3,243 research outputs found
Electronic states in a magnetic quantum-dot molecule: phase transitions and spontaneous symmetry breaking
We show that a double quantum-dot system made of diluted magnetic
semiconductor behaves unlike usual molecules. In a semiconductor double quantum
dot or in a diatomic molecule, the ground state of a single carrier is
described by a symmetric orbital. In a magnetic material molecule, new ground
states with broken symmetry can appear due the competition between the
tunnelling and magnetic polaron energy. With decreasing temperature, the ground
state changes from the normal symmetric state to a state with spontaneously
broken symmetry. Interestingly, the symmetry of a magnetic molecule is
recovered at very low temperatures. A magnetic double quantum dot with
broken-symmetry phases can be used a voltage-controlled nanoscale memory cell.Comment: 4 pages, 5 figure
Optimal laser-control of double quantum dots
Coherent single-electron control in a realistic semiconductor double quantum
dot is studied theoretically. Using optimal-control theory we show that the
energy spectrum of a two-dimensional double quantum dot has a fully
controllable transition line. We find that optimized picosecond laser pulses
generate population transfer at significantly higher fidelities (>0.99) than
conventional sinusoidal pulses. Finally we design a robust and fast charge
switch driven by optimal pulses that are within reach of terahertz laser
technology.Comment: 5 pages, 4 figure
Excited state spectroscopy in carbon nanotube double quantum dots
We report on low temperature measurements in a fully tunable carbon nanotube
double quantum dot. A new fabrication technique has been used for the top-gates
in order to avoid covering the whole nanotube with an oxide layer as in
previous experiments. The top-gates allow us to form single dots, control the
coupling between them and we observe four-fold shell filling. We perform
inelastic transport spectroscopy via the excited states in the double quantum
dot, a necessary step towards the implementation of new microwave-based
experiments.Comment: 16 pages, 6 figures, submitted to nanoletter
Conditional operation of a spin qubit
We report coherent operation of a singlet-triplet qubit controlled by the
arrangement of two electrons in an adjacent double quantum dot. The system we
investigate consists of two pairs of capacitively coupled double quantum dots
fabricated by electrostatic gates on the surface of a GaAs heterostructure. We
extract the strength of the capacitive coupling between qubit and double
quantum dot and show that the present geometry allows fast conditional gate
operation, opening pathways to multi-qubit control and implementation of
quantum algorithms with spin qubits.Comment: related papers here: http://marcuslab.harvard.ed
- …