50 research outputs found
Inducing odd-frequency triplet superconducting correlations in a normal metal
This work discusses theoretically the interplay between the superconducting
and ferromagnetic proximity effects, in a diffusive normal metal strip in
contact with a superconductor and a non-uniformly magnetized ferromagnetic
insulator. The quasiparticle density of states of the normal metal shows clear
qualitative signatures of triplet correlations with spin one (TCS1). When one
goes away from the superconduting contact, TCS1 focus at zero energy under the
form of a peak surrounded by dips, which show a typical spatial scaling
behavior. This behavior can coexist with a focusing of singlet correlations and
triplet correlations with spin zero at finite but subgap energies. The
simultaneous observation of both effects would enable an unambigous
characterization of TCS1.Comment: To appear in Physical Review Letter
Probing coherent Cooper pair splitting with cavity photons
This work discusses theoretically the behavior of a microwave cavity and a
Cooper pair beam splitter (CPS) coupled non-resonantly. The cavity frequency
pull is modified when the CPS is resonant with a microwave excitation. This
provides a direct way to probe the coherence of the Cooper pair splitting
process. More precisely, the cavity frequency pull displays an anticrossing
whose specificities can be attributed unambiguously to coherent Cooper pair
injection. This work illustrates that microwave cavities represent a powerful
tool to investigate current transport in complex nanocircuits.Comment: 7 pages, 4 figure
Conductance and current noise of a superconductor/ferromagnet quantum point contact
We study the conductance and current noise of a superconductor/ferromagnet
(S/F) single channel Quantum Point Contact (QPC) as a function of the QPC bias
voltage, using a scattering approach. We show that the Spin-Dependence of
Interfacial Phase Shifts (SDIPS) acquired by electrons upon scattering by the
QPC can strongly modify these signals. For a weakly transparent contact, the
SDIPS induces sub-gap resonances in the conductance and differential Fano
factor curves of the QPC. For high transparencies, these resonances are
smoothed, but the shape of the signals remain extremely sensitive to the SDIPS.
We show that noise measurements could help to gain more information on the
device, e.g. in cases where the SDIPS modifies qualitatively the differential
Fano factor of the QPC but not the conductance.Comment: 9 pages, 4 figure
A spin quantum bit with ferromagnetic contacts for circuit QED
We theoretically propose a scheme for a spin quantum bit based on a double
quantum dot contacted to ferromagnetic elements. Interface exchange effects
enable an all electric manipulation of the spin and a switchable strong
coupling to a superconducting coplanar waveguide cavity. Our setup does not
rely on any specific band structure and can in principle be realized with many
different types of nanoconductors. This allows to envision on-chip single spin
manipulation and read-out using cavity QED techniques
Mesoscopic admittance of a double quantum dot
We calculate the mesoscopic admittance of a double quantum dot
(DQD),which can be measured directly using microwave techniques. This quantity
reveals spectroscopic information on the DQD and is also directly sensitive to
a Pauli spin blockade effect. We then discuss the problem of a DQD coupled to a
high quality photonic resonator. When the photon correlation functions can be
developed along a random-phase-approximation-like scheme, the response of the
resonator gives an access to
Gate-dependent spin-torque in a nanoconductor-based spin-valve
This article discusses the spin-torque effect in a spin-valve made out of two
ferromagnetic leads connected through a coherent nanoconductor (NC), in the
limit where a single channel of the NC lies near the Fermi energy of the leads.
Due to quantum interferences inside the NC, the spin-torque presents clear
qualitative differences with respect to the case of a multichannel disordered
spin-valve. In particular, it can be modulated with the NC gate voltage. In
principle, this modulation can be observed experimentally, assuming that the
spin-torque affects a ferromagnetic nano-domain in direct contact with the NC
Microwave spectroscopy of a Cooper pair beam splitter
This article discusses how to demonstrate the entanglement of the split
Cooper pairs produced in a double-quantum-dot based Cooper pair beam splitter
(CPS), by performing the microwave spectroscopy of the CPS. More precisely, one
can study the DC current response of such a CPS to two on-phase microwave gate
irradiations applied to the two CPS dots. Some of the current peaks caused by
the microwaves show a strongly nonmonotonic variation with the amplitude of the
irradiation applied individually to one dot. This effect is directly due to a
subradiance property caused by the coherence of the split pairs. Using
realistic parameters, one finds that this effect has a measurable amplitude.Comment: 12 pages, 5 figure
Squeezing light with Majorana fermions
Coupling a semiconducting nanowire to a microwave cavity provides a powerfull
means to assess the presence or absence of isolated Majorana fermions in the
nanowire. These exotic bound states can cause a significant cavity frequency
shift but also a strong cavity nonlinearity leading for instance to light
squeezing. The dependence of these effects on the nanowire gate voltages gives
direct signatures of the unique properties of Majorana fermions, such as their
self-adjoint character and their exponential confinement.Comment: long version: 11 pages, 5 figure
Subradiant split Cooper pairs
We suggest a way to characterize the coherence of the split Cooper pairs
emitted by a double-quantum-dot based Cooper pair splitter (CPS), by studying
the radiative response of such a CPS inside a microwave cavity. The coherence
of the split pairs manifests in a strongly nonmonotonic variation of the
emitted radiation as a function of the parameters controlling the coupling of
the CPS to the cavity. The idea to probe the coherence of the electronic states
using the tools of Cavity Quantum Electrodynamics could be generalized to many
other nanoscale circuits.Comment: Main text + Supplemental material file (15 pages, 5 figures), to
appear in Physical Review Letter
Direct cavity detection of Majorana pairs
No experiment could directly test the particle/antiparticle duality of
Majorana fermions, so far. However, this property represents a necessary
ingredient towards the realization of topological quantum computing schemes.
Here, we show how to complete this task by using microwave techniques. The
direct coupling between a pair of overlapping Majorana bound states and the
electric field from a microwave cavity is extremely difficult to detect due to
the self-adjoint character of Majorana fermions which forbids direct energy
exchanges with the cavity. We show theoretically how this problem can be
circumvented by using photo-assisted tunneling to fermionic reservoirs. The
absence of direct microwave transition inside the Majorana pair in spite of the
light-Majorana coupling would represent a smoking gun for the Majorana
self-adjoint character.Comment: 6 pages, 4 figure