Experimental bound entanglement in a four-photon state

TL;DR

This paper reports the first experimental realization of bound entanglement in a four-photon state, demonstrating its properties and potential for entanglement unlocking in quantum information processing.

Contribution

It provides the first experimental demonstration of bound entanglement in a four-qubit state using photonic encoding, confirming theoretical predictions.

Findings

Successfully prepared a four-photon bound entangled state

Confirmed the state exhibits entanglement and undistillability

Demonstrated entanglement unlocking using the state

Abstract

Entanglement [1, 2] enables powerful new quantum technologies [3-8], but in real-world implementations, entangled states are often subject to decoherence and preparation errors. Entanglement distillation [9, 10] can often counteract these effects by converting imperfectly entangled states into a smaller number of maximally entangled states. States that are entangled but cannot be distilled are called bound entangled [11]. Bound entanglement is central to many exciting theoretical results in quantum information processing [12-14], but has thus far not been experimentally realized. A recent claim for experimental bound entanglement is not supported by their data [15]. Here, we consider a family of four-qubit Smolin states [16], focusing on a regime where the bound entanglement is experimentally robust. We encode the state into the polarization of four photons and show that our state exhibits both entanglement and undistillability, the two defining properties of bound entanglement. We then use our state to implement entanglement unlocking, a key feature of Smolin states [16].