Coherent light-matter interaction in semiconductor quantum dots
Abstract
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.