Low-noise GaAs Quantum Dots

This thesis describes the development of low-noise GaAs QDs and their applications towards practical single-photon sources. The ultra-low noise in GaAs QDs is made possible with the help of a working $n$-$i$-$p$ diode structure. The design of the diode structure is shown in Chapter 3. Compared to standard GaAs QDs in the bulk, the diode structure allows deterministic control of the QD's charge states. Separate charge plateaus of X$^{1+}$, X$^{0}$ and X$^{1-}$ can be created by an external gate voltage. Near-lifetime-limited linewidths are resolved in resonance fluorescence. Blinking is absent in the emission. Both the near-optimal linewidth and elimination of blinking are consequences of the low level of noise.
Direct evidence of the noise in the diode structure is the characterisation of the photon indistinguishability. The photons from individual GaAs QDs exhibit a high level of indistinguishability. Interestingly, the indistinguishability remains near unity even if the GaAs QD photons are created with a temporal separation of 1$\ \mu$s. The environment of the low-noise GaAs QDs is static over a long timescale.
The low-noise GaAs QDs can be connected via single photons. When interfering the photons created by separate GaAs QDs in a Hong-Ou-Mandel experiment, a visibility of 93% is realised (see Chapter 6). The near-unity visibility is a leap compared to the previous experiments on InGaAs QDs and GaAs QDs. The high visibility paves the way to employing multiple GaAs QDs as single-photon sources for advanced photonic applications. As a proof-of-principle experiment, a controlled-not gate using photons from two separate sources is realised, where high process fidelity as well as entangling ability are demonstrated.
The ultra-low noise in GaAs QDs is ideal for many relevant fundamental studies. In Chapter 4, the radiative Auger process in GaAs QDs is investigated. It was discovered that the radiative Auger process not only takes place in the QD emission but can also be optically addressed. The radiative Auger and the fundamental transitions form a $\Lambda$-system, which might open up the possibility of carrying out Terahertz spectroscopy on single quantum emitters. In Chapter 5, electron-nuclear spin interactions are experimentally studied. In a two-laser experiment, nuclear spins are shown to modify the coherent population trapping condition in the GaAs quantum-dot system depending on either the blue-Zeeman or the red-Zeeman transition is addressed. A detailed theoretical model is still under investigation.