Microwaves to entangle trapped ions

This is definitely a very important step toward practical quantum computing. Rather than using laser, which are not easy to set up as required, these groups [The work is described in Nature 476 181 and Nature 476 185.] use microwaves that can created by just alternating currents, to entangle ion spins.

However, entangling two trapped ions had required a pair of carefully aligned ultraviolet laser beams – which cannot be produced easily on an integrated circuit. To entangle two pairs of ions, two pairs of laser beams were needed and so on. A practical quantum computer would need a processor containing thousands or even millions of qubits, so scientists have long sought a way to manipulate many trapped ions without large numbers of laser beams.

In 2001 Christof Wunderlich and a colleague of the University of Hamburg had the idea of replacing the lasers with microwave and radio sources, which can be produced and controlled much more easily. Such radiation had previously been used in other trapped-ion experiments, but using it to implement quantum logic operations was a highly revolutionary suggestion. This is because the type of interaction required for quantum logic is usually very weak for this radiation. However, the researchers suggested adding a magnetic field gradient to stimulate the interaction.

Unfortunately, the need to use states that are sensitive to static magnetic fields makes the quantum states vulnerable to the magnetic noise found all around us, and the technique proved problematic. In 2008 physicists at the Ion Storage Group at the National Institute for Standards and Technology (NIST) in Boulder, Colorado, proposed eliminating the static magnetic-field gradient and using instead the oscillating field produced by the microwave source itself. The benefit being that the quantum states used in this scheme are less vulnerable to magnetic noise and more robust.

Both research groups now report significant advances in the journal Nature. Wunderlich's group, now at the University of Siegen, together with colleagues from the Institute of Theoretical Physics in Ulm, have come up with a way to produce states that, while still sensitive to the applied magnetic-field gradient, are far less vulnerable to noise and thus can be preserved more than 100 times longer. In a commentary accompanying the papers, Winfried Hensinger of the University of Sussex compares the group's scheme to a car's suspension system, which decouples the body from the wheels so that bumps in the road do not disturb the driver.

The NIST group, meanwhile, goes further and performs all of the essential quantum logical operations (albeit on only two qubits) using microwave radiation delivered via a waveguide integrated into a chip. "We've integrated the mechanism that does the entanglement between the two ions into the trapping structure," says Christian Ospelkaus, who built the experiment together with colleagues at NIST. "We no longer need to build a really complex and sophisticated laser system around the whole camp: we just send an electric current through the trap structure and that generates oscillating fields and it does all the other coherent quantum operations we need to do."[http://physicsworld.com/cws/article/news/46826]

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.