In quantum metrology, particle spin not only serves as a potent probe for measuring magnetic fields, inertia, and a variety of physical phenomena, but also holds potential for exploring new physics beyond the Standard Model. 

High-spin Schrödinger-cat state, a superposition of two oppositely directed and furthest-apart spin states, offers significant advantages over spin measurements. On one hand, the high spin quantum number amplifies the precession frequency signal. On the other hand, the cat states are insensitive to some environmental interference, suppressing measurement noise. One major technical challenge in applying cat states in experiments is how to maintain a sufficiently long coherence time.

To address this challenge, researchers first trapped 173Yb atoms with a spin of 5/2 in an optical lattice. By controlling laser pulses to induce nonlinear light shifts on the ground states of the atoms, they prepared a superposition state consisting of two spin projections, +5/2 and -5/2. 

This state, namely the Schrödinger-cat state, exhibited enhanced magnetic field sensitivity and experienced identical light shifts in the optical lattice, residing within a decoherence-free subspace. Therefore, it was immune to intensity noise and spatial variations of the lattice.

Experimental results indicated that the coherence time of this cat state exceeded 20 minutes. Through Ramsey interferometry, researchers confirmed that the phase measurement sensitivity approaches the Heisenberg limit.

This long-lived Schrödinger-cat state paves the way for atomic magnetometry, quantum computations, and the exploration of new physics beyond the Standard Model.

The spins of 173Yb atoms form a Schrödinger-cat state within a one-dimensional optical lattice. (Image by YANG Yang et al.)