116 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
Optimal power and efficiency of single quantum dot heat engines: theory and experiment
Quantum dots (QDs) can serve as near perfect energy filters and are therefore
of significant interest for the study of thermoelectric energy conversion close
to thermodynamic efficiency limits. Indeed, recent experiments in [Nat. Nano.
13, 920 (2018)] realized a QD heat engine with performance near these limits
and in excellent agreement with theoretical predictions. However, these
experiments also highlighted a need for more theory to help guide and
understand the practical optimization of QD heat engines, in particular
regarding the role of tunnel couplings on the performance at maximum power and
efficiency for QDs that couple seemingly weakly to electronic reservoirs.
Furthermore, these experiments also highlighted the critical role of the
external load when optimizing the performance of a QD heat engine in practice.
To provide further insight into the operation of these engines we use the
Anderson impurity model together with a Master equation approach to perform
power and efficiency calculations up to co-tunneling order. This is combined
with additional thermoelectric experiments on a QD embedded in a nanowire where
the power is measured using two methods. We use the measurements to present an
experimental procedure for efficiently finding the external load which
should be connected to the engine to optimize power output. Our theoretical
estimates of show a good agreement with the experimental results, and we
show that second order tunneling processes and non-linear effects have little
impact close to maximum power, allowing us to derive a simple analytic
expression for . In contrast, we find that the electron contribution to
the thermoelectric efficiency is significantly reduced by second order
tunneling processes, even for rather weak tunnel couplings
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
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
Time scales for Majorana manipulation using Coulomb blockade in gate-controlled superconducting nanowires
We numerically compute the low-energy spectrum of a gate-controlled
superconducting topological nanowire segmented into two islands, each
Josephson-coupled to a bulk superconductor. This device may host two pairs of
Majorana bound states and could provide a platform for testing Majorana fusion
rules. We analyze the crossover between (i) a charge-dominated regime
utilizable for initialization and readout of Majorana bound states, (ii) a
single-island regime for dominating inter-island Majorana coupling, (iii) a
Josephson-plasmon regime for large coupling to the bulk superconductors, and
(iv) a regime of four Majorana bound states allowing for topologically
protected Majorana manipulations. From the energy spectrum, we derive
conservative estimates for the time scales of a fusion-rule testing protocol
proposed recently [arXiv:1511.05153]. We also analyze the steps needed for
basic Majorana braiding operations in branched nanowire structures
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