Estimating the entanglement of random multipartite quantum states

TL;DR

This paper develops and compares algorithms to estimate the injective norm of random tensors, providing the first numerical insights into multipartite entanglement in various quantum state models.

Contribution

It introduces a normalized gradient descent algorithm for estimating the injective norm and applies it to various random tensor models, including complex Gaussian tensors and matrix product states.

Findings

Normalized gradient descent outperforms other algorithms in estimating the injective norm.

Numerical estimates of the average injective norm for complex Gaussian tensors and MPS are provided.

First quantitative insights into multipartite entanglement in random quantum states are achieved.

Abstract

Genuine multipartite entanglement of a given multipartite pure quantum state can be quantified through its geometric measure of entanglement, which, up to logarithms, is simply the maximum overlap of the corresponding unit tensor with product unit tensors, a quantity that is also known as the injective norm of the tensor. Our general goal in this work is to estimate this injective norm of randomly sampled tensors. To this end, we study and compare various algorithms, based either on the widely used alternating least squares method or on a novel normalized gradient descent approach, and suited to either symmetrized or non-symmetrized random tensors. We first benchmark their respective performances on the case of symmetrized real Gaussian tensors, whose asymptotic average injective norm is known analytically. Having established that our proposed normalized gradient descent algorithm generally performs best, we then use it to obtain numerical estimates for the average injective norm of complex Gaussian tensors (i.e., up to normalization, uniformly distributed multipartite pure quantum states), with or without permutation-invariance. We also estimate the average injective norm of random matrix product states constructed from Gaussian local tensors, with or without translation-invariance. All these results constitute the first numerical estimates on the amount of genuinely multipartite entanglement typically present in various models of random multipartite pure states. Finally, motivated by our numerical results, we posit two conjectures on the injective norms of random Gaussian tensors (real and complex) and Gaussian MPS in the asymptotic limit of the physical dimension.