2,526 research outputs found
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
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 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
Generation of gravitational waves during early structure formation between cosmic inflation and reheating
In the pre-reheating era, following cosmic inflation and preceding radiation
domination, the energy density may be dominated by an oscillating massive
scalar condensate, such as is the case for quadratic chaotic inflation. We have
found in a previous paper that during this period, a wide range of sub-Hubble
scale perturbations are subject to a preheating instability, leading to the
growth of density perturbations ultimately collapsing to form non-linear
structures. We compute here the gravitational wave signal due to these
structures in the linear limit and present estimates for emission in the
non-linear limit due to various effects: the collapse of halos, the tidal
interactions, the evaporation during the conversion of the inflaton condensate
into radiation and finally the ensuing turbulent cascades. The gravitational
wave signal could be rather large and potentially testable by future detectors.Comment: 11 pages, 3 figure
Auxiliary Hamiltonian representation of the nonequilibrium Dyson equation
The nonequilibrium Dyson (or Kadanoff-Baym) equation, which is an equation of
motion with long-range memory kernel for real-time Green functions, underlies
many numerical approaches based on the Keldysh formalism. In this paper we map
the problem of solving the Dyson equation in real-time onto a noninteracting
auxiliary Hamiltonian with additional bath degrees of freedom. The solution of
the auxiliary model does not require the evaluation of a memory kernel and can
thus be implemented in a very memory efficient way. The mapping is derived for
a self-energy which is local in space and is thus directly applicable within
nonequilibrium dynamical mean-field theory (DMFT). We apply the method to study
the interaction quench in the Hubbard model for an optical lattice with a
narrow confinement, using inhomogeneous DMFT in combination with second-order
weak-coupling perturbation theory. We find that, although the quench excites
pronounced density oscillations, signatures of the two-stage relaxation similar
to the homogeneous system can be observed by looking at the time-dependent
occupations of natural orbitals.Comment: 14 pages, 11 figure
Role of impact ionization in the thermalization of photo-excited Mott insulators
We study the influence of the pulse energy and fluence on the thermalization
of photo-doped Mott insulators. If the Mott gap is smaller than the width of
the Hubbard bands, the kinetic energy of individual carriers can be large
enough to produce doublon-hole pairs via a process analogous to impact
ionization. The thermalization dynamics, which involves an adjustment of the
doublon and hole densities, thus changes as a function of the energy of the
photo-doped carriers and exhibits two timescales -- a fast relaxation related
to impact ionization and a slower timescale associated with higher-order
scattering processes. The slow dynamics depends more strongly on the gap size
and the photo-doping concentration
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
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