  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.
