Science 344, 180–183 (2014)

Multiparticle entanglement is useful for enabling tasks such as quantum simulation, quantum computing and quantum-enhanced metrology. However, the engineering of large-scale entangled quantum states in atomic systems is currently limited to fewer than 20 entangled qubits. To overcome this technical limitation, Florian Haas and co-workers from École Normale Supérieure in France have developed a method based on the simultaneous, coherent interaction between about 40 87Rb atoms and the light field of an optical cavity. They used a high-finesse optical cavity formed by a small gap between the ends of two optical fibres. Precisely tuning the resonator length to match the wavelength of a weak impinging laser beam led to light transmission with a narrow linewidth. Although an ensemble of 87Rb atoms in their internal ground state was transparent to a light field resonant with the cavity, a single microwave excitation rendered the resonator opaque, heralding the presence of the entangled W state — a coherent superposition state of all possible combinations that place one quantized excitation in the excited state. The scientists created and analysed entangled states having mean atom numbers of up to 41 and experimentally demonstrated multiparticle entanglement.