467 research outputs found
Strong back-action of a linear circuit on a single electronic quantum channel
What are the quantum laws of electricity in mesoscopic circuits? This very
fundamental question has also direct implications for the quantum engineering
of nanoelectronic devices. Indeed, when a quantum coherent conductor is
inserted into a circuit, its transport properties are modified. In particular,
its conductance is reduced because of the circuit back-action. This phenomenon,
called environmental Coulomb blockade, results from the granularity of charge
transfers across the coherent conductor. Although extensively studied for a
tunnel junction in a linear circuit, it is only fully understood for arbitrary
short coherent conductors in the limit of small circuit impedances and small
conductance reduction. Here, we investigate experimentally the strong
back-action regime, with a conductance reduction of up to 90%. This is achieved
by embedding a single quantum channel of tunable transmission in an adjustable
on-chip circuit of impedance comparable to the resistance quantum
at microwave frequencies. The experiment reveals important deviations from
calculations performed in the weak back-action framework, and matches with
recent theoretical results. From these measurements, we propose a generalized
expression for the conductance of an arbitrary quantum channel embedded in a
linear circuit.Comment: 11 pages including supplementary information, to be published in
Nature Physic
Antibunched photons emitted by a dc-biased Josephson junction
We show experimentally that a dc biased Josephson junction in series with a high-enough-impedance microwave resonator emits antibunched photons. Our resonator is made of a simple microfabricated spiral coil that resonates at 4.4 GHz and reaches a 1.97kΩ characteristic impedance. The second order correlation function of the power leaking out of the resonator drops down to 0.3 at zero delay, which demonstrates the antibunching of the photons emitted by the circuit at a rate of 6×10^7 photons per second. Results are found in quantitative agreement with our theoretical predictions. This simple scheme could offer an efficient and bright single-photon source in the microwave domain
Niobium Silicon alloys for Kinetic Inductance Detectors
We are studying the properties of Niobium Silicon amorphous alloys as a
candidate material for the fabrication of highly sensitive Kinetic Inductance
Detectors (KID), optimized for very low optical loads. As in the case of other
composite materials, the NbSi properties can be changed by varying the relative
amounts of its components. Using a NbSi film with T_c around 1 K we have been
able to obtain the first NbSi resonators, observe an optical response and
acquire a spectrum in the band 50 to 300 GHz. The data taken show that this
material has very high kinetic inductance and normal state surface resistivity.
These properties are ideal for the development of KID. More measurements are
planned to further characterize the NbSi alloy and fully investigate its
potential.Comment: Accepted for publication on Journal of Low Temperature Physics.
Proceedings of the LTD15 conference (Caltech 2013
From Social Network (Centralized vs. Decentralized) to Collective Decision-Making (Unshared vs. Shared Consensus)
Relationships we have with our friends, family, or colleagues influence our personal decisions, as well as decisions we make together with others. As in human beings, despotism and egalitarian societies seem to also exist in animals. While studies have shown that social networks constrain many phenomena from amoebae to primates, we still do not know how consensus emerges from the properties of social networks in many biological systems. We created artificial social networks that represent the continuum from centralized to decentralized organization and used an agent-based model to make predictions about the patterns of consensus and collective movements we observed according to the social network. These theoretical results showed that different social networks and especially contrasted ones – star network vs. equal network - led to totally different patterns. Our model showed that, by moving from a centralized network to a decentralized one, the central individual seemed to lose its leadership in the collective movement's decisions. We, therefore, showed a link between the type of social network and the resulting consensus. By comparing our theoretical data with data on five groups of primates, we confirmed that this relationship between social network and consensus also appears to exist in animal societies
Bi-layer Kinetic Inductance Detectors for space observations between 80-120 GHz
We have developed Lumped Element Kinetic Inductance Detectors (LEKID)
sensitive in the frequency band from 80 to 120~GHz. In this work, we take
advantage of the so-called proximity effect to reduce the superconducting gap
of Aluminium, otherwise strongly suppressing the LEKID response for frequencies
smaller than 100~GHz. We have designed, produced and optically tested various
fully multiplexed arrays based on multi-layers combinations of Aluminium (Al)
and Titanium (Ti). Their sensitivities have been measured using a dedicated
closed-circle 100 mK dilution cryostat and a sky simulator allowing to
reproduce realistic observation conditions. The spectral response has been
characterised with a Martin-Puplett interferometer up to THz frequencies, and
with a resolution of 3~GHz. We demonstrate that Ti-Al LEKID can reach an
optical sensitivity of about ~ (best pixel), or
~ when averaged over the whole array. The optical
background was set to roughly 0.4~pW per pixel, typical for future space
observatories in this particular band. The performance is close to a
sensitivity of twice the CMB photon noise limit at 100~GHz which drove the
design of the Planck HFI instrument. This figure remains the baseline for the
next generation of millimetre-wave space satellites.Comment: 7 pages, 9 figures, submitted to A&
Formation of Ultracold Molecules by Merging Optical Tweezers
We demonstrate the formation of a single RbCs molecule during the merging of two optical tweezers, one containing a single Rb atom and the other a single Cs atom. Both atoms are initially predominantly in the motional ground states of their respective tweezers. We confirm molecule formation and establish the state of the molecule formed by measuring its binding energy. We find that the probability of molecule formation can be controlled by tuning the confinement of the traps during the merging process, in good agreement with coupled-channel calculations. We show that the conversion efficiency from atoms to molecules using this technique is comparable to magnetoassociation
