Ph.D Student | Schmidgall Emma |
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Subject | Quantum Optics with the Quantum-Dot-Confined Dark Exciton |

Department | Department of Physics |

Supervisor | Professor David Gershoni |

On-demand initialization and resetting of physical two-level systems (qubits) and their coherent control are basic requirements for quantum information processing. Single half-integer spin-based qubits--such as nuclei, atoms, electrons, and holes--are by far the most studied matter qubit systems. Since these spins are inherently degenerate, their utilizations require application of external magnetic fields, their initialization requires a series of repeated operations, and their full control cannot be achieved on shorter times than their Larmor precession period.

In this thesis, we investigate the feasibility of using quantum dot confined spins as qubits. Self-assembled semiconductor quantum dots are nanometer-sized islands of semiconductor material that confine charge carriers, such as single electrons or single holes, in a three-dimensional potential well. This confinement results in an atom-like set of discrete energy states, due to which these quantum dots are often called ``artificial atoms." The specific quantum dot confined spin qubit that we will explore is the dark exciton (DE). A dark exciton (DE) is an electron-hole pair where the spin projections of the two carriers are parallel. The DE has a total integer spin of 2, with projections of ±2 on the quantum dot growth axis, reflecting the difference in angular momentum and spin between the conduction and valence band states. Since photons barely interact with electronic spin, the DE is almost optically inactive and has a lifetime that is more than three orders of magnitude longer than that of the bright exciton.

We demonstrate how the spin state and population of the DE can be probed optically. We then use this probe technique to measure the coherence time of the DE spin state. We also demonstrate deterministic writing of the DE spin state using single optical pulses and picosecond pulse control of the DE spin state after photogeneration. We present a method for optical depletion of the DE from a quantum dot, increasing the efficiency of a quantum dot single photon source by nearly a factor of two. Finally, we demonstrate entanglement between the DE and a single photon, using the DE as an ``entangler'' to generate a sequence of polarization entangled photons in a cluster state. Combined with the long lifetime of the DE, these results clearly demonstrate that the DE forms an excellent matter qubit.

Additionally, we use some of the techniques and methods developed for the DE experiments to explore the three-photon radiative cascade beginning from the triexciton (three electron-hole pairs) and demonstrate how this cascade can be used to probe non-radiative relaxation processes in quantum dots.