Mediator-activated two-qubit gate with static crosstalk suppression using the multimode P-mon circuit

To scale quantum processors without sacrificing coherence, it is crucial to engineer interactions that avoid introducing new decoherence channels. Our recently developed P-mon qubit [1] achieves intrinsic protection from decoherence caused by its readout environment. Here, we extend this concept to controlled two-qubit interactions, using internal mediator modes of coupled P-mons to enable on-demand coupling while suppressing unwanted idle interactions.

The qubit modes of the two P-mons interact via a cross-Kerr interaction with their respective mediator mode, resulting in a unique mediator frequency that depends on the state of the qubit mode. A linear coupling between the two mediator modes is realized to couple the two P-mons. Since the mediator frequencies depend on the qubit state, the spectrum of the two coupled mediator modes depends on the qubit states, which we exploit to implement a microwave-activated CPHASE gate.

We implement the scheme experimentally and present the calibration and benchmarking of the gate with a fidelity of over 99.5%, while reducing ZZ interactions under idle conditions to <5 kHz. These results represent an important step toward a scalable superconducting architecture that maintains performance at scale.

[1] F. Pfeiffer et al., Phys. Rev. X 14, 041007