# Detection of On-Chip Generated Weak Microwave Radiation Using Superconducting Normal-Metal SET
## 1 Introduction  
Over the past few decades, rapid progress in nanofabrication has brought up a scope of very sensitive mesoscopic superconducting circuits operating with single quanta of physical quantities. These advances are promising for applications in electrical metrology and quantum information technology, for example, in single-electron pumping [1; 2; 3], which is the basic mechanism for the electron-counting capacitance standard [4; 5] and a quantum current standard [6]. In the superconducting qubits, single charge, single flux or single photon states interact to give rise to a variety of quantum algorithms [7; 8]. For accurate operation, the single-quantum circuits must be protected from stray electromagnetic radiation (emitted, for example, by warmer parts of the cryogenic setup) down to the utmost level of single microwave photons. Unwanted photon-induced processes were found to be responsible for the deterioration of the pumping performance by many orders of magnitude: the achieved relative uncertainty of electron transfer \(\epsilon\sim 10^{-8}\) greatly exceeded the theoretical prediction \(\epsilon\sim 10^{-20}\)[9]. A similar effect was also demonstrated in studies with a hybrid single-electron turnstile [10; 11]. Significant improvement has been recently reported for the retention time of a single-electron trap using a neatly designed mK-setup with double shielding of the sample space [12]. Furthermore, the radiation-induced generation of quasiparticles is considered to be an important decoherence mechanism in the superconducting qubit systems (see [13] andreferences therein). Therefore, studying the interaction mechanisms of quantum devices with microwave radiation and independent characterization of the microwave background of a sample at mK temperatures is of significant importance. Single microwave photon detection and spectroscopy using small tunnel junctions have been the subject of various studies (see, e.g., [14; 15; 16; 17; 18; 19]). In particular, the Coulomb blockade circuits built using Single Electron Transistors (SETs) were found to be efficient radiation detectors down to single-photon sensitivity levels in the relevant frequency range around \(f\sim 100\) GHz. Recently, appreciable microwave sensitivity has been envisaged for a Normal metal-Superconductor-Normal metal (NISIN, \"I\" stands for Insulating barrier) SET with a superconducting island [20], which we denote here as an \"NISIN detector\". A remarkable advantage of this detector appears to be due to an intrinsic signal amplification effect: each photon-excited quasiparticle in the superconducting island launches a sequence of tunneling events with many (\(\sim\)\(10^{2}\div 10^{3}\)) electrons transferred, thus boosting the device sensitivity. Up to now, the photon-assisted tunneling manifested itself in the NISIN SET as non-vanishing tunnel currents at low bias voltages in the subgap range, \(V_{\rm b}<2\Delta/e\), where \(\Delta\) is the energy gap of the superconducting island [10]. A more systematic study is, however, needed for determining the response characteristics of the detector, for which purpose weak microwave radiation with a controllable spectrum should be generated and exposed to the detector. In the present work, we report on the radiation detection peculiarities of an NISIN detector included in a complete microwave circuit with a Josephson-junction-based microwave generator coupled to the detector via the specially designed transmission line. A clear response even to very weak microwave signals has been observed and demonstrated to be tunable both in power and frequency. A high amplification factor could be evaluated through comparison of the experimental response with the calculated rates of photon-assisted tunneling of quasiparticles into the superconducting island. The frequency dependence of the detected microwave signal was investigated experimentally using two different types of coupling element.