Josephson Circuits-Based Quantum Heat Engines

Francesco Giazotto – ENS-Pise

Thermoelectric effects in metals are typically small due to the nearly perfect particle–hole symmetry around their Fermi surface. Furthermore, thermo-phase effects and linear thermoelectricity in superconducting systems have been identified only when particle–hole symmetry is explicitly broken since thermoelectric effects were considered impossible in pristine superconductors.
Here, we experimentally demonstrate that superconducting tunnel junctions develop a very large bipolar thermoelectricity in the presence of a sizable thermal gradient thanks to spontaneous particle– hole symmetry breaking [1]. Our junctions show Seebeck coefficients of up to ±300 μV K–1, which is comparable with quantum dots and roughly 105 times larger than the value expected for normal metals at subkelvin temperatures [2]. Moreover, by integrating our junctions into a Josephson interferometer, we realize a bipolar thermoelectric Josephson engine generating phase-tunable electric powers of up to ~140 nW mm–2 [2,3]. Notably, our device implements also the prototype for a persistent thermoelectric memory cell, written or erased by current injection.
We expect that our findings will lead to applications in the field of superconducting quantum technologies [4,5].
We acknowledge funding from the EU’s Horizon 2020 Research and Innovation Framework Programme under Grant No. 101057977 – SPECTRUM.

[1] G. Marchegiani, A. Braggio, and F. Giazotto, Nonlinear thermoelectricity with electron-hole symmetric systems. Phys. Rev. Lett.
124, 106801 (2020).
[2] G. Germanese, F. Paolucci, G. Marchegiani, A. Braggio, and F. Giazotto, Bipolar thermoelectric Josephson engine, Nat.
Nanotechnol. 17, 1084 (2022).
[3] G. Germanese, F. Paolucci, G. Marchegiani, A. Braggio, and F. Giazotto, Phase-control of bipolar thermoelectricity in
Josephson tunnel junctions, Phys. Rev. Applied 19, 014074 (2023).
[4] F. Paolucci, G. Germanese, A. Braggio, and F. Giazotto, A highly-sensitive broadband superconducting thermoelectric single-
photon detector, Appl. Phys. Lett. 122, 173503 (2023).
[5] C. Guarcello, R. Citro, F. Giazotto, and A. Braggio, Bipolar thermoelectrical SQUIPT (BTSQUIPT), Appl. Phys. Lett. 123, 152601
(2023).