1,180 research outputs found
Interaction effects in electric transport through self-assembled molecular monolayers
We theoretically investigate the effect of inter-molecular Coulomb
interactions on transport through molecular monolayers (or other devices based
on a large number of nanoscale conductors connected in parallel). Due to the
interactions, the current through different molecules become correlated,
resulting in distinct features in the non-linear current-voltage
characteristics, as we show by deriving and solving a type of modified master
equation, suitable for describing transport through an infinite number of
interacting conductors. Furthermore, if some of the molecules fail to bond to
both electrodes, charge traps can be induced at high voltages and block
transport through neighboring molecules, resulting in negative differential
resistance.Comment: 10 pages, 5 figures. Published versio
Using hybrid topological-spin qubit systems for two-qubit-spin gates
We investigate a hybrid quantum system involving spin qubits, based on the
spins of electrons confined in quantum dots, and topological qubits, based on
Majorana fermions. In such a system, gated control of the charge on the quantum
dots allows transfer of quantum information between the spin and topological
qubits, and the topological system can be used to facilitate transfer of spin
qubits between spatially separated quantum dots and to initialize entangled
spin-qubit pairs. Here, we show that the coupling to the topological system
also makes it possible to perform entangling two-qubit gates on spatially
separated spin qubits. The two-qubit gates are based on a combination of
topologically protected braiding operations, gate-controlled charge transfer
between the dots and edge Majorana modes, and measurements of the state of the
topological qubits.Comment: 7 pages, 1 figure. Published versio
Coupling spin qubits via superconductors
We show how superconductors can be used to couple, initialize, and read out
spatially separated spin qubits. When two single-electron quantum dots are
tunnel coupled to the same superconductor, the singlet component of the
two-electron state partially leaks into the superconductor via crossed Andreev
reflection. This induces a gate-controlled singlet-triplet splitting which,
with an appropriate superconductor geometry, remains large for dot separations
within the superconducting coherence length. Furthermore, we show that when two
double-dot singlet-triplet qubits are tunnel coupled to a superconductor with
finite charging energy, crossed Andreev reflection enables a strong two-qubit
coupling over distances much larger than the coherence length.Comment: 5 pages, 3 figures. Published versio
Scheme to measure Majorana fermion lifetimes using a quantum dot
We propose a setup to measure the lifetime of the parity of a pair of
Majorana bound states. The proposed experiment has one edge Majorana state
tunnel coupled to a quantum dot, which in turn is coupled to a metallic
electrode. When the Majorana Fermions overlap, even a small relaxation rate
qualitatively changes the non-linear transport spectrum, and for strong overlap
the lifetime can be read off directly from the height of a current peak. This
is important for the usage of Majorana Fermions as a platform for topological
quantum computing, where the parity relaxation is a limiting factor.Comment: 5 pages, 3 figures. Published versio
Parity qubits and poor man's Majorana bound states in double quantum dots
We study a double quantum dot connected via a common superconducting lead and
show that this system can be tuned to host one Majorana bound state (MBS) on
each dot. We call them "poor man's Majorana bound states" since they are not
topologically protected, but otherwise share the properties of MBS formed in
topological superconductors. We describe the conditions for the existence of
the two spatially separated MBS, which include breaking of spin degeneracy in
the two dots, with the spins polarized in different directions. Therefore, we
propose to use a magnetic field configuration where the field directions on the
two dot form an angle. By control of this angle the cross Andreev reflection
and the tunnel amplitudes can be tuned to be approximately equal, which is a
requirement for the formation of the MBS. We show that the fermionic state
encoded in the two Majoranas constitutes a parity qubit, which is non-local and
can only be measured by probing both dots simultaneously. Using a many-particle
basis for the MBS, we discuss the role of interactions and show that inter-dot
interactions always lift the degeneracy. We also show how the MBS can be probed
by transport measurements and discuss how the combination of several such
double dot systems allows for entanglement of parity qubits and measurement of
their dephasing times.Comment: 7 pages, 3 figures. Published versio
Quantum information transfer between topological and spin qubit systems
We propose a method to coherently transfer quantum information, and to create
entanglement, between topological qubits and conventional spin qubits. Our
suggestion uses gated control to transfer an electron (spin qubit) between a
quantum dot and edge Majorana modes in adjacent topological superconductors.
