Robust entanglement

Nature 504, 415–418 (2013)

Entangled states are required for a wide range of tasks in quantum optics and quantum information processing, but are fragile and easily destroyed. Controlled unitary processes have so far been the most widely used method to create entanglement deterministically, but they are susceptible to decoherence and dissipation as a result of coupling to the environment. Now, Yiheng Lin and co-workers from the USA and Denmark have developed a scheme for producing an entangled state that is inherently stable against decoherence. A ⁹Be⁺–²⁴Mg⁺–²⁴Mg⁺–⁹Be⁺ four-ion chain is confined in a linear radiofrequency Paul trap. The two ⁹Be⁺ ions serve as qubit ions, whereas the two ²⁴Mg⁺ ions are used for sympathetic cooling. Two 313 nm laser beams are frequency shifted using acousto-optic modulators and applied to the ion chain to create sideband coupling between the two qubit spin states; this coupling is required to produce steady-state entanglement. To cool the ²⁴Mg⁺ ions, laser beams with a wavelength of about 280 nm are also used. Spin-state analysis revealed that the system reaches a maximally entangled steady state after a few milliseconds with a fidelity of 0.75 when a combination of optical pulses is applied to it. Stepwise application of these pulses with a duration of 220 μs can speed up the dynamics of the scheme and achieve a fidelity of 0.89 after approximately 30 repetitions.