Bounds on adiabatic path geometry from the width class of the gap profile

Abstract

In an adiabatic evolution, the wave function follows the path of ground states $|Φ_0(s)\rangle$ of a Hamiltonian $H(s)$ as $s$ is tuned from $0$ to $1$. The geometry of this adiabatic path is described by quantities such as its length $L=\int_0^1 \|\partial_s |Φ_0(s)\rangle\|\,ds$ and its total curvature $K$. This geometry affects the evolution time $T$. For instance, $T$ is at least of order $L/Δ_*$, where $Δ_*$ is the minimum energy gap over $s\in[0,1]$. The traditional approach gives $L=\mathcal{O}(Δ_*^{-1/2})$ and $K=\mathcal{O}(Δ_*^{-1})$, which are loose in many cases. We improve these bounds by using $μ_<(γ)$, the width of the region in $s$ on which the gap is below $γ$. The gap profile is in width class $p$ when $μ_<(γ)$ falls at least as fast as $γ^{1/p}$. A typical avoided crossing is in width class $p=1$, which gives $L=\mathcal{O}(\sqrt{\log Δ_*^{-1}})$ and $K=\mathcal{O}(\log Δ_*^{-1})$. A profile in width class $p>1$ gives power laws with exponents $(p-1)/(2p)$ for $L$ and $(p-1)/p$ for $K$. The width class also bounds the evolution time. At $p=1$ we have $T=\mathcal{O}(Δ_*^{-2})$ with a schedule that advances $s$ at a constant rate, and $T=\mathcal{O}(Δ_*^{-1})$ up to a polylogarithmic correction with a schedule that traverses the path at a constant geometric speed. The bounds on $L$ hold for any twice continuously differentiable $H(s)$, and the bounds on $K$ for affine $H(s)$. We also prove the tightness of scaling of $L$ in $Δ_*$ for every integer $p\ge1$, and the scaling of $K$ at $p=1$. We demonstrate the bounds on the adiabatic Grover search, the XXZ spin chain, and molecular electronic Hamiltonians.