting of a metallic quantum dot strongly
coupled to a superconducting microwave transmission line cavity. Within the
framework of circuit quantum electrodynamics we derive a Hamiltonian for
arbitrary strong capacitive coupling between the dot and the cavity. The
dynamics of the system is described by a quantum master equation, accounting
for the electronic transport as well as the coherent, non-equilibrium
properties of the photon state. The photon state is investigated, focusing on,
for a single active mode, signatures of microwave polaron formation and the
effects of a non-equilibrium photon distribution. For two active photon modes,
intra mode conversion and polaron coherences are investigated. For the
electronic transport, electrical current and noise through the dot and the
influence of the photon state on the transport properties are at the focus. We
identify clear transport signatures due to the non-equilibrium photon
population, in particular the emergence of superpoissonian shot-noise at
ultrastrong dot-cavity couplings.Comment: 19 pages, 10 figure
Coherent current transport in wide ballistic Josephson junctions
We present an experimental and theoretical investigation of coherent current
transport in wide ballistic superconductor-two dimensional electron
gas-superconductor junctions. It is found experimentally that upon increasing
the junction length, the subharmonic gap structure in the current-voltage
characteristics is shifted to lower voltages, and the excess current at
voltages much larger than the superconducting gap decreases. Applying a theory
of coherent multiple Andreev reflection, we show that these observations can be
explained in terms of transport through Andreev resonances.Comment: 4 pages, 4 figure
Cardiac troponin T is elevated and increases longitudinally in ALS patients.
Objective: To test whether high-sensitivity cardiac troponin T (hs-cTnT) could act as a diagnostic or prognostic biomarker in ALS, comparing hs-cTnT to neurofilament light (NfL). Methods: We performed a case-control study, including 150 ALS patients, 28 ALS mimics, and 108 healthy controls, and a follow-up study of the ALS patients, during 2014-2020 in Stockholm, Sweden. We compared concentrations of hs-cTnT in plasma and NfL in the cerebrospinal fluid between cases and controls. To evaluate the diagnostic performance, we calculated the area under the curve (AUC). Hazard ratios (HRs) were estimated from Cox models to assess associations between hs-cTnT and NfL at ALS diagnosis and risk of death. The longitudinal analysis measured changes of hs-cTnT and NfL since ALS diagnosis. Results: We noted higher levels of hs-cTnT in ALS patients (median: 16.5 ng/L) than in ALS mimics (11 ng/L) and healthy controls (6 ng/L). Both hs-cTnT and NfL could distinguish ALS patients from ALS mimics, with higher AUC noted for NfL (AUC 0.88; 95%CI 0.79-0.97). Disease progression correlated weakly with hs-cTnT (Pearson's r = 0.18, p = 0.04) and moderately with NfL (Pearson's r = 0.41, p < 0.001). Shorter survival was associated with higher levels of NfL at diagnosis (HR 1.08, 95%CI 1.04-1.11), but not hs-cTnT. hs-cTnT increased (12.61 ng/L per year, 95%CI 7.14-18.06) whereas NfL decreased longitudinally since ALS diagnosis. Conclusions: NfL is a stronger diagnostic and prognostic biomarker than hs-cTnT for ALS. However, hs-cTnT might constitute a disease progression biomarker as it increases longitudinally. The underlying causes for this increase need to be investigated
Fatigue in stroke survivors: a 5-year follow-up of the Fall study of Gothenburg
Longer term knowledge of post-stroke fatigue (PSF) is limited. Our aim was to describe the prevalence of, and to identify baseline predictors associated with, PSF 5 years after stroke. We undertook a follow-up of stroke survivors from the 504 consecutively recruited participants in the observational "The Fall Study of Gothenburg", conducted between 2014 and 2016. The dependent variable, PSF, was assessed using the Swedish version of the Fatigue Assessment Scale (S-FAS) and defined as having a S-FAS score ≥ 24. The S-FAS questionnaire was mailed to potential participants in August 2020. The independent variables, previously obtained from medical records, included age; sex; comorbidities; stroke severity; hospital length of stay; body mass index (BMI); number of medications and lifestyle factors at index stroke. To identify predictors of PSF, univariable and multivariable logistic regression analyses were performed. Of the 305 eligible participants, 119 (39%) responded with complete S-FAS. Mean age at index stroke was 71 (SD 10.4) years and 41% were female. After a mean of 4.9 years after stroke, the prevalence of PSF was 52%. Among those with PSF, almost two thirds were classified as having both physical and mental PSF. In the multivariable analysis, only high BMI predicted PSF with an odds ratio of 1.25 (95% CI 1.11-1.41, p < 0.01). In conclusion, half of the participants reported PSF 5 years after index stroke and higher body mass index was identified as a predictor. The findings from this study are important for healthcare professionals, for planning health-related efforts and rehabilitation of stroke survivors.ClinicalTrials.gov, Identifier NCT02264470
Non-Equilibrium Edge Channel Spectroscopy in the Integer Quantum Hall Regime
Heat transport has large potentialities to unveil new physics in mesoscopic
systems. A striking illustration is the integer quantum Hall regime, where the
robustness of Hall currents limits information accessible from charge
transport. Consequently, the gapless edge excitations are incompletely
understood. The effective edge states theory describes them as prototypal
one-dimensional chiral fermions - a simple picture that explains a large body
of observations and calls for quantum information experiments with quantum
point contacts in the role of beam splitters. However, it is in ostensible
disagreement with the prevailing theoretical framework that predicts, in most
situations, additional gapless edge modes. Here, we present a setup which gives
access to the energy distribution, and consequently to the energy current, in
an edge channel brought out-of-equilibrium. This provides a stringent test of
whether the additional states capture part of the injected energy. Our results
show it is not the case and thereby demonstrate regarding energy transport, the
quantum optics analogy of quantum point contacts and beam splitters. Beyond the
quantum Hall regime, this novel spectroscopy technique opens a new window for
heat transport and out-of-equilibrium experiments.Comment: 13 pages including supplementary information, Nature Physics in prin
Nonequilibrium Josephson effect in mesoscopic ballistic multiterminal SNS junctions
We present a detailed study of nonequilibrium Josephson currents and
conductance in ballistic multiterminal SNS-devices. Nonequilibrium is created
by means of quasiparticle injection from a normal reservoir connected to the
normal part of the junction. By applying a voltage at the normal reservoir the
Josephson current can be suppressed or the direction of the current can be
reversed. For a junction longer than the thermal length, , the
nonequilibrium current increases linearly with applied voltage, saturating at a
value equal to the equilibrium current of a short junction. The conductance
exhibits a finite bias anomaly around . For symmetric
injection, the conductance oscillates -periodically with the phase
difference between the superconductors, with position of the minimum
( or ) dependent on applied voltage and temperature. For
asymmetric injection, both the nonequilibrium Josephson current and the
conductance becomes -periodic in phase difference. Inclusion of barriers
at the NS-interfaces gives rise to a resonant behavior of the total Josephson
current with respect to junction length with a period . Both
three and four terminal junctions are studied.Comment: 21 pages, 19 figures, submitted to Phys. Rev.
Partitioning of on-demand electron pairs
We demonstrate the high fidelity splitting of electron pairs emitted on
demand from a dynamic quantum dot by an electronic beam splitter. The fidelity
of pair splitting is inferred from the coincidence of arrival in two detector
paths probed by a measurement of the partitioning noise. The emission
characteristic of the on-demand electron source is tunable from electrons being
partitioned equally and independently to electron pairs being split with a
fidelity of 90%. For low beam splitter transmittance we further find evidence
of pair bunching violating statistical expectations for independent fermions
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