2,859 research outputs found
Fine structure and optical pumping of spins in individual semiconductor quantum dots
We review spin properties of semiconductor quantum dots and their effect on
optical spectra. Photoluminescence and other types of spectroscopy are used to
probe neutral and charged excitons in individual quantum dots with high
spectral and spatial resolution. Spectral fine structure and polarization
reveal how quantum dot spins interact with each other and with their
environment. By taking advantage of the selectivity of optical selection rules
and spin relaxation, optical spin pumping of the ground state electron and
nuclear spins is achieved. Through such mechanisms, light can be used to
process spins for use as a carrier of information
Universal quantum control of two-electron spin quantum bits using dynamic nuclear polarization
One fundamental requirement for quantum computation is to perform universal
manipulations of quantum bits at rates much faster than the qubit's rate of
decoherence. Recently, fast gate operations have been demonstrated in logical
spin qubits composed of two electron spins where the rapid exchange of the two
electrons permits electrically controllable rotations around one axis of the
qubit. However, universal control of the qubit requires arbitrary rotations
around at least two axes. Here we show that by subjecting each electron spin to
a magnetic field of different magnitude we achieve full quantum control of the
two-electron logical spin qubit with nanosecond operation times. Using a single
device, a magnetic field gradient of several hundred milliTesla is generated
and sustained using dynamic nuclear polarization of the underlying Ga and As
nuclei. Universal control of the two-electron qubit is then demonstrated using
quantum state tomography. The presented technique provides the basis for single
and potentially multiple qubit operations with gate times that approach the
threshold required for quantum error correction.Comment: 11 pages, 4 figures. Supplementary Material included as ancillary
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Nuclear spin physics in quantum dots: an optical investigation
The mesoscopic spin system formed by the 10E4-10E6 nuclear spins in a
semiconductor quantum dot offers a unique setting for the study of many-body
spin physics in the condensed matter. The dynamics of this system and its
coupling to electron spins is fundamentally different from its bulk
counter-part as well as that of atoms due to increased fluctuations that result
from reduced dimensions. In recent years, the interest in studying quantum dot
nuclear spin systems and their coupling to confined electron spins has been
fueled by its direct implication for possible applications of such systems in
quantum information processing as well as by the fascinating nonlinear
(quantum-)dynamics of the coupled electron-nuclear spin system. In this
article, we review experimental work performed over the last decades in
studying this mesoscopic,coupled electron-nuclear spin system and discuss how
optical addressing of electron spins can be exploited to manipulate and
read-out quantum dot nuclei. We discuss how such techniques have been applied
in quantum dots to efficiently establish a non-zero mean nuclear spin
polarization and, most recently, were used to reduce fluctuations of the
average quantum dot nuclear spin orientation. Both results in turn have
important implications for the preservation of electron spin coherence in
quantum dots, which we discuss. We conclude by speculating how this recently
gained understanding of the quantum dot nuclear spin system could in the future
enable experimental observation of quantum-mechanical signatures or possible
collective behavior of mesoscopic nuclear spin ensembles.Comment: 61 pages, 45 figures, updated reference list, corrected typographical
error
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
Ultrafast optical control of entanglement between two quantum dot spins
The interaction between two quantum bits enables entanglement, the
two-particle correlations that are at the heart of quantum information science.
In semiconductor quantum dots much work has focused on demonstrating single
spin qubit control using optical techniques. However, optical control of
entanglement of two spin qubits remains a major challenge for scaling from a
single qubit to a full-fledged quantum information platform. Here, we combine
advances in vertically-stacked quantum dots with ultrafast laser techniques to
achieve optical control of the entangled state of two electron spins. Each
electron is in a separate InAs quantum dot, and the spins interact through
tunneling, where the tunneling rate determines how rapidly entangling
operations can be performed. The two-qubit gate speeds achieved here are over
an order of magnitude faster than in other systems. These results demonstrate
the viability and advantages of optically controlled quantum dot spins for
multi-qubit systems.Comment: 24 pages, 5 figure
Nuclear Spin Effects in Semiconductor Quantum Dots
The interaction of an electronic spin with its nuclear environment, an issue known as the central spin problem, has been the subject of considerable attention due to its relevance for spin-based quantum computation using semiconductor quantum dots. Independent control of the nuclear spin bath using nuclear magnetic resonance techniques and dynamic nuclear polarization using the central spin itself offer unique possibilities for manipulating the nuclear bath with significant consequences for the coherence and controlled manipulation of the central spin. Here we review some of the recent optical and transport experiments that have explored this central spin problem using semiconductor quantum dots. We focus on the interaction between nuclear spins and a spin of a single electron or valence-band hole. We also review the experimental techniques as well as the key theoretical ideas and the implications for quantum information science.Physic
Spin phenomena in semiconductor quantum dots
This thesis discusses development of new semiconductor quantum dot (QD) devices and materials. Optical spectroscopy of single QDs is employed in order to investigate electronic structure and magnetic properties of these materials. First we realise self-assembled InP/GaInP QDs embedded in Schottky diode structures, with the aim to realise charge control in these nanostructures, which recently provided an important test-bed for spin phenomena on the nano-scale. By varying the bias applied to the diode, we achieve accurate control of charge states in individual QDs, and also characterise the electron-hole alignment and the lateral extent of the exciton wavefunction. Second part of the thesis explores optimum regimes for optically induced dynamic nuclear polarization (DNP) in neutral InGaAs/GaAs QDs. Very efficient DNP under ultra low optical excitation is demonstrated, and its mechanism is explained as the electron-nuclear flip-flop occurring in the second order process of the dark exciton recombination. The final part of the thesis reports on magneto-optical studies of novel individual InPAs/GaInP quantum dots studied in this work for the first time. Here the long-term aim is to realise strong carrier confinement
potentially suitable for QD operation at elevated temperatures, e.g. as a single photon emitter. Here we lay foundations for future structural studies of these dots using optically detected nuclear magnetic resonance, and explore regimes for ecient DNP in InPAs dots emitting in a wide range of wavelength 690-920 nm
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