A strong-driving toolkit for topological Floquet energy pumps with superconducting circuits

Topological Floquet energy pumps – which use periodic driving to create a topologically protected quantized energy current – have been proposed and studied theoretically, but have never been observed directly. Previous work proposed that such a pump could be realized with a strongly-driven superconducting qubit coupled to a cavity. Here, we experimentally demonstrate that the proposed hierarchy of energy scales and drive frequencies can be realized using a transmon qubit. We develop an experimental toolkit to realize the adiabatic driving field required for energy pumping using coordinated frequency modulation of the transmon and amplitude modulation of an applied resonant microwave drive. With this toolkit, we measure adiabatic evolution of the qubit under the applied field for times comparable to $T_1$, which far exceed the bare qubit dephasing time. This result paves the way for direct experimental observation of topological energy pumping.

💡 Research Summary

This paper presents the experimental realization of the essential ingredients required for a topological Floquet energy pump using a superconducting transmon circuit. Topological energy pumps are theoretically predicted to generate a quantized flow of energy (or photons) between a classical drive and a quantum cavity when the system’s Hamiltonian is periodically modulated in a way that traces a non‑trivial loop in parameter space. The quantization is protected by a Chern number, analogous to the Thouless charge pump, but here the pumped quantity is energy rather than charge.

The authors first review the theoretical model: a two‑level system (the qubit) coupled to a cavity mode with Hamiltonian
(H = \frac{1}{2}\mathbf{B}(t)\cdot\boldsymbol{\sigma} + \Delta a^\dagger a + g(a^\dagger\sigma_- + a\sigma_+)),
where (\mathbf{B}(t)) is an effective magnetic field that is the sum of a static component and two orthogonal circularly polarized drives. By choosing (\mathbf{B}(t)=B_0{\sin(\omega_{\rm mod} t),\hat{z}+