Squeezed Spin States

Today I read the classic paper Spin Squeezed States, which was highlighted as a Milestone paper by Physical Review A last year.

In this paper the authors generalize squeezing transformations (usually considered in the context of bosonic modes) to spin S systems. Squeezing allows one to reduce the fluctuations along one spin axis, at the expense of increased fluctuations along another axis, enabling precision measurements with noise below the standard quantum limit.

The ability to use squeezing to reduce quantum noise is particularly important for near-term quantum processors, potentially enabling calculations to be carried out using fewer measurements. Writing in PRX: Quantum, Pezze and Smerzi last month proposed an improved quantum phase estimation algorithm employing spin squeezed states. The key advantage of spin squeezed states compared to an earlier algorithm employing GHZ states is their enhanced robustness to noise and losses.

It is interesting to consider how spin squeezing might be useful for improving the robustness to noise and performance of noisy intermediate-scale quantum algorithms...