94 research outputs found
Supercurrent Spectroscopy of Andreev States
We measure the excitation spectrum of a superconducting atomic contact. In
addition to the usual continuum above the superconducting gap, the single
particle excitation spectrum contains discrete, spin-degenerate Andreev levels
inside the gap. Quasiparticle excitations are induced by a broadband on-chip
microwave source and detected by measuring changes in the supercurrent flowing
through the atomic contact. Since microwave photons excite quasiparticles in
pairs, two types of transitions are observed: Andreev transitions, which
consists of putting two quasiparticles in an Andreev level, and transitions to
odd states with a single quasiparticle in an Andreev level and the other one in
the continuum. In contrast to absorption spectroscopy, supercurrent
spectroscopy allows detection of long-lived odd states.Comment: typos correcte
Exciting Andreev pairs in a superconducting atomic contact
The Josephson effect describes the flow of supercurrent in a weak link, such
as a tunnel junction, nanowire, or molecule, between two superconductors. It is
the basis for a variety of circuits and devices, with applications ranging from
medicine to quantum information. Currently, experiments using Josephson
circuits that behave like artificial atoms are revolutionizing the way we probe
and exploit the laws of quantum physics. Microscopically, the supercurrent is
carried by Andreev pair states, which are localized at the weak link. These
states come in doublets and have energies inside the superconducting gap.
Existing Josephson circuits are based on properties of just the ground state of
each doublet and so far the excited states have not been directly detected.
Here we establish their existence through spectroscopic measurements of
superconducting atomic contacts. The spectra, which depend on the atomic
configuration and on the phase difference between the superconductors, are in
complete agreement with theory. Andreev doublets could be exploited to encode
information in novel types of superconducting qubits.Comment: Submitted to Natur
Theory of microwave spectroscopy of Andreev bound states with a Josephson junction
We present a microscopic theory for the current through a tunnel Josephson
junction coupled to a non-linear environment, which consists of an Andreev
two-level system coupled to a harmonic oscillator. It models a recent
experiment [Bretheau, Girit, Pothier, Esteve, and Urbina, Nature (London) 499,
312 (2013)] on photon spectroscopy of Andreev bound states in a superconducting
atomic-size contact. We find the eigenenergies and eigenstates of the
environment and derive the current through the junction due to inelastic Cooper
pair tunneling. The current-voltage characteristic reveals the transitions
between the Andreev bound states, the excitation of the harmonic mode that
hybridizes with the Andreev bound states, as well as multi-photon processes.
The calculated spectra are in fair agreement with the experimental data.Comment: 8 pages, 6 figure
Evidence for long-lived quasiparticles trapped in superconducting point contacts
We have observed that the supercurrent across phase-biased, highly
transmitting atomic size contacts is strongly reduced within a broad phase
interval around {\pi}. We attribute this effect to quasiparticle trapping in
one of the discrete sub-gap Andreev bound states formed at the contact.
Trapping occurs essentially when the Andreev energy is smaller than half the
superconducting gap {\Delta}, a situation in which the lifetime of trapped
quasiparticles is found to exceed 100 \mus. The origin of this sharp energy
threshold is presently not understood.Comment: Article (5 pages) AND Supplemental material (14 pages). To be
published in Physical Review Letter
Dynamics of quasiparticle trapping in Andreev levels
We present a theory describing the trapping and untrapping of quasiparticles
in the Andreev bound level of a single-channel weak link between two
superconductors. We calculate the rates of the transitions between even and odd
occupations of the Andreev level induced by absorption and emission of both
photons and phonons. We apply the theory to a recent experiment [Phys. Rev.
