125 research outputs found
Micro-fabricated electromagnetic filters for millikelvin experiments
In this article we report on the design, fabrication and tests of
micro-fabricated broadband filters suitable for proper electromagnetic
thermalization of electrical lines connected to sensitive quantum electronics
experiments performed at dilution fridge temperatures. Compared to previous
such miniature filters, the new design improves on performance and reliability.
These filters can be packed in space-saving cases with either single or
multi-contact connectors. Measured performance in the accessible range compares
well to simulations. We use these simulations to discuss the effectiveness of
these filters for electromagnetic thermalization at 30 mK.Comment: Available at http://www-spht.cea.fr/articles/s06/03
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
Phase controlled superconducting proximity effect probed by tunneling spectroscopy
Using a dual-mode STM-AFM microscope operating below 50mK we measured the
Local Density of States (LDoS) along small normal wires connected at both ends
to superconductors with different phases. We observe that a uniform minigap can
develop in the whole normal wire and in the superconductors near the
interfaces. The minigap depends periodically on the phase difference. The
quasiclassical theory of superconductivity applied to a simplified 1D model
geometry accounts well for the data.Comment: Accepted for publication in Physical Review Letter
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
Effect of the Tunneling Conductance on the Coulomb Staircase
Quantum fluctuations of the charge in the single electron box are
investigated. The rounding of the Coulomb staircase caused by virtual electron
tunneling is determined by perturbation theory up to third order in the
tunneling conductance and compared with precise Monte Carlo data computed with
a new algorithm. The remarkable agreement for large conductance indicates that
presently available experimental data on Coulomb charging effects in metallic
nanostructures can be well explained by finite order perturbative results.Comment: 4 pages, 5 figure
Superconducting atomic contacts under microwave irradiation
We have measured the effect of microwave irradiation on the dc
current-voltage characteristics of superconducting atomic contacts. The
interaction of the external field with the ac supercurrents leads to replicas
of the supercurrent peak, the well known Shapiro resonances. The observation of
supplementary fractional resonances for contacts containing highly transmitting
conduction channels reveals their non-sinusoidal current-phase relation. The
resonances sit on a background current which is itself deeply modified, as a
result of photon assisted multiple Andreev reflections. The results provide
firm support for the full quantum theory of transport between two
superconductors based on the concept of Andreev bound states
Proximity Induced Josephson-Quasiparticle Process in a Single Electron Transistor
We have performed the first experiments in a superconductor - normal metal -
superconductor single electron transistor in which there is an extra
superconducting strip partially overlapping the normal metal island in good
metal-to-metal contact. Superconducting proximity effect gives rise to current
peaks at voltages below the quasiparticle threshold. We interpret these peaks
in terms of the Josephson-quasiparticle process and discuss their connection
with the proximity induced energy gap in the normal metal island.Comment: 4 pages + 4 figure
High Temperature Conductance of the Single Electron Transistor
The linear conductance of the single electron transistor is determined in the
high temperature limit. Electron tunneling is treated nonperturbatively by
means of a path integral formulation and the conductance is obtained from
Kubo's formula. The theoretical predictions are valid for arbitrary conductance
and found to explain recent experimental data.Comment: 4 pages, 2 figure
The Bright Side of Coulomb Blockade
We explore the photonic (bright) side of dynamical Coulomb blockade (DCB) by
measuring the radiation emitted by a dc voltage-biased Josephson junction
embedded in a microwave resonator. In this regime Cooper pair tunneling is
inelastic and associated to the transfer of an energy 2eV into the resonator
modes. We have measured simultaneously the Cooper pair current and the photon
emission rate at the resonance frequency of the resonator. Our results show two
regimes, in which each tunneling Cooper pair emits either one or two photons
into the resonator. The spectral properties of the emitted radiation are
accounted for by an extension to DCB theory.Comment: 4 pages, 4 figures + 3 pages, 1 figure supplementary materia
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
- …