Dependence of count rate on magnetic field in superconducting thin-film TaN single-photon detectors

We have studied the magnetic-field dependence of both dark-count rates and photon-count rates in a superconducting nanowire single-photon detector made of TaN in external magnetic fields $|{\mu }_{0}H|<10$ mT perpendicular to the plane of the underlying meander structure and at $T=4$ K. The dark-count rates show a characteristic field dependence, which is asymmetric with respect to magnetic-field direction. The field and the current dependence of the dark counts can be quantitatively well explained if one assumes that the critical current is reduced to $\approx$$50\%$ at the ${180}^{\circ }$ meander turns when compared to a straight strip, and the observed asymmetry can be modeled assuming that the turnarounds are not all strictly equal. Surprisingly, the photon-count rates do not show any significant field dependence, which seems to be at odds with existing detection models invoking vortex crossings.

Figure 1

Dark counts and photon counts on logarithmic scale vs bias current. Errors are of the size of the data points or smaller except where plotted. The solid line is a least-squares fit to Eq. (51) from Ref. 6, while the dashed line is a fit to the phenomenological Eq. (1). The dashed lines connecting the photon-counts data are guides to the eye. The inset shows an electron micrograph of the measured device. In the lower left corner is a sketch of the magnetic field and bias current in relation to the meander geometry.

Figure 2

Dark counts ${\log }_{10}\left(R_{\mathrm{DC}}\right)$ for different $I_b$ plotted as a function of applied magnetic field $H$. The $|I_b|$ have been progressively increased by $0.25\mu$A. Upper panel: $I_b<0$; lower panel: $I_b>0$. The $+16.5-\mu$A data are also plotted in the upper panel, but on a reversed field axis (uppermost $H$-axis) to demonstrate the equivalence of reversing field or current direction. The corresponding $y$ axis (right axis, upper panel) has been shifted by $0.35$ to compensate the current offset. Solid lines are least-square fits according to Eq. (4) with common parameters, performed independently for negative and positive $I_b$, respectively.

Figure 3

Relative photon-count rates as functions of the applied magnetic field $H$, normalized to photon-count rates in zero field for each $I_b$. For clarity, the datasets have been shifted in $y$ direction by multiples of $0.5$. We also plotted the expected $H$ dependence for 985-nm photons and $I_b=13\mu$A (dashed black curve). Horizontal lines are to guide the eye.