Entanglement spectroscopy on a quantum computer

TL;DR

This paper introduces a quantum algorithm for computing the entanglement spectrum of quantum states, demonstrating its application to topologically-ordered states and phase transitions, and analyzing the resource tradeoffs involved.

Contribution

The paper presents a novel quantum algorithm to efficiently compute the entanglement spectrum, including the largest eigenvalues, using lower Renyi entropies and the Newton-Girard method.

Findings

Validated on Laughlin wave function for quantum Hall state

Analyzed time-qubit resource tradeoffs for large systems

Showed utility of second Renyi entropy in phase transition detection

Abstract

We present a quantum algorithm to compute the entanglement spectrum of arbitrary quantum states. The interesting universal part of the entanglement spectrum is typically contained in the largest eigenvalues of the density matrix which can be obtained from the lower Renyi entropies through the Newton-Girard method. Obtaining the $p$ largest eigenvalues ($\lambda_1>\lambda_2\ldots>\lambda_p$) requires a parallel circuit depth of $\mathcal{O}(p(\lambda_1/\lambda_p)^p)$ and $\mathcal{O}(p\log(N))$ qubits where up to $p$ copies of the quantum state defined on a Hilbert space of size $N$ are needed as the input. We validate this procedure for the entanglement spectrum of the topologically-ordered Laughlin wave function corresponding to the quantum Hall state at filling factor $\nu=1/3$. Our scaling analysis exposes the tradeoffs between time and number of qubits for obtaining the entanglement spectrum in the thermodynamic limit using finite-size digital quantum computers. We also illustrate the utility of the second Renyi entropy in predicting a topological phase transition and in extracting the localization length in a many-body localized system.