36 research outputs found
Tunable Negative Differential Resistance controlled by Spin Blockade in Single Electron Transistors
We demonstrate a tunable negative differential resistance controlled by spin
blockade in single electron transistors. The single electron transistors
containing a few electrons and spin polarized source and drain contacts were
formed in GaAs/GaAlAs heterojunctions using metallic gates. Coulomb blockade
measurements performed as a function of applied source-drain bias, electron
number and magnetic field reveal well defined regimes where a decrease in the
current is observed with increasing bias. We establish that the origin of the
negative differential regime is the spin-polarized detection of electrons
combined with a long spin relaxation time in the dot. These results indicate
new functionalities that may be utilized in nano-spintronic devices in which
the spin state is electro-statically controlled via the electron occupation
number.Comment: 8 pages, 4 figure
The Collapse of the Spin-Singlet Phase in Quantum Dots
We present experimental and theoretical results on a new regime in quantum
dots in which the filling factor 2 singlet state is replaced by new spin
polarized phases. We make use of spin blockade spectroscopy to identify the
transition to this new regime as a function of the number of electrons. The key
experimental observation is a reversal of the phase in the systematic
oscillation of the amplitude of Coulomb blockade peaks as the number of
electrons is increased above a critical number. It is found theoretically that
correlations are crucial to the existence of the new phases.Comment: REVTeX4, 4 pages, 4 figures, to appear in PR
Coherent Manipulation of Individual Electron Spin in a Double Quantum Dot Integrated with a Micro-Magnet
We report the coherent manipulation of electron spins in a double quantum dot
integrated with a micro-magnet. We performed electric dipole spin resonance
experiments in the continuous wave (CW) and pump-and-probe modes. We observed
two resonant CW peaks and two Rabi oscillations of the quantum dot current by
sweeping an external magnetic field at a fixed frequency. Two peaks and
oscillations are measured at different resonant magnetic field, which reflects
the fact that the local magnetic fields at each quantum dot are modulated by
the stray field of a micro-magnet. As predicted with a density matrix approach,
the CW current is quadratic with respect to microwave (MW) voltage while the
Rabi frequency (\nu_Rabi) is linear. The difference between the \nu_Rabi values
of two Rabi oscillations directly reflects the MW electric field across the two
dots. These results show that the spins on each dot can be manipulated
coherently at will by tuning the micro-magnet alignment and MW electric field.Comment: 5 pages, 3 figure
Voltage-tunable singlet-triplet transition in lateral quantum dots
Results of calculations and high source-drain transport measurements are
presented which demonstrate voltage-tunable entanglement of electron pairs in
lateral quantum dots. At a fixed magnetic field, the application of a
judiciously-chosen gate voltage alters the ground-state of an electron pair
from an entagled spin singlet to a spin triplet.Comment: 8.2 double-column pages, 10 eps figure
Coupling molecular spin states by photon-assisted tunneling
Artificial molecules containing just one or two electrons provide a powerful
platform for studies of orbital and spin quantum dynamics in nanoscale devices.
A well-known example of these dynamics is tunneling of electrons between two
coupled quantum dots triggered by microwave irradiation. So far, these
tunneling processes have been treated as electric dipole-allowed
spin-conserving events. Here we report that microwaves can also excite
tunneling transitions between states with different spin. In this work, the
dominant mechanism responsible for violation of spin conservation is the
spin-orbit interaction. These transitions make it possible to perform detailed
microwave spectroscopy of the molecular spin states of an artificial hydrogen
molecule and open up the possibility of realizing full quantum control of a two
spin system via microwave excitation.Comment: 13 pages, 9 figure
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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Pumped double quantum dot with spin-orbit coupling
We study driven by an external electric field quantum orbital and spin dynamics of electron in a one-dimensional double quantum dot with spin-orbit coupling. Two types of external perturbation are considered: a periodic field at the Zeeman frequency and a single half-period pulse. Spin-orbit coupling leads to a nontrivial evolution in the spin and orbital channels and to a strongly spin- dependent probability density distribution. Both the interdot tunneling and the driven motion contribute into the spin evolution. These results can be important for the design of the spin manipulation schemes in semiconductor nanostructures
Large tunable valley splitting in edge-free graphene quantum dots on boron nitride
Coherent manipulation of binary degrees of freedom is at the heart of modern
quantum technologies. Graphene offers two binary degrees: the electron spin and
the valley. Efficient spin control has been demonstrated in many solid state
systems, while exploitation of the valley has only recently been started, yet
without control on the single electron level. Here, we show that van-der Waals
stacking of graphene onto hexagonal boron nitride offers a natural platform for
valley control. We use a graphene quantum dot induced by the tip of a scanning
tunneling microscope and demonstrate valley splitting that is tunable from -5
to +10 meV (including valley inversion) by sub-10-nm displacements of the
quantum dot position. This boosts the range of controlled valley splitting by
about one order of magnitude. The tunable inversion of spin and valley states
should enable coherent superposition of these degrees of freedom as a first
step towards graphene-based qubits
Spin-orbit qubit in a semiconductor nanowire
Motion of electrons can influence their spins through a fundamental effect
called spin-orbit interaction. This interaction provides a way to electrically
control spins and as such lies at the foundation of spintronics. Even at the
level of single electrons, spin-orbit interaction has proven promising for
coherent spin rotations. Here we report a spin-orbit quantum bit implemented in
an InAs nanowire, where spin-orbit interaction is so strong that spin and
motion can no longer be separated. In this regime we realize fast qubit
rotations and universal single qubit control using only electric fields. We
enhance coherence by dynamically decoupling the qubit from the environment. Our
qubits are individually addressable: they are hosted in single-electron quantum
dots, each of which has a different Land\'e g-factor. The demonstration of a
nanowire qubit opens ways to harness the advantages of nanowires for use in
quantum computing. Nanowires can serve as one-dimensional templates for
scalable qubit registers. Unique to nanowires is the possibility to easily vary
the material even during wire growth. Such flexibility can be used to design
wires with suppressed decoherence and push semiconductor qubit fidelities
towards error-correction levels. Furthermore, electrical dots can be integrated
with optical dots in p-n junction nanowires. The coherence times achieved here
are sufficient for the conversion of an electronic qubit into a photon, the
flying qubit, for long-distance quantum communication