# Experimental quantum state certification by actively sampling photonic entangled states

## TL;DR

This paper introduces a method to verify quantum states without using all copies, enabling efficient validation for quantum technologies.

## Contribution

The novel contribution is an experimental quantum state certification protocol that samples only a subset of states and works without assuming identical distribution.

## Key findings

- Quantum state certification was implemented using active optical switches for two-photon and three-photon entangled states.
- The protocol achieves close to N−1 scaling in the number of measured states and provides real-time fidelity estimates.
- The method is device-independent and suitable for benchmarking quantum computing and communication systems.

## Abstract

Entangled quantum states are essential ingredients for many quantum technologies, but they must be validated before they are used. As a full characterization is prohibitively resource intensive, recent work has focused on developing methods to efficiently extract a few parameters of interest, in a so-called verification framework. Most existing approaches are based on preparing an ensemble of nominally identical and independently distributed (IID) quantum states and then measuring each copy of the ensemble. However, this leaves no states left for the intended quantum tasks and the IID assumptions do not always hold experimentally. To overcome these challenges, we experimentally implement quantum state certification (QSC), which measures only a subset of the ensemble, certifying the fidelity of multiple copies of the remaining states. We use active optical switches to randomly sample from sources of two-photon Bell states and three-photon GHZ (Greenberger-Horn-Zeilinger) states, reporting statistically sound fidelities in real time without destroying the entire ensemble. In addition, our QSC protocol removes the assumption that the states are identically distributed (but still assumes independent copies); can achieve close N−1 scaling, in the number of states measured N; and can be implemented in a device-independent manner. Together, these benefits make our QSC protocol suitable for benchmarking large-scale quantum computing devices and deployed quantum communication setups relying on entanglement in both standard and adversarial situations.

Quantum state certification is implemented with optimal scaling without consuming all the copies.