Magnon Squeezing in an Antiferromagnet: Reducing the Spin Noise below the Standard Quantum Limit

We report the first experimental demonstration of quantum squeezing of a collective spin-wave excitation (magnon) using femtosecond optical pulses to generate correlations involving pairs of spins in an antiferromagnetic insulator ${\mathrm{M}\mathrm{n}\mathrm{F}}_2$. In the squeezed state, the fluctuations of the magnetization of a crystallographic unit cell vary periodically in time and are reduced below that of the ground-state quantum noise.