Boson sampling beyond the dilute regime: second moments and anti-concentration

TL;DR

This paper advances the understanding of boson sampling's output statistics beyond the dilute regime, providing new analytical tools and demonstrating anti-concentration properties crucial for quantum advantage proofs.

Contribution

It introduces representation-theoretic methods to derive second moments and anti-concentration results for boson sampling in regimes of experimental interest.

Findings

Derived closed-form expressions for second moments of bosonic observables.

Established anti-concentration of output probabilities beyond the dilute regime.

Strengthened complexity-theoretic hardness guarantees for boson sampling.

Abstract

Boson sampling is a leading candidate for demonstrating quantum advantage in photonic systems. Despite significant experimental and theoretical progress, a characterization of its output statistics remains incomplete. This is especially true beyond the dilute regime, where photon collisions and bunching become significant. The associated saturated regime, characterized by mode number growing linearly with photon number, or more generally sub-quadratically, is precisely the regime of greatest experimental interest. As a consequence, anti-concentration of the output distribution--a key ingredient in hardness arguments--remains poorly understood in boson sampling. In this work, we leverage representation-theoretic tools to address this gap, obtaining closed-form expressions for second moments of generic particle-number-preserving bosonic observables. We express these quantities in terms of Hilbert-Schmidt norms of projections onto irreducible components of the operator space and show that these projection norms admit compact analytical expressions by exploiting the underlying symmetry structure. Focusing on Fock state output probabilities, we further establish anti-concentration beyond the dilute regime. Together with recent complexity-theoretic results, our findings strengthen hardness guarantees for boson sampling in experimentally interesting settings.