21,033 research outputs found
Semiconductor quantum ring as a solid-state spin qubit
The implementation of a spin qubit in a quantum ring occupied by one or a few
electrons is proposed. Quantum bit involves the Zeeman sublevels of the highest
occupied orbital. Such a qubit can be initialized, addressed, manipulated, read
out and coherently coupled to other quantum rings. An extensive discussion of
relaxation and decoherence is presented. By analogy with quantum dots, the spin
relaxation times due to spin-orbit interaction for experimentally accessible
quantum ring architectures are calculated. The conditions are formulated under
which qubits build on quantum rings can have long relaxation times of the order
of seconds. Rapidly improving nanofabrication technology have made such ring
devices experimentally feasible and thus promising for quantum state
engineering.Comment: 16 pages, 3 figure 3 table
Electrical control of spins and giant g-factors in ring-like coupled quantum dots
Emerging theoretical concepts for quantum technologies have driven a
continuous search for structures where a quantum state, such as spin, can be
manipulated efficiently. Central to many concepts is the ability to control a
system by electric and magnetic fields, relying on strong spin-orbit
interaction and a large g-factor. Here, we present a new mechanism for spin and
orbital manipulation using small electric and magnetic fields. By hybridizing
specific quantum dot states at two points inside InAs nanowires, nearly perfect
quantum rings form. Large and highly anisotropic effective g-factors are
observed, explained by a strong orbital contribution. Importantly, we find that
the orbital and spin-orbital contributions can be efficiently quenched by
simply detuning the individual quantum dot levels with an electric field. In
this way, we demonstrate not only control of the effective g-factor from 80 to
almost 0 for the same charge state, but also electrostatic change of the ground
state spin
Characteristic molecular properties of one-electron double quantum rings under magnetic fields
The molecular states of conduction electrons in laterally coupled quantum
rings are investigated theoretically. The states are shown to have a distinct
magnetic field dependence, which gives rise to periodic fluctuations of the
tunnel splitting and ring angular momentum in the vicinity of the ground state
crossings. The origin of these effects can be traced back to the Aharonov-Bohm
oscillations of the energy levels, along with the quantum mechanical tunneling
between the rings. We propose a setup using double quantum rings which shows
that Aharonov-Bohm effects can be observed even if the net magnetic flux
trapped by the carriers is zero.Comment: 16 pages (iopart format), 10 figures, accepted in J.Phys.Cond.Mat
Spin Effects in a Quantum Ring
Recent experiments are reviewed that explore the spin states of a ring-shaped
many-electron quantum dot. Coulomb-blockade spectroscopy is used to access the
spin degree of freedom. The Zeeman effect observed for states with successive
electron number allows to select possible sequences of spin ground states of
the ring. Spin-paired orbital levels can be identified by probing their
response to magnetic fields normal to the plane of the ring and electric fields
caused by suitable gate voltages. This narrows down the choice of ground-state
spin sequences. A gate-controlled singlet--triplet transition is identified and
the size of the exchange interaction matrix element is determined.Comment: 13 pages, 3 figures, Proceedings of the QD2004 conference in Banf
Finite Size Effects on the Optical Transitions in Quantum Rings under a Magnetic Field
We present a theoretical study of the energy spectrum of single electron and
hole states in quantum dots of annular geometry under a high magnetic field
along the ring axis in the frame of uncorrelated electron-hole theory. We
predict the periodic disappearance of the optical emission of the electron-hole
pair as the magnetic field increases, as a consequence of the finite height of
the barriers. The model has been applied to semiconductor rings of various
internal and external radii, giving as limiting cases the disk and antidot.Comment: 22 pages, 7 figures, LaTeX, published in Eur. Phys. J. B 53, 99-108
(2006
Laser-controlled local magnetic field with semiconductor quantum rings
We analize theoretically the dynamics of N electrons localized in a
semiconductor quantum ring under a train of phase-locked infrared laser pulses.
The pulse sequence is designed to control the total angular momentum of the
electrons. The quantum ring can be put in states characterized by strong
currents. The local magnetic field created by these currents can be used for a
selective quantum control of single spins in semiconductor systems
Controlled Coulomb effects in core-shell quantum rings
We analyse theoretically the possibilities of contactless control of in-gap
states formed by a pair of electrons confined in a triangular quantum ring. The
in-gap states are corner-localized states associated with two electrons
occupying the same corner area, and thus shifted to much higher energies than
other corner states, but still they are below the energies of
corner-side-localized states. We show how the energies, degeneracy and
splittings between consecutive levels change with the orientation of an
external electric field relatively to the polygonal cross section. We also show
how absorption changes in the presence of external electric and magnetic
fields.Comment: 4 pages, 2 figure
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