## 1 Introduction  
Great interest in the development of bolometers for radioastronomy applications has grown owing to the discoveries in the study of the cosmic microwave background (CMB) [1, 2]. The primary purpose is to measure the remnant primordial B-mode polarization after the inflation stage of the universe's evolution [3, 4]. The proximate goal is to develop a single-pixel detector with a noise equivalent power (NEP) less than the photon noise over the 40-1000 \(\upmu\)m wavelength for a wide range of optical power loads. The most widespread bolometers are the transition edge sensor (TES) [5] and the kinetic inductance detector (KID) [6]. The capacitively coupled Cold-Electron Bolometer (CEB) [7, 8] is a very attractive option suited for this purpose, since it shows high immunity against cosmic rays [9] due to the extremely small absorber volume and strong decoupling of the electron subsystem from the phonon subsystem. Besides that, the CEB demonstrates effective electron self-cooling of absorber [10] that gives opportunity to achieve the photon-noise-limited operation at 300 mK without dilution refrigerator. Arrays of CEBs were tested already for relatively high optical power load as integrating bolometer [11, 12, 13, 14]. CEB was also proposed as a single-photon counter [15]. The main problem of CEB is matching with SQUID readout due to relatively high resistance to create array of bolometers with multiplexing. The visible way is to create a parallel array of CEBs for decreasing total resistance [16]. In this paper, we report on the responsivity and electrical noise qualifications of the single-pixel CEB for a small optical power load. The uniqueness of the CEB stems from the fact that it uses the heat energy absorbed to cool down the absorbing material, contrary to other detectors where the heat energy bringsthe device to saturation. The absorbed heat energy by the absorber is estimated as the increase in the electron temperature. The CEB concept is based on removing heat from the absorber (Figure 1) [8]. The absorber heat removal, which acts as a negative electrothermal feedback for the incoming signal, is achieved by two methods. The first is through the removal of heat by the SIN tunnel junctions and the dissipation of the removed heat to the superconducting electrodes. The second method of electron cooling is accomplished by adjusting the superconducting electrodes' geometry to provide more volume for the diffusion of the tunneled quasiparticles and by connecting normal metal traps to the superconducting electrodes. The normal metal traps greatly enhance the electron cooling. We previously demonstrated an almost 200 mK decrease in electron temperature with the use of these traps [10; 17]. These modifications considerably improve the noise properties and sensitivity of the device. The CEB performance is best described through the following parameters: the responsivity, the sensitivity or noise equivalent power, and the time constant. In the current-biased mode, the responsivity, \(S_{V}\), is obtained by determining the voltage response, \(\delta V_{\omega}\), to the incident radiative power, \(\delta P_{\omega}\)[18]. \[S_{V}(\omega,I)=\frac{\delta V_{\omega}}{\delta P_{\omega}} \tag{1}\]  
The noise equivalent power (NEP) or the sensitivity is the total of all the noise sources referred to the input of the bolometer with respect to the square root of the bandwidth. The total NEP is composed of three components given by [18]  
\[NEP_{total}^{2}=\frac{\langle\delta V_{\omega}^{2}\rangle_{amp}}{S_{V}^{2}(0, I)}+10k_{B}\Sigma v\left(T_{e}^{6}+T_{ph}^{6}\right)+NEP_{SIN}^{2} \tag{2}\]  
The first expression on the right-hand side is the amplifier NEP, which is defined as the amplifier noise voltage, \(\langle\delta V_{\omega}\rangle_{amp}\), over the square of the voltage response to the incident radiative power of the bolometer, \(S_{V}(0,I)\). The second term is the NEP associated with  
Figure 1: (**a**) Principle of the CEB operation.