Trapped ions realize coupled harmonic oscillators

I highlight this work just because it was done at nearly the same time by two distant groups, one in US and the other in Austria. Both published their work in Nature. They demonstrated the potential of trapped ions in quantum computing.

[doi:10.1038/nature09800] More than 100 years ago, Hertz succeeded in transmitting signals over a few metres to a receiving antenna using an electromagnetic oscillator, thus proving the electromagnetic theory¹ developed by Maxwell. Since this seminal work, technology has developed, and various oscillators are now available at the quantum mechanical level. For quantized electromagnetic oscillations, atoms in cavities can be used to couple electric fields^{2, 3}. However, a quantum mechanical link between two mechanical oscillators (such as cantilevers^{4, 5} or the vibrational modes of trapped atoms⁶ or ions^{7, 8}) has been rarely demonstrated and has been achieved only indirectly. Examples include the mechanical transport of atoms carrying quantum information⁹ or the use of spontaneously emitted photons¹⁰. Here we achieve direct coupling between the motional dipoles of separately trapped ions over a distance of 54 micrometres, using the dipole–dipole interaction as a quantum mechanical transmission line¹¹. This interaction is small between single trapped ions, but the coupling is amplified by using additional trapped ions as antennae. With three ions in each well, the interaction is increased by a factor of seven compared to the single-ion case. This enhancement facilitates bridging of larger distances and relaxes the constraints on the miniaturization of trap electrodes. The system provides a building block for quantum computers and opportunities for coupling different types of quantum systems.

[doi:10.1038/nature09721] The harmonic oscillator is one of the simplest physical systems but also one of the most fundamental. It is ubiquitous in nature, often serving as an approximation for a more complicated system or as a building block in larger models. Realizations of harmonic oscillators in the quantum regime include electromagnetic fields in a cavity^{1, 2, 3} and the mechanical modes of a trapped atom⁴ or macroscopic solid⁵. Quantized interaction between two motional modes of an individual trapped ion has been achieved by coupling through optical fields⁶, and entangled motion of two ions in separate locations has been accomplished indirectly through their internal states⁷. However, direct controllable coupling between quantized mechanical oscillators held in separate locations has not been realized previously. Here we implement such coupling through the mutual Coulomb interaction of two ions held in trapping potentials separated by 40 μm (similar work is reported in a related paper⁸). By tuning the confining wells into resonance, energy is exchanged between the ions at the quantum level, establishing that direct coherent motional coupling is possible for separately trapped ions. The system demonstrates a building block for quantum information processing and quantum simulation. More broadly, this work is a natural precursor to experiments in hybrid quantum systems, such as coupling a trapped ion to a quantized macroscopic mechanical or electrical oscillator.