On the Dynamics of Single-Electron Tunneling in Semiconductor Quantum Dots under Microwave Radiation

Efforts are made in this thesis to reveal the dynamics of
single-electron tunneling and to realize quantum bits (qubits) in
semiconductor quantum dots. At low temperatures, confined single
quantum dots and double quantum dots are realized in the
twodimensional electron gas (2DEG) of AlGaAs/GaAs heterostructures.
For transport studies, quantum dots are coupled to the drain and
source contacts via tunnel barriers. Electron-electron interaction
in such closed quantum dots leads to Coulomb-blockade (CB) effect
and single-electron tunneling (SET) through discrete quantum
states. SET and its dynamics in single and double quantum dots are
studied using both transport and microwave spectroscopy. In
transport spectroscopy, SET is monitored by measuring the direct
tunnel current through the quantum dots in both the linear and
nonlinear transport regimes, where ground states and excited states
of the quantum dots are resolved. In a double quantum dot, bonding
and anti-bonding molecular states are formed. Quantum dots proved
to be well controlled quantum mechanical systems. In analogy to
real atoms and molecules, single quantum dots and double quantum
dots are termed artificial atoms and artificial molecules,
respectively. In microwave spectroscopy, continuous microwave
radiation is applied to quantum dots. Photon-assisted tunneling
(PAT) through the ground state and excited states is observed in
single quantum dots. In a double quantum dot, the molecular states
can be coherently superimposed by microwave photons, inducing the
Rabi oscillations and a net direct tunnel current which is
experimentally measurable. A qubit is formed in a double quantum
dot. Two new microwave spectroscopy techniques are developed in
this thesis to explore the dynamics of PAT (SET) in quantum dots.
Both techniques are called heterodyne detection of photon-induced
tunnel current (photocurrent). In one method, two coherent
continuous microwave sources with a slight frequency offset are
combined to generate a flux of microwave photons. The photon
intensity varies in time at the offset frequency. The induced
alternating photocurrent at the offset frequency is detected by a
lock-in amplifier. The in-phase component of the photocurrent
reflects the tunneling strength, and the out-of-phase component
reveals the dynamics of electron tunneling. In the other method,
two coherent pulsed, i.e., broadband, microwaves are applied to
irradiate the quantum dots, where the dynamic charge relaxation and
the pumping by microwave pulses are studied. Both techniques allow
to resolve PAT in the nonlinear transport regime. A long charge
relaxation time of single quantum dots is found by using both
techniques. No superposition of the ground state and the excited
state is achieved.