Bolometric detection of Josephson radiation

One of the most promising approaches towards large-scale quantum computation uses devices based on many Josephson junctions. Yet, even today, open questions regarding the single junction remain unsolved, such as the detailed understanding of the quantum phase transitions, the coupling of the Josephson junction to the environment or how to improve the coherence of a superconducting qubit. Here we design and build an engineered on-chip reservoir connected to a Josephson junction that acts as an efficient bolometer for detecting the Josephson radiation under non-equilibrium, that is, biased conditions. The bolometer converts the a.c. Josephson current at microwave frequencies up to about 100 GHz into a temperature rise measured by d.c. thermometry. A circuit model based on realistic parameter values captures both the current–voltage characteristics and the measured power quantitatively. The present experiment demonstrates an efficient, wide-band, thermal detection scheme of microwave photons and provides a sensitive detector of Josephson dynamics beyond the standard conductance measurements

Joule spectroscopy of hybrid superconductor–semiconductor nanodevices

Hybrid superconductor-semiconductor devices offer highly tunable platforms, potentially suitable for quantum technology applications, that have been intensively studied in the past decade. Here we establish that measurements of the superconductor-to-normal transition originating from Joule heating provide a powerful spectroscopical tool to characterize such hybrid devices. Concretely, we apply this technique to junctions in full-shell Al-InAs nanowires in the Little-Parks regime and obtain detailed information of each lead independently and in a single measurement, including differences in the superconducting coherence lengths of the leads, inhomogeneous covering of the epitaxial shell, and the inverse superconducting proximity effect; all-in-all constituting a unique fingerprint of each device with applications in the interpretation of low-bias data, the optimization of device geometries, and the uncovering of disorder in these systems. Besides the practical uses, our work also underscores the importance of heating in hybrid devices, an effect that is often overlookedWe acknowledge funding by EU through the European Research Council (ERC) Starting Grant agreement 716559 (TOPOQDot), the FET-Open contract AndQC, by the Danish National Research Foundation, Inno vation Fund Denmark, the Carlsberg Foundation, and by the Spanish AEI through Grant No. PID2020-117671GB-I00 and through the “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018- 000805-M) and the ”Ramón y Cajal” programme grant RYC 2015-1797

Heat dissipation mechanisms in hybrid superconductor−semiconductor devices revealed by Joule Spectroscopy

Understanding heating and cooling mechanisms in mesoscopic superconductor−semiconductor devices is crucial fortheir application in quantum technologies. Owing to their poor thermal conductivity, heating effects can drive superconducting-to-normal transitions even at low bias, observed as sharp conductance dips through the loss of Andreev excess currents. Tracking suchdips across magnetic field, cryostat temperature, and applied microwave power allows us to uncover cooling bottlenecks in differentparts of a device. By applying this “Joule spectroscopy” technique, we analyze heat dissipation in devices based on InAs-Al nanowiresand reveal that cooling of superconducting islands is limited by the rather inefficient electron−phonon coupling, as opposed togrounded superconductors that primarily cool by quasiparticle diffusion. We show that powers as low as 50−150 pW are able tosuppress superconductivity on the islands. Applied microwaves lead to similar heating effects but are affected by the interplay of themicrowave frequency and the effective electron−phonon relaxation timePID2020-117671GB-I00, TED2021-130292B-C4