טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentSchwartz Ido
SubjectLife and Coherence Times of the Dark Exciton in
Semiconductor Quantum Dots
DepartmentDepartment of Physics
Supervisor Professor David Gershoni
Full Thesis textFull thesis text - English Version


Abstract

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 this work, we present measurements and an analytical model that describe such a system - the dark exciton (DE). The DE is composed of an electron-heavy-hole pair with parallel spins confined inside a semiconductor quantum dot. 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). Photoluminescence taken at very low excitation intensities allows us to detect a very weak emission from the DE spectral line and thus compare it to the already known properties of the BE. An additional way to determine the DE lifetime is to excite the QD with pulses and observe the DE population at later times, similar to 'classic' lifetime measurements. However, instead of emission from radiative recombination, we observe resonant absorption to a biexcitonic transition originated from the DE. 

Finally, we developed an all-optical way to measure the presence of a DE and determine its coherent state. The detection of a photon emitted during an optical transition at a certain energy and polarization provides information on both the initial and final states of the transition. The time-dependent correlation between two such detections, i.e. the probability of detecting a photon from one transition conditional on the detection of a photon from another transition at a given earlier time which determines initial conditions, can thus provide information about the dynamics of the system between the two photon detections. Detecting a circularly-polarized photon emitted from a spin-blockaded biexciton state heralds the generation of a dark exciton as a coherent superposition of its two non-degenerate eigenstates. Then the absorption of a photon resonant to the same biexcitonic transition is dependent on the relative angle between the DE spin direction and the polarization of the exciting laser pulse. The absorption magnitude is then directly deduced from the intensity of the emission resulting from the biexciton recombination.

            These measurements determined lower bound of about 5 microseconds and 100 nanoseconds for the lifetime and the coherence time of the DE, respectively. To the best of our knowledge the coherence time of the DE has been measured here for the first time.