Bolometers for balloon and space missions have seen extensive development because of their capacity to test primordial conditions of the Universe. The major improvements consist in lowering the operating temperature to reach higher sensitivities. Here we show that an array of 192 cold-electron bolometers (CEB) demonstrates photon-noise-limited operation at the cryostat temperature of 310 mK due to effective self-cooling of the absorber. The direct electron cooling of nanoabsorber placed between normal metal-insulator-superconductor junctions has considerably higher efficiency than indirect cooling through massive suspended platform, that requires overcoming a weak electron-phonon conductance. The electron temperature reached 120 mK without a power load, and 225 mK with a 60 pW power load with self-noise of a single bolometer below 3 Á 10 À18 W Hz À1=2 at a 0.01 pW power load. This bolometer works at electron temperature less than phonon temperature, thus being a good candidate for future space missions without the use of dilution refrigerators.

We have measured a response to a black body radiation and noise of the cold-electron bolometers. The experimental results have been fitted by theoretical model with two heat-balance equations. The measured noise has been decomposed into several terms with the help of theory. It is demonstrated that the photon noise exceeds any other noise components, that allows us to conclude that the bolometers see the photon noise. Moreover, a peculiar shape of the noise dependence on the absorbed power originates completely from the photonic component according to the theory. In the additional experiment on heating of the cryostat plate together with the sample holder we have observed nearly independence of the noise on the electron temperature of the absorber, which has provided another proof of the presence of the photon noise in the first experiment.The current cosmological experiments apply very strict requirements to the detectors installed on telescopes. The Cosmic Microwave Background (CMB) is still the target of many Cosmology experiments. The main goal of the current cosmology is the understanding if an inflationary process occurred when the universe was about 10 −37 s old. Many theoretical models predict that this process should have left footprints of it in the form of a very small polarized signal called B-modes. Detecting this signal would provide important information about the primordial universe and the high energy physics.The recent joined analysis of BICEP2 experiment and Planck satellite results confirm that BICEP2 didn't detect any B-modes [1]. This work has demonstrated the stringent necessity of developing sensitive multifrequency CMB experiments to correct the observations from the dust contributions. Cold-electron bolometers [2,3] are promising detectors for cosmological applications, as they have all the qualities necessary to perform tasks: such as the high sensitivity to terahertz radiation and immunity to cosmic rays [4].The purpose of this work is experimental demonstration that cold-electron bolometers have an ultimate sensitivity, i.e. sensitive to the photon noise. Photon noise is fluctuations, inevitably present in any radiation due to the discrete nature of the photons. The photon noise turns into a voltage noise of the detector, multiplied by the volt-watt response S V of the receiver. Ideally, all the other components of the detector noise, including an internal noise, must be smaller than the photonic component. If this condition is met, the detector is limited by the photon noise. Experimental demonstration of the photon noise is necessary for the installation of this type of bolometers on telescopes.In this paper we present the results of optical experiments with a parallel-series arrays of cold-electron * Electronic address: anna.gord@list.ru bolometers (CEBs) [5,6]. In order to obtain the main characteristic of detectors -the noise equivalent power (NEP), one needs to know the absorbed power. At least three different approaches are used to determine the absorbed power in cold electron...