|Ph.D Student||Schwartz Ido|
|Subject||Characterization of the Dark Exciton and its Uses in|
|Department||Department of Physics||Supervisor||Professor David Gershoni|
|Full Thesis text|
The building block for quantum information processing is a reliable two-level system - a ’qubit.’ A candidate qubit must have a long lifetime and a long coherence time, in which its quantum state is not randomized by stochastic interactions with its environment. In addition, initializing and controlling the qubit must be possible. In this work, we present measurements that show that the dark exciton (DE) in self-assembled quantum dots is an excellent qubit. The DE is composed of an electron-heavy-hole pair with parallel spins. Since its total spin projection is ±2, it is largely optically inactive, and its lifetime is orders of magnitude longer than that of the optically-active bright exciton (BE). We developed an all-optical way to measure the presence of a DE and determine its coherent state. We further developed techniques to deterministically write a DE in any desired coherent superposition of its two eigenstates using picoseconds laser pulses. Using a DE- biexciton transition we demonstrate rotation of the DE spin state around one axis. Arbitrary rotation of the spin state is possible using two rotation pulses and the free precession of the spin. Due to its long life time, optical depletion of the DE from the quantum dot is crucial for high repetition rate experiments. We show that using a joint excited state of the bright and dark exciton we can empty the quantum dot from dark exciton. We use some of the developed techniques to show novel demonstration of a prototype device capable of producing strings of entangled photons in a cluster state.
Photonic cluster states are a resource for quantum computation based solely on single-photon measurements. We demonstrated deterministic generation of long strings of polarization-entangled photons in a cluster state by periodic timed excitation of the precessing quantum dot confined DE. In each period, an entangled photon is added to the cluster state formed by the DE and the previously emitted photons. Our prototype device produces strings of hundreds of photons in which the entanglement persists over five sequential photons. The measured process map characterizing the device has fidelity of 0.81 with that of an ideal device. Further feasible improvements of this device may reduce the resources needed for optical quantum information processing.