SPICE Workshop on Quantum Functionalities of Nanomagnets, June 17th - 19th 2025
Pascal Simon
Understanding the interplay between individual magnetic impurities and superconductivity is important for bottom-up construction of novel exotic phases of matter as well as to exploit them for quantum computing.
For decades, the description by Yu, Shiba and Rusinov (YSR) of single spins in a superconductor and its extension to include quantum effects has proven highly successful: the pair-breaking potential of the spin generates sub-gap bound states. I will first show how atomically-resolved shot noise can be used to reveal the coherent and incoherent dynamics of such sub-gap bound states and to extract its lifetime [1]. By tuning the energy of the sub-gap states through zero, the impurity screening by the superconductor makes the ground state gain or lose an electron, signaling a parity breaking quantum phase transition. I will present data and modes that explicitly invalidate the classical YSR paradigm, and propose an interpretation in terms of a multi-orbital Anderson impurity model [2].
Finally, I will discuss the quantum dynamics and such YSR when subject to an external ac magnetic field [3] and introduce and study a novel type of quantum bit, the YSR qubit, that is encoded in the energy states of a spin dimer coupled to an s-wave superconductor. I will show theoretically that both the coherent manipulation and the readout of the YSR qubit can be efficiently implemented by harnessing the dynamics of the spins that engender it.
[2] M. Uldemolins, A. Mesaros, G. D. Gu, A. Palacio-Morales, M. Aprili, P. Simon, and F. Massee, Nature Communications 15 , 8526 (2024).
[3] A Mishra, S Takei, P Simon, M Trif, Phys. Rev. B 103, L121401(2021).
[4] A Mishra, P Simon, T Hyart, M Trif, PRX Quantum 2, 040347 (2021).