78 research outputs found
Interacting electrodynamics of short coherent conductors in quantum circuits
When combining lumped mesoscopic electronic components to form a circuit,
quantum fluctuations of electrical quantities lead to a non-linear
electromagnetic interaction between the components that is not generally
understood. The Landauer-B\"uttiker formalism that is frequently used to
describe non-interacting coherent mesoscopic components is not directly suited
to describe such circuits since it assumes perfect voltage bias, i.e. the
absence of fluctuations. Here, we show that for short coherent conductors of
arbitrary transmission, the Landauer-B\"uttiker formalism can be extended to
take into account quantum voltage fluctuations similarly to what is done for
tunnel junctions. The electrodynamics of the whole circuit is then formally
worked out disregarding the non-Gaussianity of fluctuations. This reveals how
the aforementioned non-linear interaction operates in short coherent
conductors: voltage fluctuations induce a reduction of conductance through the
phenomenon of dynamical Coulomb blockade but they also modify their internal
density of states leading to an additional electrostatic modification of the
transmission. Using this approach we can account quantitatively for conductance
measurements performed on Quantum Point Contacts in series with impedances of
the order of . Our work should enable a better engineering of
quantum circuits with targeted properties
Dynamical Coulomb Blockade of Shot Noise
We observe the suppression of the finite frequency shot-noise produced by a
voltage biased tunnel junction due to its interaction with a single
electromagnetic mode of high impedance. The tunnel junction is embedded in a
quarter wavelength resonator containing a dense SQUID array providing it with a
characteristic impedance in the kOhms range and a resonant frequency tunable in
the 4-6 GHz range. Such high impedance gives rise to a sizeable Coulomb
blockade on the tunnel junction (roughly 30% reduction in the differential
conductance) and allows an efficient measurement of the spectral density of the
current fluctuations at the resonator frequency. The observed blockade of
shot-noise is found in agreement with an extension of the dynamical Coulomb
blockade theory
Fluctuation-Dissipation Relations of a Tunnel Junction Driven by a Quantum Circuit
We derive fluctuation-dissipation relations for a tunnel junction driven by a
high impedance microwave resonator, displaying strong quantum fluctuations. We
find that the fluctuation-dissipation relations derived for classical forces
hold, provided the effect of the circuit's quantum fluctuations is incorporated
into a modified non-linear curve. We also demonstrate that all
quantities measured under a coherent time dependent bias can be reconstructed
from their dc counterpart with a photo-assisted tunneling relation. We confirm
these predictions by implementing the circuit and measuring the dc current
through the junction, its high frequency admittance and its current noise at
the frequency of the resonator.Comment: Publisehd as Physical Review Letters, 114, 12680
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
Local Thermometry of Neutral Modes on the Quantum Hall Edge
A system of electrons in two dimensions and strong magnetic fields can be
tuned to create a gapped 2D system with one dimensional channels along the
edge. Interactions among these edge modes can lead to independent transport of
charge and heat, even in opposite directions. Measuring the chirality and
transport properties of these charge and heat modes can reveal otherwise hidden
structure in the edge. Here, we heat the outer edge of such a quantum Hall
system using a quantum point contact. By placing quantum dots upstream and
downstream along the edge of the heater, we can measure both the chemical
potential and temperature of that edge to study charge and heat transport,
respectively. We find that charge is transported exclusively downstream, but
heat can be transported upstream when the edge has additional structure related
to fractional quantum Hall physics.Comment: 24 pages, 18 figure
Generation of energy selective excitations in quantum Hall edge states
We operate an on-demand source of single electrons in high perpendicular
magnetic fields up to 30T, corresponding to a filling factor below 1/3. The
device extracts and emits single charges at a tunable energy from and to a
two-dimensional electron gas, brought into well defined integer and fractional
quantum Hall (QH) states. It can therefore be used for sensitive electrical
transport studies, e.g. of excitations and relaxation processes in QH edge
states
Generating Two Continuous Entangled Microwave Beams Using a dc-Biased Josephson Junction
We show experimentally that a dc-biased Josephson junction in series with two microwave resonators emits entangled beams of microwaves leaking out of the resonators. In the absence of a stationary phase reference for characterizing the entanglement of the outgoing beams, we measure second-order coherence functions to prove the entanglement. The experimental results are found in quantitative agreement with theory, proving that the low-frequency noise of the dc bias is the main limitation for the coherence time of the entangled beams. This agreement allows us to evaluate the entropy of entanglement of the resonators, estimate the entanglement flux at their output, and to identify the improvements that could bring this device closer to a useful bright source of entangled microwaves for quantum-technological applications
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
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
Electroactive biomimetic collagen-silver nanowire composite scaffolds
Electroactive biomaterials are widely explored as bioelectrodes and as scaffolds for neural and cardiac regeneration. Most electrodes and conductive scaffolds for tissue regeneration are based on synthetic materials that have limited biocompatibility and often display large discrepancies in mechanical properties with the surrounding tissue causing problems during tissue integration and regeneration. This work shows the development of a biomimetic nanocomposite material prepared from self-assembled collagen fibrils and silver nanowires (AgNW). Despite consisting of mostly type I collagen fibrils, the homogeneously embedded AgNWs provide these materials with a charge storage capacity of about 2.3 mC cm-2 and a charge injection capacity of 0.3 mC cm-2, which is on par with bioelectrodes used in the clinic. The mechanical properties of the materials are similar to soft tissues with a dynamic elastic modulus within the lower kPa range. The nanocomposites also support proliferation of embryonic cardiomyocytes while inhibiting the growth of both Gram-negative Escherichia coli and Gram-positive Staphylococcus epidermidis. The developed collagen/AgNW composites thus represent a highly attractive bioelectrode and scaffold material for a wide range of biomedical applications
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