We introduce a driven-dissipative Floquet model in which a single harmonic oscillator with modulated frequency and decay realizes a non-Hermitian synthetic lattice with an effectiveelectric field gradient in frequency space. Using the Floquet-Green’s function and its doubled-space representation, we identify a topological regime that supports directional amplification and frequency conversion, accurately captured by a local winding number. The underlying mode structure is well described by a Jackiw-Rebbi-like continuum theory with Dirac cones and solitonic zero modes in synthetic frequency. Our results establish a simple and experimentally feasible route to non-Hermitian topological amplification, naturally implementable in current quantum technologies such as superconducting circuits.
The fidelity and quantum nondemolition character of the dispersive readout in circuit QED are limited by unwanted transitions to highly excited states at specific photon numbers inthe readout resonator. This observation can be explained by multiphoton resonances between computational states and highly excited states in strongly driven nonlinear systems, analogous to multiphoton ionization in atoms and molecules. In this work, we utilize the multilevel nature of high-EJ/EC transmons to probe the excited-state dynamics induced by strong drives during readout. With up to 10 resolvable states, we quantify the critical photon number of ionization, the resulting state after ionization, and the fraction of the population transferred to highly excited states. Moreover, using pulse-shaping to control the photon number in the readout resonator in the high-power regime, we tune the adiabaticity of the transition and verify that transmon ionization is a Landau-Zener-type transition. Our experimental results agree well with the theoretical prediction from a semiclassical driven transmon model and may guide future exploration of strongly driven nonlinear oscillators.