Fast Digital Phase Detection of a Coherent Tone At GHz Frequencies

We present a detection technique based on a flux-switchable Josephson Digital Phase Detector (JDPD), which is capable of discriminating between two phase values of a coherent input tone. When suitably flux stimulated, the JDPD switches from a single-minimum to a double-minima potential and, as a result, relaxes in one of two stable configurations depending on the phase sign of the input stimulus. The outcome of this procedure is digitally encoded in the occupation probability of the phase particle in either of the two JDPD wells. In this manuscript, we demonstrate the working principle of the JDPD by discriminating the phase of a 5 GHz tone with a protocol of 300 ns of time duration. This result supports the future implementation of the JDPD to readout the state of a qubit.