Deterministic generation of a cluster state of entangled photons

Weaving an entangled cluster Entanglement is a powerful resource for quantum computation and information processing. One requirement is the ability to entangle multiple particles reliably. Schwartz et al. created an on-demand entangled cluster state of several photons by addressing a quantum dot with a sequence of laser pulses (see the Perspective by Briegel). They used an internal state of the quantum dot, a dark exciton, and its association with another internal state, a biexciton, to weave successive photons into an entangled cluster, generating entanglement between up to five photons. Science, this issue p. 434; see also p. 416 A quantum dot is used to realize entangled cluster states of up to five photons. Photonic cluster states are a resource for quantum computation based solely on single-photon measurements. We use semiconductor quantum dots to deterministically generate long strings of polarization-entangled photons in a cluster state by periodic timed excitation of a precessing matter qubit. In each period, an entangled photon is added to the cluster state formed by the matter qubit and the previously emitted photons. In our prototype device, the qubit is the confined dark exciton, and it produces strings of hundreds of photons in which the entanglement persists over five sequential photons. The measured process map characterizing the device has a fidelity of 0.81 with that of an ideal device. Further feasible improvements of this device may reduce the resources needed for optical quantum information processing.

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