43 research outputs found
Quantum-Information Processing with Semiconductor Macroatoms
An all optical implementation of quantum information processing with
semiconductor macroatoms is proposed. Our quantum hardware consists of an array
of semiconductor quantum dots and the computational degrees of freedom are
energy-selected interband optical transitions. The proposed quantum-computing
strategy exploits exciton-exciton interactions driven by ultrafast sequences of
multi-color laser pulses. Contrary to existing proposals based on charge
excitations, the present all-optical implementation does not require the
application of time-dependent electric fields, thus allowing for a
sub-picosecond, i.e. decoherence-free, operation time-scale in realistic
state-of-the-art semiconductor nanostructures.Comment: 11 pages, 5 figures, to be published in Phys. Rev. Lett., significant
changes in the text and new simulations (figure 3
Experimental realization of the one qubit Deutsch-Jozsa algorithm in a quantum dot
We perform quantum interference experiments on a single self-assembled
semiconductor quantum dot. The presence or absence of a single exciton in the
dot provides a qubit that we control with femtosecond time resolution. We
combine a set of quantum operations to realize the single-qubit Deutsch-Jozsa
algorithm. The results show the feasibility of single qubit quantum logic in a
semiconductor quantum dot using ultrafast optical control.Comment: REVTex4, 4 pages, 3 figures. Now includes more details about the
dephasing in the quantum dots. The introduction has been reworded for
clarity. Minor readability fixe
Dephasing times in quantum dots due to elastic LO phonon-carrier collisions
Interpretation of experiments on quantum dot (QD) lasers presents a
challenge: the phonon bottleneck, which should strongly suppress relaxation and
dephasing of the discrete energy states, often seems to be inoperative. We
suggest and develop a theory for an intrinsic mechanism for dephasing in QD's:
second-order elastic interaction between quantum dot charge carriers and
LO-phonons. The calculated dephasing times are of the order of 200 fs at room
temperature, consistent with experiments. The phonon bottleneck thus does not
prevent significant room temperature dephasing.Comment: 4 pages, 1 figure, accepted for Phys. Rev. Let
Continuous quantum measurement of two coupled quantum dots using a point contact: A quantum trajectory approach
We obtain the finite-temperature unconditional master equation of the density
matrix for two coupled quantum dots (CQD) when one dot is subjected to a
measurement of its electron occupation number using a point contact (PC). To
determine how the CQD system state depends on the actual current through the PC
device, we use the so-called quantum trajectory method to derive the
zero-temperature conditional master equation. We first treat the electron
tunneling through the PC barrier as a classical stochastic point process (a
quantum-jump model). Then we show explicitly that our results can be extended
to the quantum-diffusive limit when the average electron tunneling rate is very
large compared to the extra change of the tunneling rate due to the presence of
the electron in the dot closer to the PC. We find that in both quantum-jump and
quantum-diffusive cases, the conditional dynamics of the CQD system can be
described by the stochastic Schr\"{o}dinger equations for its conditioned state
vector if and only if the information carried away from the CQD system by the
PC reservoirs can be recovered by the perfect detection of the measurements.Comment: 14 pages, 1 figures, RevTex, onecolumn, to appear in Phys. Rev.
Probing Single-Electron Spin Decoherence in Quantum Dots using Charged Excitons
We propose to use optical detection of magnetic resonance (ODMR) to measure
the decoherence time T_{2} of a single electron spin in a semiconductor quantum
dot. The electron is in one of the spin 1/2 states and a circularly polarized
laser can only create an optical excitation for one of the electron spin states
due to Pauli blocking. An applied electron spin resonance (ESR) field leads to
Rabi spin flips and thus to a modulation of the photoluminescence or,
alternatively, of the photocurrent. This allows one to measure the ESR
linewidth and the coherent Rabi oscillations, from which the electron spin
decoherence can be determined. We study different possible schemes for such an
ODMR setup, including cw or pulsed laser excitation.Comment: 8 pages, 7 figures. Proceedings of the PASPS3 conference, Santa
Barbara, CA (USA). To appear in the Journal of Superconductivit
Decoherence of quantum registers
The dynamical evolution of a quantum register of arbitrary length coupled to
an environment of arbitrary coherence length is predicted within a relevant
model of decoherence. The results are reported for quantum bits (qubits)
coupling individually to different environments (`independent decoherence') and
qubits interacting collectively with the same reservoir (`collective
decoherence'). In both cases, explicit decoherence functions are derived for
any number of qubits. The decay of the coherences of the register is shown to
strongly depend on the input states: we show that this sensitivity is a
characteristic of types of coupling (collective and independent) and not
only of the collective coupling, as has been reported previously. A non-trivial
behaviour ("recoherence") is found in the decay of the off-diagonal elements of
the reduced density matrix in the specific situation of independent
decoherence. Our results lead to the identification of decoherence-free states
in the collective decoherence limit. These states belong to subspaces of the
system's Hilbert space that do not get entangled with the environment, making
them ideal elements for the engineering of ``noiseless'' quantum codes. We also
discuss the relations between decoherence of the quantum register and
computational complexity based on the new dynamical results obtained for the
register density matrix.Comment: Typos corrected. Discussion and references added. 1 figure + 3 tables
added. This updated version contains 13 (double column) pages + 8 figures.
