Spin squeezing
Collective quantum correlations reduce spin noise along a useful measurement axis.
$$\xi_R^2=\frac{N(\Delta J_\perp)^2}{\lvert\langle\mathbf J\rangle\rvert^2}$$
Physical intuition
Collective quantum correlations reduce spin noise along a useful measurement axis.
Deep dive: physical mechanism
Nonlinear interactions redistribute collective spin uncertainty; the squeezed quadrature improves suitable phase estimates.
What is measured experimentally?
Measure spin variance relative to a coherent spin state.
Assumptions, noise and limitations
Squeezing must be aligned with the signal and preserved through readout.
Research and simulation exercise
Simulate one-axis twisting for a small spin ensemble.
Connect the ideas
Explore the chapter overview, learning path, and references.