Microwave-activated Controlled-Z gates with Fluxonium

Fluxonium [1] has emerged as a promising superconducting qubit platform, combining long co-

herence times with high-fidelity single- and two-qubit operations [2–6]. A key challenge, however,

remains the scalability of the control and suppression of residual qubit–qubit interactions. While

tunable couplers are often used in Transmon-based processors to dynamically mitigate these inter-

actions, the Fluxonium qubit enables a coupler-free strategy. In this approach, microwave drives ex-

ploit the AC Stark shift to both suppress and enhance effective coupling, thereby naturally realizing

a controlled-Z (CZ) gate [5, 6].

In this work, we investigate the scalability of the microwave-activated CZ gate within a three-qubit

Fluxonium architecture. We first characterize a two-qubit subsystem, demonstrating robust coherence

and high-fidelity single-qubit control. By applying carefully chosen microwave tones, we show that

the effective interaction between qubits can be dynamically tuned—from near-complete suppression

to significant enhancement—depending on the drive parameters. These results lay the groundwork

for extending microwave-activated entangling gates to multi-qubit Fluxonium processors and provide

key insights into their scalability and control in larger architectures.