Coherent light-matter interaction in semiconductor quantum dots
Zhe Xian, Koong
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Coherent light-matter interaction allows for population control of a single quantum emitter. Detection of the photons emitted immediately after the interaction is equivalent to reading out the state of the emitter. Frequency and time-domain measurements on these photons reveal the information about the coherence of the emitter, typically imprinted as the indistinguishability of the scattered photons. This thesis focuses on the optical spectroscopy of semiconductor quantum dots at cryogenic temperature (4 K) under coherent driving. By analyzing the coherence and the statistics of the scattered photons, the population inversion, the fundamental dephasing mechanisms, and the coherent coupling amongst emitters can be probed. First, the experimental data indicates that the coupling of the atom-like transitions to the vibronic transitions of the crystal lattice is independent of the driving strength, even for detuned excitation using the spin-Λ configuration. This imposes a fundamental limit to the coherence of the photons emitted from solid-state emitters. Next, the coherent dynamics of a two-level quantum emitter driven by a pair of symmetrically detuned phase-locked pulses is studied. The spectroscopic results of a solid-state two-level system show that coherent population control and a large amount of population inversion are possible using asymmetric dichromatic excitation, which is achieved by adjusting the relative weighting between the red- and blue-detuned pulses. Furthermore, this technique can be extended to multi-level systems like the biexciton-exciton cascade, such that a pair of suitably detuned laser pulses, each resonant to the biexciton-exciton or the exciton-ground state transition, can be used to achieve population inversion from the ground state to the excited (biexciton) state. In addition, coherent control of cooperative emission arising from two distant but indistinguishable solid-state emitters due to path erasure is demonstrated via the results from the photon correlations, measured with Hanbury Brown-Twiss and Hong-Ou-Mandel interferometers. Finally, applications of these single-photon emitters integrated in deterministically-positioned nanowires and micropillar cavities are discussed. The former allows for the demonstration of the parallel spectroscopy of up to 7 emitters using a multi-core-fiber-based confocal microscope. In the latter case, the coherence, indistinguishability as well as photon-number distribution of the scattered photons from a neutral exciton resonantly coupled to the cavity resonance are characterized, before they are converted to the telecommunication C-band via quantum frequency conversion. With these photons, the single-photon BB84 protocol is implemented and a secure key rate of ∼ 1 kHz after propagating through 150 km of optical fiber is observed. This constitutes a key step towards integration of this single-photon source for fiber-based quantum networking.