Efficient detection of useful long-range entanglement in imperfect cluster states

TL;DR

This paper introduces a benchmarking method to detect long-range entanglement in imperfect photonic cluster states generated by quantum dots, enabling useful quantum computing resources despite high photon loss.

Contribution

The authors propose a new measurement-based benchmarking technique that estimates long-range entanglement in photonic cluster states with low detection efficiency.

Findings

Estimates three-qubit correlation <ZXZ> to bound entanglement.

Demonstrates <ZXZ> > 2/3 as a criterion for useful entanglement.

Enables experimental verification of entanglement with current technology.

Abstract

Photonic cluster states are a crucial resource for optical quantum computing. Recently a quantum dot single photon source has been demonstrated to produce strings of photons in a linear cluster state, but high photon loss rates make it impossible to characterize the entanglement generated by conventional methods. We present a benchmarking method for such sources that can be used to demonstrate useful long-range entanglement with currently available collection/detection efficiencies below 1%. Measurement of the polarization state of single photons in different bases can provide an estimate for the three-qubit correlation function <ZXZ>. This value constrains correlations spanning more than three qubits, which in turn provide a lower bound for the localizable entanglement between any two qubits in the large state produced by the source. Finite localizable entanglement can be established by demonstrating <ZXZ> > 2/3. This result enables photonic experiments demonstrating computationally useful entanglement with currently available technology.