  We propose a new phase detection technique based on a flux-switchable
superconducting circuit, the Josephson digital phase detector (JDPD), which is
capable of discriminating between two phase values of a coherent input tone.
When properly excited by an external flux, the JDPD is able to switch from a
single-minimum to a double-minima potential and, consequently, relax in one of
the two stable configurations depending on the phase sign of the input tone.
The result of this operation is digitally encoded in the occupation probability
of a phase particle in either of the two JDPD wells. In this work, we
demonstrate the working principle of the JDPD up to a frequency of 400 MHz with
a remarkable agreement with theoretical expectations. As a future scenario, we
discuss the implementation of this technique to superconducting qubit readout.
We also examine the JDPD compatibility with the single-flux-quantum
architecture, employed to fast-drive and measure the device state.
