# THz Range Low-Noise SIS Receivers for Space and Ground-Based Radio Astronomy +
## 1 Introduction  
The development of ultra-sensitive terahertz (THz) receivers is nowadays one of the most intensively and successfully explored areas of superconducting electronics [1; 2]. Superconducting elements offer an extremely high characteristic frequency and very strong nonlinearity. This makes it possible to develop systems for receiving a THz signal with unique parameters unattainable for devices based on other principles. Many applications require a spectral resolution of \(\Delta t/t\)-\(10^{6}\); this can only be achieved with heterodyne receiving systems. The heterodyne mixer down-converts the weak input signal of interest to a lower intermediate frequency (IF) without loss of phase; the spectrum of IF signal is the same as the input one, but is shifted down in frequency by LO frequency. A heterodyne receiver is sensitive to two input frequency bands, spaced on either side of the LO frequency by the IF--double sideband (DSB) mode. The signal of interest usually appears in only one of these bands (signal band); unwanted noise and spurious radiation received in the other sideband (image) can be suppressed by eliminating image frequencies--single sideband (SSB) mode. Currently, the most developed are two types of superconducting mixers: SIS mixers [1; 2; 3; 4; 5; 6] for frequencies from 0.2 to 1.2 THz and hot-electron bolometer (HEB) mixers [7; 8; 9; 10; 11] for frequencies of 1-5 THz. Both types of mixers are mainly used in radio astronomy, where low receiver noise is a critical issue. The parameters requested by astronomers can only be realized using superconducting mixing elements with extremely low intrinsic noise, determined by the nature of the elements and cryogenic operating temperature. The mixers based on SIS tunnel junctions are the most developed input devices for heterodyne receivers at frequencies \(f\) from 0.1 to approximately 1.2 THz. The sharp nonlinearity in the tunneling current of the SIS junction is used for the mixer operation; this nonlinearity results from a single-electron tunneling process between two superconductors separated by a very thin oxide insulating layer. To describe this tunneling current under the influence of a local oscillator (LO) signal a quantum-mechanical description is required [3; 4].This process is known as photon assisted tunneling [4], as a result quasi-particle steps appear in the SIS current-voltage characteristic (IVC). Intrinsically the SIS mixers can exhibit conversion gain; important advantages of such mixers are a low LO power requirement and very low intrinsic noise [1; 2; 3; 4; 5]. Mixer noise temperature (DSB) is limited by the quantum value \(\mathit{hf}/(2k_{B})\)[6], where \(h\) and \(k_{B}\) are the Plank and Boltzmann constants, respectively. The SIS mixers were already successfully used both for space missions like Hershel HIFI [12] and for the ground-based telescopes (including the largest ALMA multi-element interferometer [13], the European NOEMA interferometer [14], and the APEX telescope [15]). The development of superconducting electronics makes it possible to create a superconducting integrated receiver (SIR) [16], which combines all receiver components in one microcircuit: a planar antenna, an SIS mixer, a superconducting local oscillator based on a flux-flow oscillator (FFO), and a SIS harmonic mixer for FFO phase locking. SIR sensitivity and spectral resolution were demonstrated by s atmospheric limb sounding on board a high-altitude balloon [17]. SIR has been successfully implemented in the laboratory for detailed spectral measurements of THz radiation emitted from the BSCCO mesa at frequencies up to 750 GHz [18], as well as for recording sub-THz radiation of the human body under physiological stress [19]. The Russian space observatory \"Millimetron\" [20] with a 10-m coolable telescope, a successor of the Russian ground-space interferometer \"Radioastron\", is designed to study different objects in the universe, including black holes, at sub-THz and THz frequencies.