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 photonassisted tunneling as an effective mechanism of single photon detection. High sensitivity down to a very small oscillation amplitude v J $ 10 nV ( E act Շ hf J and down to low photon absorption rates C ph $ (1-50) Hz, as well as a clear threshold type of operation with an activation energy E act $ 400 leV, is demonstrated for the trap with respect to the microwave photons of frequency f J $ (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. Operation of single-electron tunneling (SET) circuits is known to be significantly influenced by microwave radiation coupled to tunnel junctions. 1 The related phenomena are regularly observed in experiments in the form of noise-induced charge tunneling, and so-called photon-assisted tunneling (PAT) 2 mechanisms have been extensively studied over the last two decades, both in the normal conducting 3,4 and in the superconducting 5-10 systems with tunnel junctions. A straightforward demonstration of PAT in an electron pump has been provided in Ref. 4, where an external source of the highfrequency signal was used. Recently, several studies have been performed which involve compact experimental arrangements combining on the same chip a tunnel-junction-based source of the microwaves with a strongly coupled (also tunnel-junction-based) photon detector. 10,11 On the other hand, a dramatic reduction in the background PAT rates has been achieved with the help of a double-shielded cryogenic sample holder 12 or on-chip line filtering 13 employed for the tunneling experiments.In this letter, we report on the direct observation of the photon activated tunneling of charge in a hybrid Josephson-SET circuit, integrating on the same chip a tunable microwave source and an SET-based single-photon detector coupled to this source via a superconducting coplanar waveguide. The experimental layout is shown in Fig. 1 and includes a DC-SQUID-based Josephson oscillator, a two-wire transmission line, and a two-junction charge trap read out by an SET electrometer. 9 We demonstrate a strong effect of controllable microwave irradiation on the switching statistics in the trap. A comparison of the experimental data with the PAT theory 1,2 clearly demonstrates the quantum nature of interaction between the weak electromagnetic wave and the electron tunneling system. In contrast to our previous experiment reported in Ref. 10, the detector is placed clearly apart from the source, so that the possible acoustic (phonon-mediated) component of the signal (see, e.g., Ref. 14) reaching our detector is deliberately reduced.The oscil...