Because of the spin polarization of the Majorana modes, the electron transfer
translates spin superposition states into superposition states of the Majorana
system, and vice versa. Furthermore, we show how a topological superconductor
can be used to facilitate long-distance quantum information transfer and
entanglement between spatially separated spin qubits.Comment: 4+ pages, 2 figures, published versio
Coupling and braiding Majorana bound states in networks defined in proximitized two-dimensional electron gases
Two-dimensional electron gases with strong spin-orbit coupling covered by a
superconducting layer offer a flexible and potentially scalable platform for
Majorana networks. We predict Majorana bound states (MBSs) to appear for
experimentally achievable parameters and realistic gate potentials in two
designs: either underneath a narrow stripe of a superconducting layer
(S-stripes) or where a narrow stripe has been removed from a uniform layer
(N-stripes). The coupling of the MBSs can be tuned for both types in a wide
range (10 eV) using gates placed adjacent to the stripes. For
both types, we numerically compute the local density of states for two parallel
Majorana-stripe ends as well as Majorana trijunctions formed in a tuning-fork
geometry. The MBS coupling between parallel Majorana stripes can be suppressed
below 1 neV for potential barriers in the meV range for separations of about
200 nm. We further show that the MBS couplings in a trijunction can be
gate-controlled in a range similar to the intra-stripe coupling while
maintaining a sizable gap to the excited states (tens of eV). Altogether,
this suggests that braiding can carried out on a time scale of 10-100 ns
Errors and uncertainties in microwave link rainfall estimation explored using drop size measurements and high-resolution radar data
Microwave links can be used for the estimation of path-averaged rainfall by using either the path-integrated attenuation or the difference in attenuation of two signals with different frequencies and/or polarizations. Link signals have been simulated using measured time series of raindrop size distributions (DSDs) over a period of nearly 2 yr, in combination with wind velocity data and Taylor’s hypothesis. For this purpose, Taylor’s hypothesis has been tested using more than 1.5 yr of high-resolution radar data. In terms of correlation between spatial and temporal profiles of rainfall intensities, the validity of Taylor’s hypothesis quickly decreases with distance. However, in terms of error statistics, the hypothesis is seen to hold up to distances of at least 10 km. Errors and uncertainties (mean bias error and root-mean-square error, respectively) in microwave link rainfall estimates due to spatial DSD variation are at a minimum at frequencies (and frequency combinations) where the power-law relation for the conversion to rainfall intensity is close to linear. Errors generally increase with link length, whereas uncertainties decrease because of the decrease of scatter about the retrieval relations because of averaging of spatially variable DSDs for longer links. The exponent of power-law rainfall retrieval relations can explain a large part of the variation in both bias and uncertainty, which means that the order of magnitude of these error statistics can be predicted from the value of this exponent, regardless of the link length
Distinguishing Majorana bound states from localized Andreev bound states by interferometry
Experimental evidence for Majorana bound states (MBSs) is so far mainly based
on the robustness of a zero-bias conductance peak. However, similar features
can also arise due to Andreev bound states (ABSs) localized at the end of an
island. We show that these two scenarios can be distinguished by an
interferometry experiment based on embedding a Coulomb-blockaded island into an
Aharonov-Bohm ring. For two ABSs, when the ground state is nearly degenerate,
cotunneling can change the state of the island and interference is suppressed.
By contrast, for two MBSs the ground state is nondegenerate and cotunneling has
to preserve the island state, which leads to -periodic conductance
oscillations with magnetic flux. Such interference setups can be realized with
semiconducting nanowires or two-dimensional electron gases with
proximity-induced superconductivity and may also be a useful spectroscopic tool
for parity-flip mechanisms
Non-linear thermoelectrics of molecular junctions with vibrational coupling
We present a detailed study of the non-linear thermoelectric properties of a
molecular junction, represented by a dissipative Anderson-Holstein model. A
single orbital level with strong Coulomb interaction is coupled to a localized
vibrational mode and we account for both electron and phonon exchange with both
electrodes, investigating how these contribute to the heat and charge
transport. We calculate the efficiency and power output of the device operated
as a heat to electric power converter and identify the optimal operating
conditions, which are found to be qualitatively changed by the presence of the
vibrational mode. Based on this study of a generic model system, we discuss the
desirable properties of molecular junctions for thermoelectric applications.Comment: 8 pages, 5 figure
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