  We propose a passive single-photon detector based on the bipolar
thermoelectric effect occurring in tunnel junctions between two different
superconductors thanks to spontaneous electron-hole symmetry breaking. Our
thermoelectric detector (TED) converts a finite temperature difference caused
by the absorption of a single photon into an open circuit thermovoltage.
Designed with feasible parameters, our TED is able to reveal single-photons of
frequency ranging from about 15 GHz to about 150 PHz depending on the chosen
design and materials. In particular, this detector is expected to show values
of signal-to-noise ratio SNR about 15 at {\nu} = 50 GHz when operated at a
temperature of 10 mK. Interestingly, this device can be viewed as a digital
single-photon detector, since it generates an almost constant voltage VS for
the full operation energies. Our TED can reveal single photons in a frequency
range wider than 4 decades with the possibility to discern the energy of the
incident photon by measuring the time persistence of the generated
thermovoltage. Its broadband operation suggests that our TED could find
practical applications in several fields of quantum science and technology,
such as quantum computing, telecommunications, optoelectronics, THz
spectroscopy and astro-particle physics.
