Remote Entangling Gates for Spin Qubits in Quantum Dots using an Offset-Charge-Sensitive Transmon Coupler

Modular processors comprising spin qubits confined in quantum dots are a promising candidate for scalable quantum computers. Superconducting circuits are reasonable choices for quantum interconnects connecting distant spin qubit processors, due to their larger length scale and their ability to couple to electric dipoles induced in quantum dots [1]. One widely used strategy is to use a resonator to mediate this coupling, but the reported coupling strengths have been comparable to decoherence rates due to materials and fabrication limitations – for instance, the achievable impedance of high kinetic-inductance resonators. In this work, we propose a method to bypass this limitation: we use an offset-charge-sensitive transmon to realize microwave-activated CZ gates [2]. The coupling is set to be purely longitudinal so that the qubit logical states condition the coupler transition frequency. A conditional phase shift on the qubits is achieved by driving the coupler transition. We investigate two gate schemes: a rapid, off-resonant pulse with constant amplitude [3], and a dynamically decoupled gate to mitigate charge noise, optimized by envelope engineering techniques. Non-Markovian time-domain simulation results suggest that a CZ gate fidelity exceeding 90% is possible with realistic parameters and noise models.

[1] X. Mi. et al., Science 355 (2017)

[2] H. H. Kang et al., arXiv:2409.08915

[3] I. A. Simakov et al., PRX Quantum 4 (2023)