The Constant Geometric Speed Schedule for Adiabatic State Preparation: Towards Quadratic Speedup without Prior Spectral Knowledge

Abstract

The efficiency of adiabatic quantum evolution is governed by the evolution time $T$, which typically scales as $\mathcal{O}(Δ^{-2})$ with the minimum energy gap $Δ$. However, the rigorous lower bound is $\mathcal{O}(LΔ^{-1})$, where $L$ is the adiabatic path length. Although $L$ is formally upper-bounded by $\mathcal{O}(Δ^{-1})$, such a bound is often too loose in practice, and $L$ can be bounded independently of $Δ$. This indicates the potential for a quadratic speedup through adiabatic schedule construction. Here, we introduce the constant geometric speed (CGS) schedule, which traverses the adiabatic path at a uniform rate. We show that this approach reduces the scaling of the evolution time by a factor of $Δ^{-1}$, provided $L$ remains bounded independently of $Δ$. We propose a segmented CGS protocol where path segment lengths are computed from eigenstate overlaps on the fly, eliminating the need for prior spectral knowledge. Numerical tests on adiabatic unstructured search, N$_2$, and a [2Fe-2S] cluster demonstrate the optimal $Δ^{-1}$ scaling, confirming a quadratic speedup over the standard linear schedule.