120 research outputs found
Quantum Dynamics of Electron-Nuclei Coupled System in Quantum Dots
We have investigated the dynamics of the electron-nuclei coupled system in
quantum dots. The bunching of results of the electron spin measurements and the
revival in the conditional probabilities are salient features of the nuclear
spin memory. The underlying mechanism is the squeezing of the nuclear spin
state and the correlations between the successive electron spin measurements.
Further we make a proposal for the preparation and detection of superposition
states of nuclear spins merely relying on electron spin measurements. For
unpolarized, completely random nuclear spin state one can still trace the
quantum interference effects. We discuss the realization of these schemes for
electron spins on both single and double QDs.Comment: 4 pages,3 figure
Cyclotron-resonant exciton transfer between the nearly free and strongly localized radiative states of a two-dimensional hole gas in a high magnetic field
Avoided crossing of the emission lines of a nearly free positive trion and a
cyclotron replica of an exciton bound to an interface acceptor has been
observed in the magneto-photoluminescence spectra of p-doped GaAs quantum
wells. Identification of the localized state depended on the precise mapping of
the anti-crossing pattern. The underlying coupling is caused by an exciton
transfer combined with a resonant cyclotron excitation of an additional hole.
The emission spectrum of the resulting magnetically tunable coherent state
probes weak localization in the quantum well.Comment: 5 pages, 5 figure
Polarization Properties of Single Quantum Dots in Nanowires
We study the absorption and emission polarization of single semiconductor
quantum dots in semiconductor nanowires. We show that the polarization of light
absorbed or emitted by a nanowire quantum dot strongly depends on the
orientation of the nanowire with respect to the directions along which light is
incident or emitted. Light is preferentially linearly polarized when directed
perpendicular to the nanowire elongation. In contrast, the degree of linear
polarization is low for light directed along the nanowire. This result is vital
for photonic applications based on intrinsic properties of quantum dots, such
as generation of entangled photons. As an example, we demonstrate optical
access to the spin states of a single nanowire quantum dot.Comment: 4 pages, 4 figure
Spin relaxation in the impurity band of a semiconductor in the external magnetic field
Spin relaxation in the impurity band of a 2D semiconductor with spin-split
spectrum in the external magnetic field is considered. Several mechanisms of
spin relaxation are shown to be relevant. The first one is attributed to
phonon-assisted transitions between Zeeman sublevels of the ground state of an
isolated impurity, while other mechanisms can be described in terms of spin
precession in a random magnetic field during the electron motion over the
impurity band. In the later case there are two contributions to the spin
relaxation: the one given by optimal impurity configurations with the
hop-waiting time inversely proportional to the external magnetic field and
another one related to the electron motion on a large scale. The average spin
relaxation rate is calculated
Hyperfine interaction in InAs/GaAs self-assembled quantum dots : dynamical nuclear polarization versus spin relaxation
We report on the influence of hyperfine interaction on the optical
orientation of singly charged excitons X+ and X- in self-assembled InAs/GaAs
quantum dots. All measurements were carried out on individual quantum dots
studied by micro-photoluminescence at low temperature. We show that the
hyperfine interaction leads to an effective partial spin relaxation, under
50kHz modulated excitation polarization, which becomes however strongly
inhibited under steady optical pumping conditions because of dynamical nuclear
polarization. This optically created magnetic-like nuclear field can become
very strong (up to ~4 T) when it is generated in the direction opposite to a
longitudinally applied field, and exhibits then a bistability regime. This
effect is very well described by a theoretical model derived in a perturbative
approach, which reveals the key role played by the energy cost of an electron
spin flip in the total magnetic field. Eventually, we emphasize the
similarities and differences between X+ and X- trions with respect to the
hyperfine interaction, which turn out to be in perfect agreement with the
theoretical description.Comment: 10 pages, 5 figure
Dynamics of impurity, local and non-local information for two non identical qubits
From the separability point of view the problem of two atoms interact with a
single cavity mode is investigated. The density matrix is calculated and used
to discuss the entanglement and to examine the dynamics of the local and
non-local information. Our examination concentrated on the variation in the
