# Ubiquitous Complexity of Entanglement Spectra

## TL;DR

This paper rigorously analyzes the computational complexity of entanglement spectra in quantum states, revealing that counting sizable elements is fundamentally a0P-complete, indicating intrinsic difficulty.

## Contribution

It provides a complexity-theoretic proof that counting elements in entanglement spectra is a0P-complete for various quantum states, highlighting fundamental computational limitations.

## Key findings

- Counting entanglement spectra is a0P-complete.
- The problem is as hard as computing Ising model partition functions.
- This complexity is intrinsic, not just technical.

## Abstract

In recent years, the entanglement spectra of quantum states have been identified to be highly valuable for improving our understanding on many problems in quantum physics, such as classification of topological phases, symmetry-breaking phases, and eigenstate thermalization, etc. However, it remains a major challenge to fully characterize the entanglement spectrum of a given quantum state. An outstanding problem is whether the difficulty is intrinsically technical or fundamental? Here using the tools in computational complexity, we perform a rigorous analysis to pin down the counting complexity of entanglement spectra of (i) states generated by polynomial-time quantum circuits, (ii) ground states of gapped 5-local Hamiltonians, and (iii) projected entangled-pair states (PEPS). We prove that despite the state complexity, the problems of counting the number of sizable elements in the entanglement spectra all belong to the class $\mathsf{\# P}$-complete, which is as hard as calculating the partition functions of Ising models. Our result suggests that the absence of an efficient method for solving the problem is fundamental in nature, from the point of view of computational complexity theory.