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IVC of the HEB device at 325 mK (zero magnetic field, B = 0) and 100 mK (B > 0). A linear slope at low voltage bias is due to the residual resistance (normal metal connections between the HEB and the bias circuit), Rres = 0.14 Ω. The IVC peak current (150 nA at 325 mK and 58 nA at 100 mK) roughly corresponds to the critical current, IC . Single photon detection at 100 mK was observed only in the bias range corresponding to the resistive state (i.e., Vb > 30 nV). An inset shows the circuit diagram.
Noise spectrum of the HEB device at 100 mK. The dotted line shows the frequency dependence, with two plateaus. This is typical for a TES. The sharp cutoff at ∼10 kHz is due to an external low-pass filter. Below ∼400 Hz, the noise shown by the fitted line is a sum of the TEF and the Johnson and SQUID noise. Above 400 Hz, the TEF noise rolls off and only the Johnson and the SQUID noise remain.
Amplitude histogram for the HEB device at 50 mK with different QCL pulse durations. τQCL sets the average number of absorbed photon per pulse, μ. For these three cases, μ = 0.13, 0.65, and 1.3. The labeled photon number peaks, k = 0, 1, are shown for all three values of μ.
Photon count histogram for the HEB device at 100 mK. Both the dark count and photon count statistics fit well with a Gaussian function with an rms deviation σ = 1.2 nA. The solid line is the modeling of the count statistics using a combination of the Poisson and Gaussian distributions. An average number of absorbed photons per pulse in the Poisson distribution μ = 0.47. The labels show photon number peaks k = 0, 1, 2, and 3.
Dependence of the current on the electron temperature derived from experimental IVCs and a heat-balance equation (Eq. (2)). Each curve corresponds to a different bath temperature T (labeled by the curve). A nearly linear variation of the current vs. Te for T = 100 mK explains the total number of the observed photon number peaks and their equidistant positions (see Fig. 4). An inset shows R(Te ) curves recovered from the same data set (the labels indicate the bath temperature for each curve). They almost coincide thus indicating the validity of the thermal model assuming that the bolometer is a lumped element and all its characteristics can be described by an empirical R(Te ) dependence and Eq. (2).
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