921 research outputs found
Spin-valley blockade in carbon nanotube double quantum dots
We present a theoretical study of the Pauli or spin-valley blockade for
double quantum dots in semiconducting carbon nanotubes. In our model we take
into account the following characteristic features of carbon nanotubes: (i)
fourfold (spin and valley) degeneracy of the quantum dot levels, (ii) the
intrinsic spin-orbit interaction which is enhanced by the tube curvature, and
(iii) valley-mixing due to short-range disorder, i.e., substitutional atoms,
adatoms, etc. We find that the spin-valley blockade can be lifted in the
presence of short-range disorder, which induces two independent random (in
magnitude and direction) valley-Zeeman-fields in the two dots, and hence acts
similarly to hyperfine interaction in conventional semiconductor quantum dots.
In the case of strong spin-orbit interaction, we identify a parameter regime
where the current as the function of an applied axial magnetic field shows a
zero-field dip with a width controlled by the interdot tunneling amplitude, in
agreement with recent experiments.Comment: 15 pages, 6 figures, 2 tables; v2: published versio
Interference of heavy holes in an Aharonov-Bohm ring
We study the coherent transport of heavy holes through a one-dimensional ring
in the presence of spin-orbit coupling. Spin-orbit interaction of holes, cubic
in the in-plane components of momentum, gives rise to an angular momentum
dependent spin texture of the eigenstates and influences transport. We analyze
the dependence of the resulting differential conductance of the ring on hole
polarization of the leads and the signature of the textures in the
Aharonov-Bohm oscillations when the ring is in a perpendicular magnetic field.
We find that the polarization-resolved conductance reveals whether the dominant
spin-orbit coupling is of Dresselhaus or Rashba type, and that the cubic
spin-orbit coupling can be distinguished from the conventional linear coupling
by observing the four-peak structure in the Aharonov-Bohm oscillations.Comment: 12 pages, 11 figure
Enhancement of electron spin coherence by optical preparation of nuclear spins
We study a large ensemble of nuclear spins interacting with a single electron
spin in a quantum dot under optical excitation and photon detection. When a
pair of applied laser fields satisfy two-photon resonance between the two
ground electronic spin states, detection of light scattering from the
intermediate exciton state acts as a weak quantum measurement of the effective
magnetic (Overhauser) field due to the nuclear spins. If the spin were driven
into a coherent population trapping state where no light scattering takes
place, then the nuclear state would be projected into an eigenstate of the
Overhauser field operator and electron decoherence due to nuclear spins would
be suppressed: we show that this limit can be approached by adapting the laser
frequencies when a photon is detected. We use a Lindblad equation to describe
the time evolution of the driven system under photon emission and detection.
Numerically, we find an increase of the electron coherence time from 5 ns to
500 ns after a preparation time of 10 microseconds.Comment: 5 pages, 4 figure
Circuit theory for decoherence in superconducting charge qubits
Based on a network graph analysis of the underlying circuit, a quantum theory
of arbitrary superconducting charge qubits is derived. Describing the
dissipative elements of the circuit with a Caldeira-Leggett model, we calculate
the decoherence and leakage rates of a charge qubit. The analysis includes
decoherence due to a dissipative circuit element such as a voltage source or
the quasiparticle resistances of the Josephson junctions in the circuit. The
theory presented here is dual to the quantum circuit theory for superconducting
flux qubits. In contrast to spin-boson models, the full Hilbert space structure
of the qubit and its coupling to the dissipative environment is taken into
account. Moreover, both self and mutual inductances of the circuit are fully
included.Comment: 8 pages, 3 figures; v2: published version; typo in Eq.(30) corrected,
minor changes, reference adde
Signatures of spin blockade in the optical response of a charged quantum dot
We model spin blockade for optically excited electrons and holes in a charged
semiconductor quantum dot. We study the case where the quantum dot is initially
charged with a single electron and is then filled with an additional, optically
excited electron-hole pair, thus forming a charged exciton (trion). To make
contact with recent experiments, we model an optical pump-probe setup, in which
the two lowest quantum dot levels (s and p shells) are photo excited. Using the
Lindblad master equation, we calculate the differential transmission spectrum
as a function of the pump-probe time delay. Taking into account both spin
conserving and spin-flip intraband relaxation processes, we find that the
presence of the ground-state electron spin leads to an optical spin blockade at
short delay times which is visible as a crossover between two exponential
decays of the differential transmission. To make predictions for future
experiments, we also study the dependence of the spin-blockade on an external
magnetic field.Comment: 8 pages, 8 figure
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
Spin exchange interaction with tunable range between graphene quantum dots
We study the spin exchange between two electrons localized in separate
quantum dots in graphene. The electronic states in the conduction band are
coupled indirectly by tunneling to a common continuum of delocalized states in
the valence band. As a model, we use a two-impurity Anderson Hamiltonian which
we subsequently transform into an effective spin Hamiltonian by way of a
two-stage Schrieffer-Wolff transformation. We then compare our result to that
from a Coqblin-Schrieffer approach as well as to fourth order perturbation
theory.Comment: 8 pages, 3 figure
Coherent Adiabatic Spin Control in the Presence of Charge Noise Using Tailored Pulses
We study finite-time Landau-Zener transitions at a singlet-triplet level
crossing in a GaAs double quantum dot, both experimentally and theoretically.
Sweeps across the anticrossing in the high driving speed limit result in
oscillations with a small visibility. Here we demonstrate how to increase the
oscillation visibility while keeping sweep times shorter than T2* using a
tailored pulse with a detuning dependent level velocity. Our results show an
improvement of a factor ~2.9 for the oscillation visibility. In particular, we
were able to obtain a visibility of ~0.5 for St\"uckelberg oscillations, which
demonstrates the creation of an equally weighted superposition of the qubit
states.Comment: Related papers at http://pettagroup.princeton.ed
Asymmetry and decoherence in a double-layer persistent-current qubit
Superconducting circuits fabricated using the widely used shadow evaporation
technique can contain unintended junctions which change their quantum dynamics.
We discuss a superconducting flux qubit design that exploits the symmetries of
a circuit to protect the qubit from unwanted coupling to the noisy environment,
in which the unintended junctions can spoil the quantum coherence. We present a
theoretical model based on a recently developed circuit theory for
superconducting qubits and calculate relaxation and decoherence times that can
be compared with existing experiments. Furthermore, the coupling of the qubit
to a circuit resonance (plasmon mode) is explained in terms of the asymmetry of
the circuit. Finally, possibilities for prolonging the relaxation and
decoherence times of the studied superconducting qubit are proposed on the
basis of the obtained results.Comment: v.2: published version; 8 pages, 12 figures; added comparison with
experiment, improved discussion of T_ph
Scaling Behavior of Entanglement in Two- and Three-Dimensional Free Fermions
Exactly solving a spinless fermionic system in two and three dimensions, we
investigate the scaling behavior of the block entropy in critical and
non-critical phases. The scaling of the block entropy crucially depends on the
nature of the excitation spectrum of the system and on the topology of the
Fermi surface. Noticeably, in the critical phases the scaling violates the area
law and acquires a logarithmic correction \emph{only} when a well defined Fermi
surface exists in the system. When the area law is violated, we accurately
verify a conjecture for the prefactor of the logarithmic correction, proposed
by D. Gioev and I. Klich [quant-ph/0504151].Comment: 4 pages, 4 figure
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