1,063 research outputs found
Coulomb scattering cross-section in a 2D electron gas and production of entangled electrons
We calculate the Coulomb scattering amplitude for two electrons injected with
opposite momenta in an interacting 2DEG. We include the effect of the Fermi
liquid background by solving the 2D Bethe-Salpeter equation for the
two-particle Green function vertex, in the ladder and random phase
approximations. This result is used to discuss the feasibility of producing
spin EPR pairs in a 2DEG by collecting electrons emerging from collisions at a
pi/2 scattering angle, where only the entangled spin-singlets avoid the
destructive interference resulting from quantum indistinguishability.
Furthermore, we study the effective 2D electron-electron interaction due to the
exchange of virtual acoustic and optical phonons, and compare it to the Coulomb
interaction. Finally, we show that the 2D Kohn-Luttinger pairing instability
for the scattering electrons is negligible in a GaAs 2DEG.Comment: 19 pages, 10 figure
Spin-based quantum gating with semiconductor quantum dots by bichromatic radiation method
A potential scheme is proposed for realizing a two-qubit quantum gate in
semiconductor quantum dots. Information is encoded in the spin degrees of
freedom of one excess conduction electron of each quantum dot. We propose to
use two lasers, radiation two neighboring QDs, and tuned to blue detuning with
respect to the resonant frequencies of individual excitons. The two-qubit phase
gate can be achieved by means of both Pauli-blocking effect and dipole-dipole
coupling between intermediate excitonic states.Comment: Europhysics Letters 66 (2004) 1
Charge qubit entanglement in double quantum dots
We study entanglement of charge qubits in a vertical tunnel-coupled double
quantum dot containing two interacting electrons. Exact diagonalization is used
to compute the negativity characterizing entanglement. We find that
entanglement can be efficiently generated and controlled by sidegate voltages,
and describe how it can be detected. For large enough tunnel coupling, the
negativity shows a pronounced maximum at an intermediate interaction strength
within the Wigner molecule regime.Comment: revised version of the manuscript, as published in EPL, 7 pages, 4
figure
Multi-Qubit Gates in Arrays Coupled by 'Always On' Interactions
Recently there has been interest in the idea of quantum computing without
control of the physical interactions between component qubits. This is highly
appealing since the 'switching' of such interactions is a principal difficulty
in creating real devices. It has been established that one can employ 'always
on' interactions in a one-dimensional Heisenberg chain, provided that one can
tune the Zeeman energies of the individual (pseudo-)spins. It is important to
generalize this scheme to higher dimensional networks, since a real device
would probably be of that kind. Such generalisations have been proposed, but
only at the severe cost that the efficiency of qubit storage must *fall*. Here
we propose the use of multi-qubit gates within such higher-dimensional arrays,
finding a novel three-qubit gate that can in fact increase the efficiency
beyond the linear model. Thus we are able to propose higher dimensional
networks that can constitute a better embodiment of the 'always on' concept - a
substantial step toward bringing this novel concept to full fruition.Comment: 20 pages in preprint format, inc. 3 figures. This version has fixed
typos and printer-friendly figures, and is to appear in NJ
Universal quantum computation with ordered spin-chain networks
It is shown that anisotropic spin chains with gapped bulk excitations and
magnetically ordered ground states offer a promising platform for quantum
computation, which bridges the conventional single-spin-based qubit concept
with recently developed topological Majorana-based proposals. We show how to
realize the single-qubit Hadamard, phase, and pi/8 gates as well as the
two-qubit CNOT gate, which together form a fault-tolerant universal set of
quantum gates. The gates are implemented by judiciously controlling Ising
exchange and magnetic fields along a network of spin chains, with each
individual qubit furnished by a spin-chain segment. A subset of single-qubit
operations is geometric in nature, relying on control of anisotropy of spin
interactions rather than their strength. We contrast topological aspects of the
anisotropic spin-chain networks to those of p-wave superconducting wires
discussed in the literature.Comment: 9 pages, 3 figure
Magnetization transport and quantized spin conductance
We analyze transport of magnetization in insulating systems described by a
spin Hamiltonian. The magnetization current through a quasi one-dimensional
magnetic wire of finite length suspended between two bulk magnets is determined
by the spin conductance which remains finite in the ballistic limit due to
contact resistance. For ferromagnetic systems, magnetization transport can be
viewed as transmission of magnons and the spin conductance depends on the
temperature T. For antiferromagnetic isotropic spin-1/2 chains, the spin
conductance is quantized in units of order at T=0.
