Achievement 4

Electronic thermometry at the micro-second time scale

Q-NET team at AALTO has demonstrated radiofrequency thermometry below 100 mK. The device is based on a normal metal-insulator-superconductor (NIS) tunnel junction (see Fig.1(a)) coupled to a resonator with transmission readout. A rf-transmission readout of the NIS tunnel junction provides 90μK/√Hz thermometry with a bandwidth of 10 MHz.

Usually, the high impedance of the junction and stray capacitance that come from the measurement cables limit the bandwidth to the kHz range. In order to achieve a fast readout, the NIS junction is connected to the LC resonant circuit working at a frequency of f0 = 625 MHz. At low input power, the resonator probes the differential conductance G = ∂I/∂Vb of the junction at the bias point Vb. In Fig. 1(b), the resonance peak is shown to respond to changes in Vb. In Fig. 1(c), the transmittance |s21|2 is plotted as a function of Vb for a set of bath temperatures Tbath in the range of 20 to 325 mK. The corresponding electronic temperature Te versus Vb is extracted from the traces in the main panel, and plotted in the inset (triangles).

At base temperature Tbath = 20 mK the team finds that Te  ≈ 85 mK. This saturation possibly occurs due to imperfect shielding from high temperature electromagnetic environment or low frequency noise in the dc lines. Conversely, at high temperatures, Te closely follows Tbath, as the electron-phonon heat conductance provides a strong thermal anchoring to the electrons in the Cu island. The agreement between Te and Tbath enables the NIS junction as an rf-NIS electron thermometer. Such fast electron-thermometry can provide a way to detect single photons in the nearest future.


Figure: (a) Scanning-electron micrograph of the NIS device, (b) transmittance as a function of frequency at three different Vb values, (c) transmittance versus Vb for different temperatures Tbath, inset shows the extracted electronic temperature Te vs Vb.


Reference: "Fast electron thermometry towards ultra-sensitive calorimetric detection", S. Gasparinetti, K. L. Viisanen, O.-P. Saira, T. Faivre, M. Arzeo, M. Meschke, and J. P. Pekola, arxiv:1405.7568v2, Physical Review Applied 3, 014007 (2015).



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