  We fabricated and tested a squelch circuit consisting of a copper powder
filter with an embedded Josephson junction connected to ground. For small
signals (squelch-ON), the small junction inductance attenuates strongly from DC
to at least 1 GHz, while for higher frequencies dissipation in the copper
powder increases the attenuation exponentially with frequency. For large
signals (squelch-OFF) the circuit behaves as a regular metal powder filter. The
measured ON/OFF ratio is larger than 50dB up to 50 MHz. This squelch can be
applied in low temperature measurement and control circuitry for quantum
nanostructures such as superconducting qubits and quantum dots.
