## 3 Experimental Results  
In Figure 2, the current-voltage characteristics of CEBs are shown for the case of no signal (no bias of YBCO oscillator) and the case under YBCO JJ power load. Bolometer response is defined as a voltage difference at a fixed bias current through the CEBs array with and without the JJ radiation. Based on the response characteristic (the inset of Figure 2) the bias current for the bolometer has been chosen aside from the maximum response since there is a minimal value of noise-equivalent-power [3]. Changing the current through the Josephson junction (for zero current through the coil, creating the magnetic field), the signal from the bolometer and the voltage on the YBCO oscillator (which is connected with the oscillation frequency via the Josephson relation) have been measured, see Figure 3. The detected signal consists of two components. The first one, smooth, corresponds to infrared radiation from an overheated Josephson junction and magnetic coil. It can be well described by a spline line \(V_{CEB}=aV_{YBCO}^{2}+bV_{YBCO}+c\) (dashed curve in Figure 3). The second component, uneven, is a generation signal at the Josephson frequency \(f=2 eV_{YBCO}/h\) and is sensitive to a magnetic field. To reduce the first component, the measurements were carried out as quickly as possible, which was allowed without loss of accuracy thanks to the fast response of CEBs. The adjustment of JJ bias and  
Figure 1: The view of the experimental setup, its schematic representation and samples design. measurement time of the bolometer response to the JJ radiation of a given frequency is milliseconds, while the duration of the thermal response due to system overheating is seconds. Subtracting the component of the background heating signal, the response of the bolometer to the YBCO JJ radiation as a function of frequency was obtained (blue solid curve in Figure 4, marked as YBCO, for the case of zero magnetic field when the only bias current through JJ was varied). It is seen that the response consists of several peaks with different heights. Comparing it with the CEBs amplitude-frequency characteristic (AFC) obtained by the calibrated backward-wave oscillator (BWO) source (green symbols in Figure 4), one can conclude that the peak positions are determined both by characteristics of the receiving and the radiating systems. If in addition to bias current, one starts to vary the magnetic field (current through the coil), the response grows significantly and larger peaks can be visible (red dashed curve in Figure 4). It should be noted that the CEB AFC measurements were performed with BWO source placed outside the cryostat, and the signal was strongly attenuated both by attenuators placed at BWO, and also by null-density filters at the cryostat windows to prevent from 0.3 K plate overheating due to incoming THz signal. The internal inhomogeneity of the BWO amplitude-frequency characteristic was properly taken into account when plotting the corresponding curve of Figure 4.  
Figure 3: Left: CEBs voltage (solid curve) and a fitting spline (dashed curve). Right: current-voltage characteristic of YBCO oscillator. Figure 2: Current-voltage characteristic of bolometer for the case of no signal and under the YBCO JJ radiation. The inset: CEBs response as a function of bias current. The mark corresponds to the used bias. Figure 5 presents the experimental IV curves of YBCO JJ for various values of the applied magnetic field. The color indicates the level of CEBs response. We observe field dependent steps, characteristic to the travelling wave regime in a long Josephson junction. It should be noted that in difference with Nb junctions [38], for YBCO structures, due to larger damping, it is rather difficult to distinguish between various step types, such as displaced linear slopes, Fiske steps and velocity matching (VM) steps [51, 52]. The separate peaks, repeated at the same frequency for all magnetic fields, are attributed to overlapping responses between different harmonics of antennas of YBCO oscillator and CEBs array.