Lett. 106, 257003 (2011)] in which the dynamics of the trapping of
quasiparticles in the Andreev levels of superconducting atomic contacts coupled
to a Josephson junction was measured. We show that the plasma energy
of the Josephson junction defines a rather abrupt transition between a fast
relaxation regime dominated by coupling to photons and a slow relaxation regime
dominated by coupling to phonons. With realistic parameters the theory provides
a semi-quantitative description of the experimental results.Comment: 11 pages, 9 figures. Accepted for publication in Physical Review
A quantitative mesoscale characterization of the mechanical behaviour of Ceramic Matrix Composites
An experimental micro-macro kinematic description of matrix cracking in unidirectionnal ceramic matrix composites is proposed. It has been enlighten by observations performed during an in situ tensile test in a Scanning Electron Microscope. The characterization of matrix crack nucleation, propagation and coalescence has been done with new parameters and related to the macroscopic behaviour
Coherent manipulation of Andreev states in superconducting atomic contacts
Coherent control of quantum states has been demonstrated in a variety of
superconducting devices. In all these devices, the variables that are
manipulated are collective electromagnetic degrees of freedom: charge,
superconducting phase, or flux. Here, we demonstrate the coherent manipulation
of a quantum system based on Andreev bound states, which are microscopic
quasiparticle states inherent to superconducting weak links. Using a circuit
quantum electrodynamics setup we perform single-shot readout of this "Andreev
qubit". We determine its excited state lifetime and coherence time to be in the
microsecond range. Quantum jumps and parity switchings are observed in
continuous measurements. In addition to possible quantum information
applications, such Andreev qubits are a testbed for the physics of single
elementary excitations in superconductors.Comment: Supplementary Materials at the end of the fil
Superconducting Quantum Point Contacts
We review our experiments on the electronic transport properties of atomic
contacts between metallic electrodes, in particular superconducting ones.
Despite ignorance of the exact atomic configuration, these ultimate quantum
point contacts can be manipulated and well characterized in-situ. They allow
performing fundamental tests of the scattering theory of quantum transport. In
particular, we discuss the case of the Josephson effect
Microwave studies of the fractional Josephson effect in HgTe-based Josephson junctions
The rise of topological phases of matter is strongly connected to their
potential to host Majorana bound states, a powerful ingredient in the search
for a robust, topologically protected, quantum information processing. In order
to produce such states, a method of choice is to induce superconductivity in
topological insulators. The engineering of the interplay between
superconductivity and the electronic properties of a topological insulator is a
challenging task and it is consequently very important to understand the
physics of simple superconducting devices such as Josephson junctions, in which
new topological properties are expected to emerge. In this article, we review
recent experiments investigating topological superconductivity in topological
insulators, using microwave excitation and detection techniques. More
precisely, we have fabricated and studied topological Josephson junctions made
of HgTe weak links in contact with two Al or Nb contacts. In such devices, we
have observed two signatures of the fractional Josephson effect, which is
expected to emerge from topologically-protected gapless Andreev bound states.
We first recall the theoretical background on topological Josephson junctions,
then move to the experimental observations. Then, we assess the topological
origin of the observed features and conclude with an outlook towards more
advanced microwave spectroscopy experiments, currently under development.Comment: Lectures given at the San Sebastian Topological Matter School 2017,
published in "Topological Matter. Springer Series in Solid-State Sciences,
vol 190. Springer
Quantum coherent control of a hybrid superconducting circuit made with graphene-based van der Waals heterostructures
Quantum coherence and control is foundational to the science and engineering
of quantum systems. In van der Waals (vdW) materials, the collective coherent
behavior of carriers has been probed successfully by transport measurements.
However, temporal coherence and control, as exemplified by manipulating a
single quantum degree of freedom, remains to be verified. Here we demonstrate
such coherence and control of a superconducting circuit incorporating
graphene-based Josephson junctions. Furthermore, we show that this device can
be operated as a voltage-tunable transmon qubit, whose spectrum reflects the
electronic properties of massless Dirac fermions traveling ballistically. In
addition to the potential for advancing extensible quantum computing
technology, our results represent a new approach to studying vdW materials
using microwave photons in coherent quantum circuits
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