  We introduce and experimentally characterize a superconducting
single-sideband modulator compatible with cryogenic microwave circuits, and
propose its use for frequency domain multiplexing of superconducting qubit
readout. The monolithic single-quadrature modulators that comprise the device
are formed with purely reactive elements (capacitors and Josephson junction
inductors) and require no microwave-frequency control tones. Microwave signals
in the 4 to 8 GHz band, with power up to -85 dBm, are converted up or down in
frequency by as much as 120 MHz. Spurious harmonics in the device can be
suppressed by up to 25 dB for select probe and modulation frequencies.