mean photon number and the ratio of the coupling parameters. Furthermore, we
have also assumed that the atomic system is initially in the ground states as
well as in the intermediate states. It has been shown that the local
information is transferred to non-local information when the impurity of one
qubit or both is maximum
Observation of Faraday rotation from a single confined spin
Ability to read-out the state of a single confined spin lies at the heart of
solid-state quantum information processing. While all-optical spin measurements
using Faraday rotation has been successfully implemented in ensembles of
semiconductor spins, read-out of a single semiconductor spin has only been
achieved using transport measurements based on spin-charge conversion. Here, we
demonstrate an all-optical dispersive measurement of the spin-state of a single
electron trapped in a semiconductor quantum dot. We obtain information on the
spin state through conditional Faraday rotation of a spectrally detuned optical
field, induced by the polarization- and spin-selective trion (charged quantum
dot) transitions. To assess the sensitivity of the technique, we use an
independent resonant laser for spin-state preparation. An all-optical
dispersive measurement on single spins has the important advantage of
channeling the measurement back-action onto a conjugate observable, thereby
allowing for repetitive or continuous quantum nondemolition (QND) read-out of
the spin-state. We infer from our results that there are of order unity
back-action induced spin-flip Raman scattering events within our measurement
timescale. Therefore, straightforward improvements such as the use of a
solid-immersion lens and higher efficiency detectors would allow for
back-action evading spin measurements, without the need for a cavity
Towards coherent optical control of a single hole spin: rabi rotation of a trion conditional on the spin state of the hole
A hole spin is a potential solid-state q-bit, that may be more robust against nuclear spin induced dephasing than an electron spin. Here we propose and demonstrate the sequential preparation, control and detection of a single hole spin trapped on a self-assembled InGaAs/GaAs quantum dot. The dot is embedded in a photodiode structure under an applied electric field. Fast, triggered, initialization of a hole spin is achieved by creating a spin-polarized electron-hole pair with a picosecond laser pulse, and in an applied electric field, waiting for the electron to tunnel leaving a spin-polarized hole. Detection of the hole spin with picoseconds time resolution is achieved using a second picosecond laser pulse to probe the positive trion transition, where a trion is created conditional on the hole spin being detected as a change in photocurrent. Finally, using this setup we observe a Rabi rotation of the hole-trion transition that is conditional on the hole spin, which for a pulse area of 2 pi can be used to impart a phase shift of pi between the hole spin states, a non-general manipulation of the hole spin. (C) 2009 Elsevier Ltd. All rights reserved
Triplet-Singlet Spin Relaxation via Nuclei in a Double Quantum Dot
The spin of a confined electron, when oriented originally in some direction,
will lose memory of that orientation after some time. Physical mechanisms
leading to this relaxation of spin memory typically involve either coupling of
the electron spin to its orbital motion or to nuclear spins. Relaxation of
confined electron spin has been previously measured only for Zeeman or exchange
split spin states, where spin-orbit effects dominate relaxation, while spin
flips due to nuclei have been observed in optical spectroscopy studies. Using
an isolated GaAs double quantum dot defined by electrostatic gates and direct
time domain measurements, we investigate in detail spin relaxation for
arbitrary splitting of spin states. Results demonstrate that electron spin
flips are dominated by nuclear interactions and are slowed by several orders of
magnitude when a magnetic field of a few millitesla is applied. These results
have significant implications for spin-based information processing
Millisecond-range electron spin memory in singly-charged InP quantum dots
We report millisecond-range spin memory of resident electrons in an ensemble
of InP quantum dots (QDs) under a small magnetic field of 0.1 T applied along
the optical excitation axis at temperatures up to about 5 K. A pump-probe
photoluminescence (PL) technique is used for optical orientation of electron
spins by the pump pulses and for study of spin relaxation over the long time
scale by measuring the degree of circular polarization of the probe PL as a
function of pump-probe delay. Dependence of spin decay rate on magnetic field
and temperature suggests two-phonon processes as the dominant spin relaxation
mechanism in this QDs at low temperatures.Comment: 3 pages, 4 figures, submitted to Appl. Phys. Let
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