Absence of spin-boson quantum phase transition for transmon qubits
Kuljeet Kaur, Théo Sépulcre, Nicolas Roch, Izak Snyman, Serge Florens, Soumya Bera
Superconducting circuits are currently developed as a versatile platform for
the exploration of many-body physics, both at the analog and digital levels.
Their building blocks are often idealized as two-level qubits, drawing powerful
analogies to quantum spin models. For a charge qubit that is capacitively
coupled to a transmission line, this analogy leads to the celebrated spin-boson
description of quantum dissipation. We put here into evidence a failure of the
two-level paradigm for realistic superconducting devices, due to electrostatic
constraints which limit the maximum strength of dissipation. These prevent the
occurence of the spin-boson quantum phase transition for transmons, even up to
relatively large non-linearities. A different picture for the many-body ground
state describing strongly dissipative transmons is proposed, showing unusual
zero point fluctuations.