Scalable Multiparticle Entanglement of Trapped Ions

Schrödinger's hypothetical cat was both dead and alive thanks to a paradox of quantum mechanics, in which a system exists in two or more states at once in a ‘superposition’ of entangled states. Creating this situation experimentally is very difficult, especially for systems made up of many particles, as interactions with the environment destroy superposition in a process called decoherence. So far, entangled states of just a handful of atoms or photons have been achieved. Now, two groups have extended the limits of quantum state engineering by creating the largest entangled atomic systems to date. Working with atoms held in an ion trap by an electromagnetic field to limit decoherence, a team from the National Institute of Standards and Technology in Boulder, Colorado, has created ‘cat states’ of up to six beryllium atoms. A second group, based at Innsbruck University in Austria, has achieved a similar feat by making a related entangled state, a ‘W state’, containing up to eight particles. As the states are created ‘on demand’ and should be scalable to many more particles, there is hope that this technology will pave the way for building a large-scale quantum computer. The generation, manipulation and fundamental understanding of entanglement lies at the very heart of quantum mechanics. Entangled particles are non-interacting but are described by a common wavefunction; consequently, individual particles are not independent of each other and their quantum properties are inextricably interwoven1,2,3. The intriguing features of entanglement become particularly evident if the particles can be individually controlled and physically separated. However, both the experimental realization and characterization of entanglement become exceedingly difficult for systems with many particles. The main difficulty is to manipulate and detect the quantum state of individual particles as well as to control the interaction between them. So far, entanglement of four ions4 or five photons5 has been demonstrated experimentally. The creation of scalable multiparticle entanglement demands a non-exponential scaling of resources with particle number. Among the various kinds of entangled states, the ‘W state’6,7,8 plays an important role as its entanglement is maximally persistent and robust even under particle loss. Such states are central as a resource in quantum information processing9 and multiparty quantum communication. Here we report the scalable and deterministic generation of four-, five-, six-, seven- and eight-particle entangled states of the W type with trapped ions. We obtain the maximum possible information on these states by performing full characterization via state tomography10, using individual control and detection of the ions. A detailed analysis proves that the entanglement is genuine. The availability of such multiparticle entangled states, together with full information in the form of their density matrices, creates a test-bed for theoretical studies of multiparticle entanglement. Independently, ‘Greenberger–Horne–Zeilinger’ entangled states11 with up to six ions have been created and analysed in Boulder12.