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.

Figure 1

Relative differential transmission as a function of pump-probe delay. The Fourier transform spectrum in the inset shows peaks due to coherent two-magnon excitations (squeezed magnons) and coherent phonons.

Figure 2

(a) Pump-probe data showing two-magnon oscillations. The LP fit is shown in red. (b) Comparison between the Fourier transform of the LP fit (red) and the Raman spectrum recorded at 3 K using 140 mW of 514.5 nm ${\mathrm{A}\mathrm{r}}^{+}$ laser light. (c) Magnon dispersion for wave vectors in the $[100]$ and $[001]$ directions (from Ref. 18). To compare with the Raman experiments, the frequency scale has been multiplied by a factor of 2.

Figure 3

Schematic representation of magnon squeezing in ${\mathrm{M}\mathrm{n}\mathrm{F}}_2$. The arrows and cones attached to the manganese ions represent, respectively, the spins and their angular fluctuations at zero temperature. The inset shows the time dependence of the noise (represented by the angle $\theta$, which fluctuates about an expectation value of zero) and the square root of the expectation value of ${\theta }^2$, which is finite. The arrow indicates the time at which the laser pulse impinges on the solid.