Experimental Quantum State Certification by Actively Sampling Photonic Entangled States

TL;DR

This paper demonstrates an experimental method for certifying quantum entangled states efficiently by actively sampling and measuring only a subset of states, enabling real-time fidelity assessment without destroying the entire ensemble.

Contribution

The authors experimentally implement a quantum state certification protocol that removes IID assumptions, uses active sampling, and provides real-time fidelity estimates for entangled states.

Findings

Successfully certifies fidelity of entangled states in real time

Removes IID assumption in quantum state verification

Achieves near optimal scaling with number of measurements

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 independent (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) proposed by Gocanin \textit{et al.}, 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 states, reporting statistically-sound fidelities in real time without destroying the entire ensemble. Additionally, 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. Altogether, 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.