# Fully Superconducting Josephson Bolometers for Gigahertz Astronomy
## 1 Introduction  
The universe is permeated by radiation extending over almost all the electromagnetic spectrum, the so-called diffuse extragalactic background radiation (DEBRA) [1]. Particular interest is posed on the portion of the DEBRA covering energies lower than 1 eV, since this range contains information regarding the formation and the evolution of the universe. Indeed, the temperature and the polarization of the cosmic microwave background (CMB) [2; 3; 4], ranging from 300 MHz to 630 GHz, reveal the early stages of the universe and represent the smoking gun proof of the Big Bang. Furthermore, the emission from stars/galaxies formation [5] and the active galactic nuclei (AGNs) [6] gives rise to the cosmic infrared background (CIB) [7; 8], that shows 10% of the power of the CMB. As a consequence, sensitive gigahertz photon detection is fundamental to shed light on different radioastronomy phenomena [9], such as atomic vibrations in galaxy clusters [10], hydrogen atom emission in the galaxy clusters [11], radio burst sources [12], comets [13], gigahertz-peaked spectrum radio sources [14] and supermassive black holes [15]. Figure 1 resumes the frequency and energy range for selected radio-astronomy source in the GHz band. State-of-the-art GHz/THz detectors for astronomy are the transition edge sensors (TESs) [16; 17; 18] and the kinetic inductance detectors (KIDs) [19], since they are extremely sensitive and robust. Indeed, TES-based bolometers reached a noise equivalent power (NEP) of the order of \(10^{-19}\) W/\(\sqrt{\text{Hz}}\)[20], while KIDs showed an NEP of about \(10^{-18}\) W/\(\sqrt{\text{Hz}}\)[21]. To push detection technology towards lower values of NEP, it is necessary a strong reduction of the thermal exchange of the sensor active region, i.e., the portion of the device absorbing the incident radiation, with all the other thermal sinks. To this end, miniaturized superconducting sensors have been realized [22]. Furthermore, several hybrid nano-structuresexploiting the Josephson effect [23] have been proposed and realized [24; 25]. In particular, cold electron bolometers (CEBs) in normal metal/superconductor tunnel systems were experimentally demonstrated to show a \(NEP\sim 3\times 10^{-18}\)\(\mathrm{W}/\sqrt{\mathrm{Hz}}\)[26], while gate-tunable CEBs can be realized by substituting the metal with silicon [27] or graphene [28]. Furthermore, detectors based on proximity effect demonstrated a NEP of the order of \(10^{-20}\)\(\mathrm{W}/\sqrt{\mathrm{Hz}}\) in superconductor/normal metal/superconductor (SNS) junctions [29; 30] and \(7\times 10^{-19}\)\(\mathrm{W}/\sqrt{\mathrm{Hz}}\) in superconductor/graphene/superconductor (SGS) junctions [31]. Finally, an innovative sensor based on the exotic temperature-to-phase conversion (TPC) in a superconducting ring is expected to provide a \(NEP\sim 10^{-23}\)\(\mathrm{W}/\sqrt{\mathrm{Hz}}\)[32]. Despite the high sensitivity, all these innovative bolometers do not fulfill the strict requirements for being employed in ultra-sensitive space telescopes. In fact, materials, fabrication procedures and structures need to be extensively tested to survive the extreme stress (vibrations, acceleration, radiation, etc.) typical of space missions. Instead, we review recent developments of superconducting radiation sensors based on a structure that shares many technological aspects with the already employed TESs and KIDs. Indeed, the nanoscale transition edge sensor (nano-TES) [33] and the Josephson escape sensor (JES) [34] are envisioned by downsizing the active region of a TES to the nano-scale. In particular, they employ a one dimensional fully superconducting Josephson junction (1D-JJ) as radiation absorber. In full analogy with the widespread TES, these sensors take advantage of the stark variation of the resistance of a superconductor while transitioning to the normal-state. Furthermore, the resistance versus temperature characteristics of a 1D-JJ can be tuned by varying the bias current.