Spin-Boson Quantum Phase Transition in Multilevel Superconducting Qubits

Superconducting circuits are currently developed as a versatile platform for the exploration of manybody physics, by building on nonlinear elements that are often idealized as two-level qubits. A classic example is given by a charge qubit that is capacitively coupled to a transmission line, which leads to the celebrated spin-boson description of quantum dissipation. We show that the intrinsic multilevel structure of superconducting qubits drastically restricts the validity of the spin-boson paradigm due to phase localization, which spreads the wave function over many charge states. Numerical renormalization group simulations also show that the quantum critical point moves out of the physically accessible range in the multilevel regime. Imposing charge discreteness in a simple variational state accounts for these multilevel effects, which are relevant for a large class of devices.

Automated summary

Superconducting circuits are developed as a versatile platform for exploring manybody physics, based on nonlinear elements idealized as two-level qubits. However, the intrinsic multilevel structure of superconducting qubits restricts the validity of the spin-boson paradigm. Numerical renormalization group simulations show that the quantum critical point moves out of the physically accessible range in the multilevel regime. Imposing charge discreteness in a simple variational state accounts for these multilevel effects.