Magnetization currents produce an electric field and hence can be measured
directly. For magnetization transport in electric fields phenomena analogous to
the Hall effect emerge.Comment: 4 pages, 3 figures, minor change
Dissipation effects in spin-Hall transport of electrons and holes
We investigate the spin-Hall effect of both electrons and holes in
semiconductors using the Kubo formula in the correct zero-frequency limit
taking into account the finite momentum relaxation time of carriers in real
semiconductors. This approach allows to analyze the range of validity of recent
theoretical findings. In particular, the spin-Hall conductivity vanishes for
vanishing spin-orbit coupling if the correct zero-frequency limit is performed.Comment: 5 pages, no figures, version to appear in Phys. Rev.
Spin electric effects in molecular antiferromagnets
Molecular nanomagnets show clear signatures of coherent behavior and have a
wide variety of effective low-energy spin Hamiltonians suitable for encoding
qubits and implementing spin-based quantum information processing. At the
nanoscale, the preferred mechanism for control of quantum systems is through
application of electric fields, which are strong, can be locally applied, and
rapidly switched. In this work, we provide the theoretical tools for the search
for single molecule magnets suitable for electric control. By group-theoretical
symmetry analysis we find that the spin-electric coupling in triangular
molecules is governed by the modification of the exchange interaction, and is
possible even in the absence of spin-orbit coupling. In pentagonal molecules
the spin-electric coupling can exist only in the presence of spin-orbit
interaction. This kind of coupling is allowed for both and
spins at the magnetic centers. Within the Hubbard model, we find a relation
between the spin-electric coupling and the properties of the chemical bonds in
a molecule, suggesting that the best candidates for strong spin-electric
coupling are molecules with nearly degenerate bond orbitals. We also
investigate the possible experimental signatures of spin-electric coupling in
nuclear magnetic resonance and electron spin resonance spectroscopy, as well as
in the thermodynamic measurements of magnetization, electric polarization, and
specific heat of the molecules.Comment: 31 pages, 24 figure
Andreev-Tunneling, Coulomb Blockade, and Resonant Transport of Non-Local Spin-Entangled Electrons
We propose and analyze a spin-entangler for electrons based on an s-wave
superconductor coupled to two quantum dots each of which is tunnel-coupled to
normal Fermi leads. We show that in the presence of a voltage bias and in the
Coulomb blockade regime two correlated electrons provided by the Andreev
process can coherently tunnel from the superconductor via different dots into
different leads. The spin-singlet coming from the Cooper pair remains preserved
in this process, and the setup provides a source of mobile and nonlocal
spin-entangled electrons. The transport current is calculated and shown to be
dominated by a two-particle Breit-Wigner resonance which allows the injection
of two spin-entangled electrons into different leads at exactly the same
orbital energy, which is a crucial requirement for the detection of spin
entanglement via noise measurements. The coherent tunneling of both electrons
into the same lead is suppressed by the on-site Coulomb repulsion and/or the
superconducting gap, while the tunneling into different leads is suppressed
through the initial separation of the tunneling electrons. In the regime of
interest the particle-hole excitations of the leads are shown to be negligible.
The Aharonov-Bohm oscillations in the current are shown to contain single- and
two-electron periods with amplitudes that both vanish with increasing Coulomb
repulsion albeit differently fast.Comment: 11 double-column pages, 2 figures, REVTeX, minor revision
Molecular states in carbon nanotube double quantum dots
We report electrical transport measurements through a semiconducting
single-walled carbon nanotube (SWNT) with three additional top-gates. At low
temperatures the system acts as a double quantum dot with large inter-dot
tunnel coupling allowing for the observation of tunnel-coupled molecular states
extending over the whole double-dot system. We precisely extract the tunnel
coupling and identify the molecular states by the sequential-tunneling line
shape of the resonances in differential conductance.Comment: 5 pages, 4 figure
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