85 research outputs found
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
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
The relaxation time of a chiral quantum R-L circuit
We report on the GHz complex admittance of a chiral one dimensional ballistic
conductor formed by edge states in the quantum Hall regime. The circuit
consists of a wide Hall bar (the inductor L) in series with a tunable resistor
(R) formed by a quantum point contact. Electron interactions between edges are
screened by a pair of side gates. Conductance steps are observed on both real
and imaginary parts of the admittance. Remarkably, the phase of the admittance
is transmission-independent. This shows that the relaxation time of a chiral
R-L circuit is resistance independent. A current and charge conserving
scattering theory is presented that accounts for this observation with a
relaxation time given by the electronic transit time in the c cuit
An On-Demand Coherent Single Electron Source
We report on the electron analog of the single photon gun. On demand single
electron injection in a quantum conductor was obtained using a quantum dot
connected to the conductor via a tunnel barrier. Electron emission is triggered
by application of a potential step which compensates the dot charging energy.
Depending on the barrier transparency the quantum emission time ranges from 0.1
to 10 nanoseconds. The single electron source should prove useful for the
implementation of quantum bits in ballistic conductors. Additionally periodic
sequences of single electron emission and absorption generate a quantized
AC-current
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