Strong coupling between a microwave photon and a singlet-triplet qubit*

Spin qubits in semiconuctors are promising contenders for realizing scalable quantum computers because of their small footprint, long coherence times and fast gate operations. However, entangling gates between spin qubits are short range, limiting the scale-up towards larger quantum processors.

In circuit quantum-electrodynamics (QED), on the other hand, superconducting qubits are routinely interconnected with each other exploiting resonators as quantum buses. Implementing circuit QED techniques in the context of spin qubits is challenging and recent advances rely on micromagnets complicating scale-up.

Here, we present a different approach based on intrinsic spin-orbit interaction that is naturally present in zincblende InAs nanowires. Intrinsic spin-orbit interaction leviates the limitations introduced by micromagnets. We utilize a zincblende InAs nanowire in which a double quantum dot (DQD) is defined by epitaxially-grown crystal-phase barriers. The DQD is integrated in a circuit QED architecture featuring a magnetic field-resilient and high quality NbTiN resonator. The resonator is characterized by a large kinetic inductance giving rise to a large impedance of 2 kΩ enhancing the photon-qubit interaction. We investigate the hybrid DQD-resonator system and demonstrate clear indications of a coherent coupling between an electron singlet-triplet qubit and a resonator mode in the single photon limit.