טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentKodriano Yaron
SubjectOptical Control of Single Spins in Semiconductor
Quantum Dots
DepartmentDepartment of Physics
Supervisor Professor David Gershoni
Full Thesis textFull thesis text - English Version


Abstract

Quantum information processing has invoked a lot of interest in the past few decades. There are several candidates for the basic quantum information unit, the qubit. Realizations of qubits in solid state and in particular in semiconductors are desired for stability, scalability and compatibility with contemporary technologies. One promising candidate is the spin of confined charge carriers in semiconductor quantum dots. These nanometric objects localize charge carriers in three dimensions, thereby discretizing their energy spectrum. The advantage of quantum dots is in their addressability by optical and electrical means. Thus, they form an excellent channel for quantum information exchange between photons, “flying” qubits, and carriers' spins- “anchored”, matter-qubits.

This thesis presents experimental demonstration of rotation by any desired angle, about any desired axis (universal gate operation) of the direction of a matter spin qubit, using a single optical pulse. High fidelity, universal gate operation which is orders of magnitude shorter than the qubit life and coherence times is essential for realizations of quantum information processing. So far, this was achieved by two, variably delayed rotations, during which the qubit coherently precesses (Ramsey interference). The duration of this three step operation was determined by the qubit's precession period and the loss of fidelity was accumulated. In contrast, we provide high fidelity universal qubit operation using only one picosecond long optical pulse. The pulse duration defines the gate duration, its polarization defines the spin rotation axis, and its detuning from resonance defines the magnitude of the rotation angle. The operation is applied on a qubit formed by the spin of an optically excited electron (bright exciton) confined within a semiconductor quantum dot.

At the last part of the thesis, we report on measurements of the life- and coherence-times of a quantum dot confined dark exciton. The dark exciton cannot be directly excited optically, nor can it decay radiatively. Therefore its lifetime is orders of magnitude longer than that of the bright exciton. We used intensity autocorrelation measurements of the radiative transition, which heralds the formation of a dark exciton state, in order to extract these times. Lower bounds of 1 μsec and 20 nsec for the life and coherence-time of the dark exciton, respectively, are inferred by our measurements. This establishes that the dark exciton can be used as a viable matter spin qubit.