Unique Games hardness of Quantum Max-Cut, and a vector-valued Borell's inequality

The Gaussian noise stability of a function $f:\mathbb{R}^n \to \{-1, 1\}$ is the expected value of $f(\boldsymbol{x}) \cdot f(\boldsymbol{y})$ over $\rho$-correlated Gaussian random variables $\boldsymbol{x}$ and $\boldsymbol{y}$. Borell's inequality states that for $-1 \leq \rho \leq 0$, this is minimized by the halfspace $f(x) = \mathrm{sign}(x_1)$. In this work, we generalize this result to hold for functions $f:\mathbb{R}^n \to S^{k-1}$ which output $k$-dimensional unit vectors. Our main result shows that the expected value of $\langle f(\boldsymbol{x}), f(\boldsymbol{y})\rangle$ over $\rho$-correlated Gaussians $\boldsymbol{x}$ and $\boldsymbol{y}$ is minimized by the function $f(x) = x_{\leq k} / \Vert x_{\leq k} \Vert$, where $x_{\leq k} = (x_1, \ldots, x_k)$. As an application, we show several hardness of approximation results for Quantum Max-Cut, a special case of the local Hamiltonian problem related to the anti-ferromagnetic Heisenberg model. Quantum Max-Cut is a natural quantum analogue of classical Max-Cut and has become testbed for designing quantum approximation algorithms. We show the following: 1. The integrality gap of the basic SDP is $0.498$, matching an existing rounding algorithm. Combined with existing approximation results for Quantum Max-Cut, this shows that the basic SDP does not achieve the optimal approximation ratio. 2. It is Unique Games-hard (UG-hard) to compute a $(0.956+\varepsilon)$-approximation to the value of the best product state, matching an existing approximation algorithm. This result may be viewed as applying to a generalization of Max-Cut where one seeks to assign $3$-dimensional unit vectors to each vertex; we also give tight hardness results for the analogous $k$-dimensional generalization of Max-Cut. 3. It is UG-hard to compute a $(0.956+\varepsilon)$-approximation to the value of the best (possibly entangled) state.