PRA in pres
The Aharonov-Bohm effect for an exciton
We study theoretically the exciton absorption on a ring shreded by a magnetic
flux. For the case when the attraction between electron and hole is
short-ranged we get an exact solution of the problem. We demonstrate that,
despite the electrical neutrality of the exciton, both the spectral position of
the exciton peak in the absorption, and the corresponding oscillator strength
oscillate with magnetic flux with a period ---the universal flux
quantum. The origin of the effect is the finite probability for electron and
hole, created by a photon at the same point, to tunnel in the opposite
directions and meet each other on the opposite side of the ring.Comment: 13 RevTeX 3.0 pages plus 4 EPS-figures, changes include updated
references and an improved chapter on possible experimental realization
Dynamics of a mesoscopic qubit under continuous quantum measurement
We present the conditional quantum dynamics of an electron tunneling between
two quantum dots subject to a measurement using a low transparency point
contact or tunnel junction. The double dot system forms a single qubit and the
measurement corresponds to a continuous in time readout of the occupancy of the
quantum dot. We illustrate the difference between conditional and unconditional
dynamics of the qubit. The conditional dynamics is discussed in two regimes
depending on the rate of tunneling through the point contact: quantum jumps, in
which individual electron tunneling current events can be distinguished, and a
diffusive dynamics in which individual events are ignored, and the
time-averaged current is considered as a continuous diffusive variable. We
include the effect of inefficient measurement and the influence of the relative
phase between the two tunneling amplitudes of the double dot/point contact
system.Comment: 12 pages (one-column Revtex), 7 figure
Hilbert space structure of a solid state quantum computer: two-electron states of a double quantum dot artificial molecule
We study theoretically a double quantum dot hydrogen molecule in the GaAs
conduction band as the basic elementary gate for a quantum computer with the
electron spins in the dots serving as qubits. Such a two-dot system provides
the necessary two-qubit entanglement required for quantum computation. We
determine the excitation spectrum of two horizontally coupled quantum dots with
two confined electrons, and study its dependence on an external magnetic field.
In particular, we focus on the splitting of the lowest singlet and triplet
states, the double occupation probability of the lowest states, and the
relative energy scales of these states. We point out that at zero magnetic
field it is difficult to have both a vanishing double occupation probability
for a small error rate and a sizable exchange coupling for fast gating. On the
other hand, finite magnetic fields may provide finite exchange coupling for
quantum computer operations with small errors. We critically discuss the
applicability of the envelope function approach in the current scheme and also
the merits of various quantum chemical approaches in dealing with few-electron
problems in quantum dots, such as the Hartree-Fock self-consistent field
method, the molecular orbital method, the Heisenberg model, and the Hubbard
model. We also discuss a number of relevant issues in quantum dot quantum
computing in the context of our calculations, such as the required design
tolerance, spin decoherence, adiabatic transitions, magnetic field control, and
error correction.Comment: 22 2-column pages, 11 figures. Published versio
A database of the coseismic effects following the 30 October 2016 Norcia earthquake in Central Italy
We provide a database of the coseismic geological surface effects following the Mw 6.5 Norcia earthquake that hit central Italy on 30 October 2016. This was one of the strongest seismic events to occur in Europe in the past thirty years, causing complex surface ruptures over an area of >400 km 2. The database originated from the collaboration of several European teams (Open EMERGEO Working Group; about 130 researchers) coordinated by the Istituto Nazionale di Geofisica e Vulcanologia. The observations were collected by performing detailed field surveys in the epicentral region in order to describe the geometry and kinematics of surface faulting, and subsequently of landslides and other secondary coseismic effects. The resulting database consists of homogeneous georeferenced records identifying 7323 observation points, each of which contains 18 numeric and string fields of relevant information. This database will impact future earthquake studies focused on modelling of the seismic processes in active extensional settings, updating probabilistic estimates of slip distribution, and assessing the hazard of surface faulting