Solid-state systems which mimic two-level atoms are being actively developed. Improving the quantum coherence of these systems, for instance spin qubits or single photon emitters using semiconductor quantum dots, involves dealing with noise. The sources of noise are inherent to the semiconductor and are complex. 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. We investigate noise in an ultra-pure semiconductor using a minimally-invasive, ultra-sensitive, local probe: resonance fluorescence from a single quantum dot. We distinguish between charge noise and spin noise via a crucial difference in their optical signatures. Noise spectra for both electric and magnetic fields are derived. The noise spectrum of the charge noise can be fully described by the fluctuations in an ensemble of localized charge defects in the semiconductor. We demonstrate the "semiconductor vacuum" for the optical transition at frequencies above 50 kHz: by operating the device at high enough frequencies, we demonstrate transform-limited quantum dot optical linewidths

Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-innanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells. S emiconductor quantum dots have been shown to be excellent building blocks for quantum photonics applications, such as single-photon sources and nano-sensing. Desirable properties of a single-photon emitter include high-fidelity anti-bunching (very small g 2 (t = 0)), narrow emission lines (ideally transform limited to a few microelectronvolt) and high brightness (>1 MHz count rate on standard detector). For simplicity, these properties should be achieved either with electrical injection or non-resonant optical excitation. Desirable properties of a nano-sensor include a high sensitivity to local electric and magnetic fields, with the quantum dot located as close as possible to the target region. A popular realization involves Stranski-Krastanow InGaAs quantum dots embedded in a three-dimensional matrix, which are excellent building blocks for the realization of practical singlephoton sources 1 . However, the photon extraction out of the bulk semiconductor is highly inefficient on account of the large mismatch in refractive indices of GaAs and vacuum. An attractive way forward is to embed the quantum dots in a nanowire 2 . To solve the extraction problem, the nanowire is designed to operate as a single-mode waveguide, a so-called photonic nanowire, with a taper as photon out-coupler 3 . Also, for nano-sensing applications, a quantum dot in a nanowire can be located much closer to the active medium. Top-down fabrication of the photonic waveguide is technologically complex, however. Bottom-up fabrication of the photonic waveguide is very attractive 4-6 , but it is at present challenging to self-assemble quantum dots in the nanowires with narrow linewidths and high yields 7,8 . Nano-sensing applications are at present not highly developed. Other degrees of freedom of the quantum-dot-in-nanowire system that can be usefully exploited are the mechanical modes ...

Developing a quantum photonics network requires a source of very-high-fidelity single photons. An outstanding challenge is to produce a transform-limited single-photon emitter to guarantee that single photons emitted far apart in the time domain are truly indistinguishable. This is particularly difficult in the solid-state as the complex environment is the source of noise over a wide bandwidth. A quantum dot is a robust, fast, bright and narrow-linewidth emitter of single photons; layer-by-layer growth and subsequent nano-fabrication allow the electronic and photonic states to be engineered. This represents a set of features not shared by any other emitter but transform-limited linewidths have been elusive. Here, we report transform-limited linewidths measured on second timescales, primarily on the neutral exciton but also on the charged exciton close to saturation. The key feature is control of the nuclear spins, which dominate the exciton dephasing via the Overhauser field.