# Fully Superconducting Josephson Bolometers for Gigahertz Astronomy
## 6 Conclusions  
This paper reviews a new class of superconducting bolometers owing the possibility of being finely in situ tuned by a bias current. This family of sensors includes the nanoscale transition edge sensor (nano-TES) [33] and the Josephson escape sensor (JES) [34], which take advantage of the strong resistance variation of a superconductor when transitioning to the normal-state. These bolometers employ a one dimensional fully superconducting Josephson junction (1D-JJ) as the active region. On the one hand, this enables the modulation of the \(R(T)\) characteristics of the device by varying \(I\). On the other hand, the lateral superconducting electrodes ensure exponentially suppressed thermal losses, the so-called Andreev mirrors effect.  
The 1D-JJ is theoretically analyzed from an electronic and thermal transport point of view. In particular, the current dependence of the resistance versus temperature characteristics and of the electron-phonon thermalization are predicted. Furthermore, the equations of the main figures of merit of a bolometer, such as the noise equivalent power and response time are calculated both in the nano-TES and JES operations.  
A complete series of electronic and thermal experiments allowed extracting all the parameters of the device active region, such as the current-dependent \(R(T)\) characteristics, the density of states and the electron-phonon thermal relaxation. These data allow determining the performance of the sensing element the nano-TES and the JES while irradiated with continuous radiation, that is as a bolometer. The nano-TES active region reaches a total noise equivalent power of about \(5\times 10^{-20}\) W/\(\sqrt{\rm{Hz}}\) limited by thermal fluctuations, thus suggesting the possibility to produce detectors with higher sensitivity than state-of-the-art conventional transition edge sensors [20]. In addition, the negative electrothermal feedback is expected to ensure a very fast sensing response \(\tau_{eff}\simeq 10\) ns. The JES showed the possibility of in situ tuning the sensitivity by changing the biasing current. The intrinsic thermal fluctuation limitation to NEP can be lowered down to the value of \(\sim\)\(1\times 10^{-25}\) W/\(\sqrt{\rm{Hz}}\). On the contrary, the sensing speed decreases significantly (up to about 100 ms), due to the poor thermalization of the active region when operated  
Figure 6: Deduced properties of a JES. (**a**) Thermal fluctuations limited NEP as a function of the ratio \(I_{C,0}/I\) for two different 1D-JJs. The NEP improves of about six orders of magnitude by increasing \(I\). (**b**) JES bolometer time constant (\(\tau_{1/2}\)) as a function of the ratio \(I_{C,0}/I\) for two different 1D-JJs. The high current operation decreases the speed of the bolometer.  
fully in the superconducting state. Thus, the JES might be the basis for unprecedented sensitive bolometers operating in the GHz range.  
The nano-TES and the JES are expected to have a strong impact on radio astronomy [1; 9]. In particular, studies of the CMB [2; 3; 4], CIB [7; 8], AGNs [6], galaxies [10; 11], comets [13], gigahertz-peaked spectrum radio sources [14], supermassive black holes [15] and radio burst sources [12] need new and hyper-sensitive radiation sensors operating in the gigahertz range. Furthermore, the nano-TES and the JES might be employed as well in imaging for medical purposes [35], quality controls in industrial plants [36] and security scanners [37].  
This research was funded by the European Union's Horizon 2020 research and innovation programme under the grant No. 777222 ATTRACT (Project T-CONVERSE) and under FET-Open grant agreement No. 800923-SUPERTED, and by the CSN V of INFN under the technology innovation grant SIMP.  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
We acknowledge [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] for fruitful discussions.  
The authors declare no conflict of interest.