  Cryogenic confocal microscopy is a powerful method for studying solid state
quantum devices such as single photon sources and optically controlled qubits.
While the vast majority of such studies have been conducted at temperatures of
a few Kelvin, experiments involving fragile quantum effects often require lower
operating temperatures. To also allow for electrical dynamic control, microwave
connectivity is required. For polarization-sensitive studies, free space
optical access is advantageous compared to fiber coupling. Here we present a
confocal microscope in a dilution refrigerator providing all the above features
at temperatures below 100 mK. The installed high frequency cabling meets the
requirements for state of the art spin qubit experiments. As another unique
advantage of our system, the sample fitting inside a large puck can be
exchanged while keeping the cryostat cold with minimal realignment. Assessing
the performance of the instrument, we demonstrate confocal imaging,
sub-nanosecond modulation of the emission wavelength of a suitable sample and
an electron temperature of 76 mK. While the instrument was constructed
primarily with the development of optical interfaces to electrically controlled
qubits in mind, it can be used for many experiments involving quantum
transport, solid state quantum optics and microwave-optical transducers.
