  For the purposes of measuring the high-frequency complex conductivity of
correlated-electron materials at low temperatures and high magnetic fields, a
method is introduced for performing coherent time-domain terahertz spectroscopy
directly in the cryogenic bore of existing dc and pulsed magnets. Miniature
fiber-coupled THz emitters and receivers are constructed and are demonstrated
to work down to 1.5 Kelvin and up to 17 Tesla, for eventual use in higher-field
magnets. Maintaining the sub-micron alignment between fiber and antenna during
thermal cycling, obtaining ultrafast (${<200}$~fs) optical gating pulses at the
end of long optical fibers, and designing highly efficient devices that work
well with low-power optical gating pulses constitute the major technical
challenges of this project. Data on a YBCO superconducting thin film and a high
mobility 2D electron gas is shown.
