Entangled mechanical oscillators

Entanglement is one of the hallmarks of quantum mechanics and is a key tool in the burgeoning field of quantum information processing. Generating entangled states has become routine in the quantum realms of photons and of electron and atomic spins. Now John Jost and colleagues at NIST in Boulder, Colorado, the Weizmann Institute of Science, and Lockheed Martin Corp have demonstrated entanglement in separated mechanical oscillators. Each oscillator consists of a pair of ions—one ⁹Be⁺ and one ²⁴ Mg ⁺—that behave like two unequal masses connected by a spring 4 µm long. The pairs are separated by 240 µm, so their individual vibrational motions are decoupled. To entangle those vibrational modes, the researchers cool the four ions in one zone of a multizone ion trap (shown here) while coaxing them with electrode voltages to line up in a specific order: a Be ion at each end. They next entangle the spins of the two Be ions and then separate the pairs into different trap zones. Lasers tuned to the Mg ions recool the separated pairs while maintaining the Be entanglement. The team finally uses laser pulses to coherently transfer the entanglement from the Be spin states onto the pairs’ motional states. The end product is the entangled superposition of vibrational oscillations in the pairs’ ground and first excited states. Along the way, the team also demonstrated the entanglement between one ion’s spin state and the motion of the other ion pair. Mechanical entanglement and the tools developed to achieve it will be important ingredients for scaling up quantum information processing with trapped ions. (J. D. Jost et al., Nature 459 , 683, 2009 http://dx.doi.org/10.1038/nature08006 .)