39 research outputs found
Nuclear spin state narrowing via gate--controlled Rabi oscillations in a double quantum dot
We study spin dynamics for two electrons confined to a double quantum dot
under the influence of an oscillating exchange interaction. This leads to
driven Rabi oscillations between the --state and the
--state of the two--electron system. The width of the
Rabi resonance is proportional to the amplitude of the oscillating exchange. A
measurement of the Rabi resonance allows one to narrow the distribution of
nuclear spin states and thereby to prolong the spin decoherence time. Further,
we study decoherence of the two-electron states due to the hyperfine
interaction and give requirements on the parameters of the system in order to
initialize in the --state and to perform a
operation with unit fidelity.Comment: v1:9 pages, 1 figure; v2: 13 pages, 2 figures, added section on
measurement, to appear in Phys. Rev.
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-Orbit Coupling and Time-Reversal Symmetry in Quantum Gates
We study the effect of spin-orbit coupling on quantum gates produced by
pulsing the exchange interaction between two single electron quantum dots.
Spin-orbit coupling enters as a small spin precession when electrons tunnel
between dots. For adiabatic pulses the resulting gate is described by a unitary
operator acting on the four-dimensional Hilbert space of two qubits. If the
precession axis is fixed, time-symmetric pulsing constrains the set of possible
gates to those which, when combined with single qubit rotations, can be used in
a simple CNOT construction. Deviations from time-symmetric pulsing spoil this
construction. The effect of time asymmetry is studied by numerically
integrating the Schr\"odinger equation using parameters appropriate for GaAs
quantum dots. Deviations of the implemented gate from the desired form are
shown to be proportional to dimensionless measures of both spin-orbit coupling
and time asymmetry of the pulse.Comment: 10 pages, 3 figure
Quantum Entanglement of Identical Particles
We consider entanglement in a system of fixed number of identical particles.
Since any operation should be symmetrized over all the identical particles and
there is the precondition that the spatial wave functions overlap, the meaning
of identical-particle entanglement is fundamentally different from that of
distinguishable particles. The identical-particle counterpart of the Schmidt
basis is shown to be the single-particle basis in which the one-particle
reduced density matrix is diagonal. But it does not play a special role in the
issue of entanglement, which depends on the single-particle basis chosen. The
nonfactorization due to (anti)symmetrization is naturally excluded by using the
(anti)symmetrized basis or, equivalently, the particle number representation.
The natural degrees of freedom in quantifying the identical-particle
entanglement in a chosen single-particle basis are occupation numbers of
different single particle basis states. The entanglement between effectively
distinguishable spins is shown to be a special case of the occupation-number
entanglement.Comment: 5 pages, revtex4. A sentence is improve
Spin decay and quantum parallelism
We study the time evolution of a single spin coupled inhomogeneously to a
spin environment. Such a system is realized by a single electron spin bound in
a semiconductor nanostructure and interacting with surrounding nuclear spins.
We find striking dependencies on the type of the initial state of the nuclear
spin system. Simple product states show a profoundly different behavior than
randomly correlated states whose time evolution provides an illustrative
example of quantum parallelism and entanglement in a decoherence phenomenon.Comment: 6 pages, 4 figures included, version to appear in Phys. Rev.
Double-Occupancy Errors, Adiabaticity, and Entanglement of Spin-Qubits in Quantum Dots
Quantum gates that temporarily increase singlet-triplet splitting in order to
swap electronic spins in coupled quantum dots, lead inevitably to a finite
double-occupancy probability for both dots. By solving the time-dependent
Schr\"odinger equation for a coupled dot model, we demonstrate that this does
not necessarily lead to quantum computation errors. Instead, the coupled dot
ground state evolves quasi-adiabatically for typical system parameters so that
the double-occupancy probability at the completion of swapping is negligibly
small. We introduce a measure of entanglement which explicitly takes into
account the possibilty of double occupancies and provides a necessary and
sufficient criterion for entangled states.Comment: 9 pages, 4 figures include
Electron spin evolution induced by interaction with nuclei in a quantum dot
We study the decoherence of a single electron spin in an isolated quantum dot
induced by hyperfine interaction with nuclei for times smaller than the nuclear
spin relaxation time. The decay is caused by the spatial variation of the
electron envelope wave function within the dot, leading to a non-uniform
hyperfine coupling . We show that the usual treatment of the problem based
on the Markovian approximation is impossible because the correlation time for
the nuclear magnetic field seen by the electron spin is itself determined by
the flip-flop processes.
The decay of the electron spin correlation function is not exponential but
rather power (inverse logarithm) law-like. For polarized nuclei we find an
exact solution and show that the precession amplitude and the decay behavior
can be tuned by the magnetic field. The decay time is given by ,
where is the number of nuclei inside the dot. The amplitude of precession,
reached as a result of the decay, is finite. We show that there is a striking
difference between the decoherence time for a single dot and the dephasing time
for an ensemble of dots.Comment: Revtex, 11 pages, 5 figure
Zitterbewegung of electrons and holes in III-V semiconductor quantum wells
The notion of zitterbewegung is a long-standing prediction of relativistic
quantum mechanics. Here we extend earlier theoretical studies on this
phenomenon for the case of III-V zinc-blende semiconductors which exhibit
particularly strong spin-orbit coupling. This property makes nanostructures
made of these materials very favorable systems for possible experimental
observations of zitterbewegung. Our investigations include electrons in n-doped
quantum wells under the influence of both Rashba and Dresselhaus spin-orbit
interaction, and also the two-dimensional hole gas. Moreover, we give a
detailed anaysis of electron zitterbewegung in quantum wires which appear to be
particularly suited for experimentally observing this effect.Comment: 10 pages, 3 figures include
Test of Bell's Inequality using the Spin Filter Effect in Ferromagnetic Semiconductor Micro-structures
A theoretical proposal for testing Bell's inequality in mesoscopic systems is
presented. We show that the entanglement of two electron spins can be detected
in the spin filter effect in the mesoscopic semiconductor / ferromagnetic
semiconductor / semiconductor junction. The current-current correlation
function is calculated by use of the quantum scattering theory and we compare
it with the local hidden variable theory. We also discuss the influence of an
imperfect spin filter and derive the condition to see the violation of Bell's
inequality experimentally.Comment: 6 pages, 4 figures, submitted to J. Phys. Soc. Jp
Quantum-Hall Quantum-Bits
Bilayer quantum Hall systems can form collective states in which electrons
exhibit spontaneous interlayer phase coherence. We discuss the possibility of
using bilayer quantum dot many-electron states with this property to create
two-level systems that have potential advantages as quantum bits.Comment: 4 pages, 4 figures included, version to appear in Phys. Rev. B (Rapid
Communications