Understanding and Improving Qubit Coherence in a Dense Nuclear Spin Environment Using Cluster Correlation Expansion Methods

Future quantum networks will pioneer fundamentally secure communication and enable distributed quantum computing. Of the network node platforms, single rare-earth ions in crystals exhibit great potential due to their long coherence times and integrability into nanophotonic structures. ¹⁷¹Yb:YVO₄ is especially appealing as Yb³⁺ has a simple hyperfine structure with optical clock transitions and a zero magnetic dipole moment at zero magnetic field that shields the qubit manifold from surrounding magnetic fields, ensuring both long spin and optical coherence times.

Despite the first-order magnetic insensitivity, the Yb³⁺ qubit has a limited coherence time currently understood to be set by interactions with nearby spins.¹ We construct a model using cluster correlation expansion methods to study noise generated by the dense V⁵⁺ nuclear spin bath and paramagnetic defects in the crystal.² The model also enables us to design pulse sequences to combat undesired interactions.

The V⁵⁺ spins, while a noise source, offer resources for ensemble-based memories for quantum states. Control of the second shell of nearest neighbor V⁵⁺ spins has been demonstrated.³ As we develop control techniques for first shell spins, our model will provide insight into how the spin environment affects the manipulation and storage of these new quantum memories.

This research serves as a solid foundation for understanding how to optimize qubit coherence and harness new memory resources as we work towards a future quantum internet.

1. Kindem, J.M., Ruskuc, A., Bartholomew, J.G. et al. Control and single-shot readout of an ion embedded in a nanophotonic cavity. Nature 580, 201–204 (2020).

2. Onizhuk, M. and Galli, G. PyCCE: A Python Package for Cluster Correlation Expansion Simulations of Spin Qubit Dynamics. Adv. Theory Simul., 4: 2100254 (2021).

3. Ruskuc, A., Wu, CJ., Rochman, J. et al. Nuclear spin-wave quantum register for a solid-state qubit. Nature 602, 408–413 (2022).