Nature Photon. 8, 364–368 (2014)

Putting the power of quantum entanglement at our disposal is the stuff of quantum theorists' dreams. More often than not, however, reality doesn't look quite as elegant as the mathematics — decoherence is largely to blame.

One clever theoretical solution to the problem of constructing more robust quantum states relies on concatenating layers of entanglement — using smaller states as building blocks for larger ones. Now, it's no longer just theory: He Lu and colleagues have demonstrated the creation of concatenated Greenberger–Horne–Zeilinger (GHZ) multi-partite entangled states using many photons.

In their experimental set-up, the team encoded logical qubits in polarization Bell states of photon pairs. In this way, six photons were used to create a three-qubit GHZ state, whose robustness against decoherence was reinforced by the underlying additional bi-partite entanglement layer. The fragile multi-partite quantum correlations thereby lasted measurably longer than those established in any other system thus far.