  We present the design, control system, and noise analysis of a 6-axis
seismometer comprising a mass suspended by a single fused silica fibre. We
utilise custom-made, compact Michelson interferometers for the readout of the
mass motion relative to the table and successfully overcome the sensitivity of
existing commercial seismometers by over an order of magnitude in the angular
degrees of freedom. We develop the sensor for gravitational-wave observatories,
such as LIGO, Virgo, and KAGRA, to help them observe intermediate-mass black
holes, increase their duty cycle, and improve localisation of sources. Our
control system and its achieved sensitivity makes the sensor suitable for other
fundamental physics experiments, such as tests of semiclassical gravity,
searches for bosonic dark matter, and studies of the Casimir force.
