Laser Cooling of Nuclear Magnons

The initialization of nuclear spin to its ground state is challenging due to its small energy scale compared with thermal energy, even at cryogenic temperature. In this Letter, we propose an optonuclear quadrupolar effect, whereby two-color optical photons can efficiently interact with nuclear spins. Leveraging such an optical interface, we demonstrate that nuclear magnons, the collective excitations of nuclear spin ensemble, can be cooled down optically. Under feasible experimental conditions, laser cooling can suppress the population and entropy of nuclear magnons by more than 2 orders of magnitude, which could facilitate the application of nuclear spins in quantum information science.

Figure 1

The ONQ effect in zinc-blende GaAs. (a) Yellow (green) bubbles denote positive (negative) changes in electron charge density when an electric field ${\mathcal{E}}_x$ is applied. Pink (blue) spheres are Ga (As) atoms. (b) $\mathrm{\Delta }{\mathcal{V}}_{ij}$ at the site of As nuclei as a function of ${\mathcal{E}}_x$.

Figure 2

(a) A semiclassical one-dimensional illustration of the NM mode. (b) Band dispersion of the NMs (not to scale). One has ${\gamma }_n\mathcal{B}\gg \mathcal{J}I[z_c-\mathcal{Z}(\mathit{k})]$. (c) Illustration of the four-NM scattering process.

Figure 3

Laser cooling dynamics. The initial NM population is $n_{\mathrm{th}}=1$. (a) Time evolution of $n_0$ in the weak-coupling regime. (b) $n_0^{\text{steady}}$ as a function of ${\kappa }_0$ and ${\kappa }_h$ in the weak-coupling regime. ${\mathcal{G}}_h=10\mathrm{kHz}$. (c) Time evolution of $n_0$ and $n_h$ in the strong coupling regime. ${\mathcal{G}}_h=1\mathrm{MHz}$. Inset of (c) shows possible transitions of the NMs. Circles denote optical photons or lasers with frequencies marked inside. The hexagon denotes the NM. Green and red arrows denote ONQ transitions. Gray wavy lines denote coupling with the heat bath. (d) $n_0^{\text{steady}}$ as a function of ${\mathcal{G}}_h$. The red (green) curve denotes laser cooling without (with) $Q$ switching of the optical cavity. The cyan-shaded area corresponds to the strong-coupling regime. In (a), (c), (d), ${\kappa }_0=0.1\mathrm{kHz}$ and ${\kappa }_h=1\mathrm{MHz}$ are used.