## 4 Measurement Results
It can be seen that the noise level does not change due to rather weak external signal power (on the order of 1 pW, which was got from fitting as in [23]), received by the CEB, and narrow spectral linewidth of YBCO oscillator, operating at 4 K temperature. The total voltage noise of a bolometer \(\delta V_{\text{tot}}\) consists of three components: the noise of the amplifier \(\delta V_{\text{amp}}\), the noise of the bolometer itself \(\delta V_{\text{CEB}}\) and the noise of an external signal \(\delta V_{\text{ex}}\), thus \(\delta V_{\text{tot}}{}^{2}=\delta V_{\text{amp}}{}^{2}+\delta V_{\text{CEB}} {}^{2}+\delta V_{\text{ex}}{}^{2}\). Measured total noise (pedestal level) in the absence of an external signal at the frequency of 120 Hz is 8.5 nV/sqrt(Hz) (see Figure 5). Taking into account that the amplifier noise \(\delta V_{\text{amp}}=5\) nV/sqrt(Hz), this means that the intrinsic noise of the bolometer \(\delta V_{\text{CEB}}=6.9\) nV/sqrt(Hz) is slightly larger  
Figure 4: The amplitude-frequency characteristics of cold-electron bolometers with different capacitance C and lengths L of coplanar lines. Solid curves—rectangular coplanars, dashed curves—rounded coplanars, crosses—theory for the parameters close to the experiment for green solid curve with diamonds. than the amplifier noise \(\delta\mathrm{V_{amp}}\). When an external high-frequency signal is applied, the total noise of the bolometer may increase due to the amplitude and phase noise of the external generator (as it was in our experiments with the backward wave oscillator [15]) or due to photon noise \(\delta\mathrm{V_{ph}}\). The absence of changes in our case, Figure 5, indicates high stability of the source based on the high-temperature YBCO Josephson junction. On the other hand, we also do not observe the contribution of photon noise. It was shown [23] that for a power load of 5 pW the total noise-equivalent power of a single CEB with room temperature amplifiers AD745 equals \(10^{-16}\) W/sqrt(Hz) and is larger than photon NEP. That is why we did not see noise growth in our experiments. Finally, on the basis of this single cell, we are developing an array of resonant CEBs which are able to work in the photon noise limited regime for larger signal powers. Such regime was demonstrated in our previous works [7; 8] for parallel-series arrays of cold-electron bolometers [26]. Multi-Cell Double-Folded Slot Antenna with Coplanar Lines and CEBs for Ancillary Frequency Channels of LSPE  
In this section, we consider rounded arrays of double-folded slot antenna cells with resonant cold-electron bolometers for secondary frequency channels of SWIPE instrument, designed for the LSPE project. These frequency channels are set to 220 and 240 GHz with the required bandwidths around 5% for both channels and intended for receiving signals from a horn. It is quite a challenging task to create such a narrowband array of antenna cells for so high frequency. While there are works on antenna metasurfaces [27; 28; 29; 30; 31; 32; 33], normally the resulting bandwidths are much above 10%. In difference with standard metasurfaces, where the signal from all antennas is fed to a single receiver, due to the small size of CEBs of order 1 um, it is possible to place the bolometer inside each antenna in the array, so we deal with the antenna array with distributed receivers. Here we show that certain progress in narrowband arrays is possible with double-folded slot cells, where a coplanar line is used for better impedance matching. Previously, similar design of a current-biased array of double-slot cells with coplanar lines showed rather good results for creating a narrowband amplitude-frequency characteristic, as shown in simulations [20]. Now these antenna cells, grouped into rounded array, are connected in parallel with DC wires to work in a voltage-biased mode. DC wires are  
Figure 5: Voltage noise of CEB under the influence of YBaCuO signal and in the absence of external GHz radiation.