Simultaneous High-Fidelity Readout and Strong Coupling for a Donor-Based Spin Qubit

TL;DR

This paper demonstrates that for donor-based flip-flop spin qubits, optimal intermediate tunnel couplings enable simultaneous high-fidelity readout and strong coupling to superconducting resonators, advancing scalable quantum architectures.

Contribution

It identifies optimal operating points balancing coupling strength and coherence, and shows how squeezed input fields can mitigate experimental constraints.

Findings

High-fidelity readout and strong coupling achieved simultaneously.

Optimal intermediate tunnel couplings identified for flip-flop qubits.

Squeezed input fields mitigate charge-photon coupling and photon loss issues.

Abstract

Superconducting resonators coupled to solid-state qubits offer a scalable architecture for long-range entangling operations and fast, high-fidelity readout. Realizing this requires low photon-loss rates and qubits with tunable electric dipole moments that couple strongly to the resonator's electric field while maintaining long coherence times. For spin qubits, spin-photon coupling is typically achieved via spin-charge hybridization. However, this introduces a fundamental trade-off: a large spin-charge admixture enhances the coupling strength, which boosts readout and resonator-mediated gate speeds, but exposes the qubit to increased decoherence, thereby increasing the threshold required for strong coupling and limiting the time available for accurate state measurement. This makes it essential to identify optimal operating points for each qubit platform. We address this for the donor-based flip-flop qubit, whose microwave-controllable electron-nuclear spin states make it suitable for coupling to microwave resonators. We demonstrate that, by choosing intermediate tunnel couplings that balance strong interaction with long qubit lifetimes, high-fidelity readout and strong coupling are simultaneously achievable. We also map out the respective charge-photon couplings and photon-loss rates required. Furthermore, we show that experimental constraints on charge-photon coupling and photon loss can be mitigated using squeezed input fields. As similar trade-offs appear in quantum-dot-based qubits, our methods and insights extend naturally to these platforms, offering a potential route toward scalable architectures.