Charge noise and spin noise in a semiconductor quantum device

Self-assembled quantum dots are very attractive as the building blocks for quantum light sources and
spin qubits. For instance, a single quantum dot is a robust, fast, narrow-linewidth source of single
photons, features not shared by any other emitter. A spin qubit is implemented by a single electron
or hole confined to a quantum dot. Fundamental quantum mechanics have been explored in
experiments with single quantum dots and spectacular success has been achieved. Future
developments however demand an enhanced quantum coherence. For instance, indistinguishable
single photons and coherent spins are required to implement a quantum repeater. For quantum-dot-
based single photon sources, the linewidths are in the best case typically a factor of two larger than
the transform limit in which the linewidth is determined only by the radiative decay time. Photons
generated far apart in the time domain are therefore not indistinguishable. Spin coherence is
presently limited to microsecond timescales. Improving the quantum coherence involves dealing
with the noise inherent to the device. Charge noise results in a fluctuating electric field, spin noise in
a fluctuating magnetic field at the location of the qubit, and both can lead to dephasing and
decoherence of optical and spin states. Here, the noise and strategies to circumvent its deleterious
effects are explored in order to optimize the performance of solid-state quantum systems.
This thesis is divided into five parts. The first chapter describes in detail the main experimental tool
to explore noise in the solid-state: resonance fluorescence from single quantum dots. A polarization-
based dark-field microscope is realized allowing background-free resonance fluorescence detection
while operating in a set-and-forget mode.
Chapter 2 investigates charge fluctuations in a semiconductor. The origin of the main source of
charge noise in the commonly used optical field-effect devices is pinned down: charge fluctuations at
a GaAs/AlAs interface nearby the quantum dots. These defects are moved further away from the
quantum dots in an improved sample design resulting in close-to-transform limited optical
linewidths.
Even with the improved heterostructures, the transform limit is not reached. Noise spectra of both
charge noise and spin noise provide powerful insights into the noise inherent to the semiconductor,
discussed in chapter 3. A time trace of the resonance fluorescence from a single quantum dot is
translated into a noise spectrum. A crucial difference in their optical signatures allows the nature of
the noise, charge or spin, to be identified. The charge noise is centred at low frequencies, the spin
noise is centred at high frequencies. This technique is able to reveal the entire spectrum of the spin
noise. The combined noise falls rapidly with frequency becoming insignificant above 50 kHz for the
quantum dot optical transition as signalled by transform-limited linewidths.
The low frequency noise, charge noise, results in considerable noise in the emission frequency of the
single photons. This problem is solved in chapter 4 with a dynamic feedback technique that locks the
quantum emission frequency to a reference. The charge noise and its deleterious effects are highly
reduced. A frequency-stabilized source of single photons in the solid-state is realized.
The low frequency linewidths are in the best case typically a factor of two larger than the transform
limit. It is shown in chapter 5 that spin noise in the host material is the dominant exciton dephasing
mechanism. This applies to both the neutral and charged excitons. For the neutral exciton, the spin
noise increases with increasing excitation power. Conversely for the charged exciton, spin noise
decreases with increasing excita­tion power. This effect is exploited to demonstrate transform-
limited linewidths for the charged exciton even when the measurement is performed very slowly.