  We report on the design and sensitivity of a new torsion pendulum for
measuring the performance of ultra-precise inertial sensors and for the
development of associated technologies for space-based gravitational wave
observatories and geodesy missions. The apparatus comprises a 1 m-long, 50
um-diameter, tungsten fiber that supports an inertial member inside a vacuum
system. The inertial member is an aluminum crossbar with four hollow cubic test
masses at each end. This structure converts the rotation of the torsion
pendulum into translation of the test masses. Two test masses are enclosed in
capacitive sensors which provide readout and actuation. These test masses are
electrically insulated from the rest of the cross-bar and their electrical
charge is controlled by photoemission using fiber-coupled ultraviolet light
emitting diodes. The capacitive readout measures the test mass displacement
with a broadband sensitivity of 30 nm / sqrt(Hz), and is complemented by a
laser interferometer with a sensitivity of about 0.5 nm / sqrt(Hz). The
performance of the pendulum, as determined by the measured residual torque
noise and expressed in terms of equivalent force acting on a single test mass,
is roughly 200 fN / sqrt(Hz) around 2 mHz, which is about a factor of 20 above
the thermal noise limit of the fiber.
