  Using a Josephson junction interferometer (DC SQUID) as a microwave source
for irradiating a single-electron trap, both devices fabricated on the same
chip, we study the process of photon-assisted tunneling as an effective
mechanism of single photon detection. High sensitivity down to a very small
oscillation amplitude $v_{\rm J} \sim$ 10 nV$\ll E_{\rm act} \lesssim hf_{\rm
J}$ and down to low photon absorption rates $\Gamma_{\rm ph} \sim$ (1--50)Hz,
as well as a clear threshold type of operation with an activation energy
$E_{\rm act} \sim$ 400 $\mu$eV, are demonstrated for the trap with respect to
the microwave photons of frequency $f_{\rm J} \sim$ (100--200)GHz. Tunable
generation is demonstrated with respect to the power and frequency of the
microwave signal produced by the SQUID source biased within the subgap voltage
range. A much weaker effect is observed at the higher junction voltages along
the quasiparticle branch of the $I-V$ curve; this response mostly appears due
to the recombination phonons.