Reaching a Consensus: Terminology and Concepts Used in Coordination and Decision-Making Research
Research on coordination and decision-making in humans and nonhuman primates has increased considerably throughout the last decade. However, terminology has been used inconsistently, hampering the broader integration of results from different studies. In this short article, we provide a glossary containing the central terms of coordination and decision-making research. The glossary is based on previous definitions that have been critically revised and annotated by the participants of the symposium “Where next? Coordination and decision-making in primate groups” at the XXIIIth Congress of the International Primatological Society (IPS) in Kyoto, Japan. We discuss a number of conceptual and methodological issues and highlight consequences for their implementation. In summary, we recommend that future studies on coordination and decision-making in animal groups do not use the terms “combined decision” and “democratic/despotic decision-making.” This will avoid ambiguity as well as anthropocentric connotations. Further, we demonstrate the importance of 1) taxon-specific definitions of coordination parameters (initiation, leadership, followership, termination), 2) differentiation between coordination research on individual-level process and group-level outcome, 3) analyses of collective action processes including initiation and termination, and 4) operationalization of successful group movements in the field to collect meaningful and comparable data across different species
Scheduling Task-parallel Applications in Dynamically Asymmetric Environments
Shared resource interference is observed by applications as dynamic
performance asymmetry. Prior art has developed approaches to reduce the impact
of performance asymmetry mainly at the operating system and architectural
levels. In this work, we study how application-level scheduling techniques can
leverage moldability (i.e. flexibility to work as either single-threaded or
multithreaded task) and explicit knowledge on task criticality to handle
scenarios in which system performance is not only unknown but also changing
over time. Our proposed task scheduler dynamically learns the performance
characteristics of the underlying platform and uses this knowledge to devise
better schedules aware of dynamic performance asymmetry, hence reducing the
impact of interference. Our evaluation shows that both criticality-aware
scheduling and parallelism tuning are effective schemes to address interference
in both shared and distributed memory applicationsComment: Published in ICPP Workshops '2
Malaria control – two years' use of insecticide treated bednets compared with insecticide house spraying in Kwazulu-Natal
Objectives_ The objective of this study was to produce data indicating whether insecticide-treated bednets should replac insecticide house spraying as a malaria control method in South Africa_ We report 2 years of preliminary data on malaria incidence comparing areas receiving insecticidetreated bednets and those subjected to house spraying in northern KwaZulu-Natal.Design, setting and subjects. In order to measure significant reductions in malaria incidence between the two interventions, a geographical information system (GIS) was used to identify and create seven pairs of geographical blood ; (areas) in the malaria high-risk areas of Ndumu and Makani in Ingwavuma magisterial district, KwaZulu-Natal, Individual blocks were then randomly allocated to either insecticide-treated bednets or house spraying with deltamethrin. Malaria cases were either routinely recorded by surveillance agents at home or were reported to the nearest health facility_Results and conclusions. The results show that 2 years' use of insecticide-treated bednets by communities in Ndumu and Makanis, KwaZulu-Natal, significantly reduced the malaria incidence both in 1997 (rate ratio (RR) =0_879, 95% confidence interval (Cn 0.80 - 0.95, P =0.04) and in 1998 (RR = 0.667, Cl 0_61 - 0.72, P = 0.0001). Using a t-test, these significant reductions were further confirmed by an assessment of the rate of change between 1996 and 1998, showing a 16% reduction in malaria incidence in blocks using bednets and an increase of 45% in sprayed areas (t = 2.534, P = 0.026 (12 df». In order to decide whether bednets : should replace house spraying in South Africa, we need more : data on the efficacy of treated bednets, their long-term acceptability and the cost of the two interventions
Light echoes reveal an unexpectedly cool Eta Carinae during its 19th-century Great Eruption
Eta Carinae (Eta Car) is one of the most massive binary stars in the Milky
Way. It became the second-brightest star in the sky during its mid-19th century
"Great Eruption," but then faded from view (with only naked-eye estimates of
brightness). Its eruption is unique among known astronomical transients in that
it exceeded the Eddington luminosity limit for 10 years. Because it is only 2.3
kpc away, spatially resolved studies of the nebula have constrained the ejected
mass and velocity, indicating that in its 19th century eruption, Eta Car
ejected more than 10 M_solar in an event that had 10% of the energy of a
typical core-collapse supernova without destroying the star. Here we report the
discovery of light echoes of Eta Carinae which appear to be from the 1838-1858
Great Eruption. Spectra of these light echoes show only absorption lines, which
are blueshifted by -210 km/s, in good agreement with predicted expansion
speeds. The light-echo spectra correlate best with those of G2-G5 supergiant
spectra, which have effective temperatures of ~5000 K. In contrast to the class
of extragalactic outbursts assumed to be analogs of Eta Car's Great Eruption,
the effective temperature of its outburst is significantly cooler than allowed
by standard opaque wind models. This indicates that other physical mechanisms
like an energetic blast wave may have triggered and influenced the eruption.Comment: Accepted for publication by Nature; 4 pages, 4 figures, SI: 6 pages,
3 figures, 5 table
- …