Quantum dots are both excellent single-photon sources and hosts for single spins. This combination enables the deterministic generation of Raman-photons—bandwidth-matched to an atomic quantum-memory—and the generation of photon cluster states, a resource in quantum communication and measurement-based quantum computing. GaAs quantum dots in AlGaAs can be matched in frequency to a rubidium-based photon memory, and have potentially improved electron spin coherence compared to the widely used InGaAs quantum dots. However, their charge stability and optical linewidths are typically much worse than for their InGaAs counterparts. Here, we embed GaAs quantum dots into an n-i-p-diode specially designed for low-temperature operation. We demonstrate ultra-low noise behaviour: charge control via Coulomb blockade, close-to lifetime-limited linewidths, and no blinking. We observe high-fidelity optical electron-spin initialisation and long electron-spin lifetimes for these quantum dots. Our work establishes a materials platform for low-noise quantum photonics close to the red part of the spectrum.

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In a multi-electron atom, an excited electron can decay by emitting a photon. Typically, the leftover electrons are in their ground state. In a radiative Auger process, the leftover electrons are in an excited state and a red-shifted photon is created [1][2][3][4] . In a quantum dot, radiative Auger is predicted for charged excitons 5 . Here, we report the observation of radiative Auger on trions in single quantum dots. For a trion, a photon is created on electron-hole recombination, leaving behind a single electron. The radiative Auger process promotes this additional (Auger) electron to a higher shell of the quantum dot. We show that the radiative Auger effect is a powerful probe of this single electron: the energy separations between the resonance fluorescence and the radiative Auger emission directly measure the single-particle splittings of the electronic states in the quantum dot with high precision. In semiconductors, these single-particle splittings are otherwise hard to access by optical means as particles are excited typically in pairs, as excitons. After the radiative Auger emission, the Auger carrier relaxes back to the lowest shell. Going beyond the original theoretical proposals, we show how applying quantum optics techniques to the radiative Auger photons gives access to the single-electron dynamics, notably relaxation and tunnelling. This is also hard to access by optical means: even for quasi-resonant p-shell excitation, electron relaxation takes place in the presence of a hole, complicating the relaxation dynamics. The radiative Auger effect can be exploited in other semiconductor nanostructures and quantum emitters in the solid state to determine the energy levels and the dynamics of a single carrier.

Initial age-related degradation mechanisms for InAs quantum dot lasers grown on silicon substrates emitting at 1.3 µm are investigated. The rate of degradation is observed to increase for devices operated at higher carrier densities and is therefore dependent on gain requirement or cavity length. While carrier localization in quantum dots minimizes degradation, an increase in the number of defects in the early stages of aging can increase the internal optical-loss that can initiate rapid degradation of laser performance due to the rise in threshold carrier density. Population of the two-dimensional states is considered the major factor for determining the rate of degradation, which can be significant for lasers requiring high threshold carrier densities. This is demonstrated by operating lasers of different cavity lengths with a constant current and measuring the change in threshold current at regular intervals. A segmented-contact device, which can be used to measure the modal absorption and also operate as a laser, is used to determine how the internal optical-loss changes in the early stages of degradation. Structures grown on silicon show an increase in internal optical loss, whereas the same structure grown on GaAs shows no signs of increase in internal optical loss when operated under the same conditions.

Our GaAs quantum dots device exhibits ultra-low noise as evidenced by optical linewidths close-to the ideal limit, an elimination of blinking, charge locked by Coulomb blockade, high-fidelity spin initialization, and a long electron-spin lifetime.

In this submission, we discuss the growth of charge-controllable GaAs quantum dots embedded in an n-i-p diode structure, from the perspective of a molecular beam epitaxy grower. The QDs show no blinking and narrow linewidths. We show that the parameters used led to a bimodal growth mode of QDs resulting from low arsenic surface coverage. We identify one of the modes as that showing good properties found in previous work. As the morphology of the fabricated QDs does not hint at outstanding properties, we attribute the good performance of this sample to the low impurity levels in the matrix material and the ability of n- and p-doped contact regions to stabilize the charge state. We present the challenges met in characterizing the sample with ensemble photoluminescence spectroscopy caused by the photonic structure used. We show two straightforward methods to overcome this hurdle and gain insight into QD emission properties.

In a radiative Auger process, optical decay leaves other carriers in excited states, resulting in weak red-shifted satellite peaks in the emission spectrum. The appearance of radiative Auger in the emission directly leads to the question if the process can be inverted: simultaneous photon absorption and electronic demotion. However, excitation of the radiative Auger transition has not been shown, neither on atoms nor on solid-state quantum emitters. Here, we demonstrate the optical driving of the radiative Auger transition, linking few-body Coulomb interactions and quantum optics. We perform our experiments on a trion in a semiconductor quantum dot, where the radiative Auger and the fundamental transition form a Λ-system. On driving both transitions simultaneously, we observe a reduction of the fluorescence signal by up to 70%. Our results suggest the possibility of turning resonance fluorescence on and off using radiative Auger as well as THz spectroscopy with optics close to the visible regime.

Lowering the threshold gain of InAs quantum dot lasers grown on Silicon, significantly extends device lifetime. Measurements on degraded devices show increased optical mode loss is responsible for degradation and a consequent shortening of lasing wavelength. Reliable and efficient electrically-pumped silicon-based lasers are currently required as sources in silicon photonic integrated circuits and ultimately to enable full integration of photonics and electronics. While wafer bonding of Compound Semiconductors (CS) and Silicon is the favoured immediate solution, in the longer term epitaxial growth of CS on silicon is seen as the ideal for large scale manufacturing. Recent results using Quantum Dots (QDs) show promising lifetimes [e.g. [1] [2]] even though epitaxially grown CS Quantum well (QW) laser on Si structures degrade within seconds. It is understood that both the number and size of defects present in a laser structure can increase during operation and this in-turn degrades performance and ultimately shortens the device lifetime. In QW lasers it has been shown that the primary mechanism for degradation is the increased non-radiative recombination rate that accompanies the defects, although an increased internal optical mode loss can also contribute. QDs bring two major advantages, firstly, they can effectively deflect defects or pin them preventing loop formation, and secondly, they localise charge carriers preventing them from diffusing laterally and recombining non-radiatively at a defect site. The latter, reduces the rate of degradation because it suppresses recombination enhanced defect reactions (REDR), a process which can cause dislocation climb, further reducing device performance. QDs are effective at isolating carriers and limiting the recombination at defect sites, however, the optical mode propagates along the entire length of the laser cavity and therefore will always suffer from the increase in dislocations via optical scattering, characterised by the internal optical mode loss. In this study, we test the reliability of deep-etched ridgewaveguide QD lasers grown directly on Si substrates and investigate how factors such as cavity length, and hence carrier density, affect degradation rate whilst running in continuous wave (CW) operation. Using a segmented contact structure, that can be run as either a laser or a single-pass gain structure, we show that measured optical mode loss increases for devices on life test and is a major contributing factor on the eventual degradation of performance in these QD-on-Si lasers. The samples examined here are similar designs to those used in reference [1], but with a larger dot size distribution and consequently somewhat higher threshold current density for similar cavity lengths. However, most importantly the approaches to minimise defects propagating towards the active region, comprising 5 layers of InAs dots each grown in a dot-in-a-well (DWELL) on In0.15Ga0.85As surrounded by a GaAs core, are similar. These include Si(100) wafers with 4° offcut to the [011...