Researchers use µwaves to quantum-entangle ions

Researchers use µwaves to quantum-entangle ions

New technique could lead to pocket-sized quantum computers based on commercial microwave technology

For the first time, the quantum properties of two separated ions have been manipulated by microwaves, instead of the usual laser beams. Experiments performed by researchers at the U.S. National Institute of Standards and Technology, or NIST (www.nist.gov) in Boulder, CO, indicate it may be possible to significantly simplify and reduce the size of quantum computers, reducing a room-sized quantum computing “laser park” to a pocket-sized system based on commercial microwave technology similar to that used in smart phones. “It’s conceivable a modest-sized quantum computer could eventually look like a smart phone combined with a laser pointer-like device, while sophisticated machines might have an overall footprint comparable to a regular desktop PC,” says physicist Dietrich Leibfried of the NIST group working on trapped-ion quantum information.

Ions are a leading candidate for use as quantum bits (qubits) the basic unit of information in a quantum computer because they can be controlled with better accuracy and less loss of information. Use of microwaves reduces errors introduced by power and laser-beam-pointing instabilities, as well as laser-induced spontaneous emissions by the ions. Experimentally, use of ions is at a more advanced stage than other qubit candidates such as superconducting circuits, which can also be manipulated on chips with microwaves.

While microwaves have been used previously to manipulate single ions, the NIST researchers are the first to enable quantum entanglement by positioning microwaves sources close enough to the ions — just 30 μm away — so as to rotate the “spins” of individual magnesium ions and entangle the spins of an ion pair. Leibfried points out that this is a “universal” set of quantum logic operations because rotations and entanglement can be combined in sequence to perform any calculation allowed by quantum mechanics. The same NIST research group has already used ions and lasers to demonstrate many basic components and processes for a quantum computer.

The latest experiments integrate wiring for microwave sources directly on a chip-sized ion trap and use a desktop-scale table of lasers, mirrors, and lenses that is only about one-tenth the size previously required (seefigure ). Currently, low-power UV lasers are needed to cool the ions and observe experimental results, but those lasers might eventually be made as small as ones used in portable DVD players.

In the microwave apparatus used for NIST quantum computing experiments, a pair of magnesium ions are trapped by electric fields and manipulated with microwaves inside a glass chamber (illuminated by a green light-emitting diode for visual effect) at the center of the apparatus. A UV-laser beam (colorized to appear blue) is used to cool the ions and detect their quantum state. Inside the chamber, two ions are made to hover above the middle of the square (7.4 mm on a side) gold trap (inset) on aluminum nitride backing. (Photo credit: Y. Colombe, NIST.)

In the experiments, two ions were held by electromagnetic fields, hovering above an ion trap chip consisting of gold electrodes electroplated onto an aluminum-nitride backing. Activating some of the electrodes with 1 to 2-GHz microwave pulses produces magnetic fields that are used to rotate the ions’ spins, so that spin orientation can be used to represent binary data. Adapting a technique first developed with lasers, the researchers gradually increase the magnetic fields across the ions in just the right way, so as to excite the ions’ motion depending on the spin orientations and, in the process, entangling the spins. The properties of the entangled ions thus produced are quantum linked, such that a measurement of one ion would reveal the instantaneous state of the other.

The group found the right combination of settings for the three electrodes to produce the optimal change in magnetic fields oscillation while minimizing other, unwanted effects. However, microwave operations still need to be improved to enable practical quantum computations or simulations: the NIST researchers achieved entanglement 76% of the time (well above the minimum threshold of 50% defining the onset of quantum properties), less than the 99.3% entanglement of the best laser-controlled operations.

In addition to improving microwave operations by reducing unwanted ion motion, the NIST team also plans to study how to suppress cross-talk between different information processing zones on the same chip. Different frequencies could be used for logic operations and control of other nearby qubits, for instance. Smaller traps could enable faster operations if unwanted heating can be suppressed.

Quantum computers could solve certain problems that are currently intractable even with supercomputers — such as breaking complex data encryption codes. A nearer-term goal is to design quantum simulations of physical phenomenon, such as high-temperature superconductivity, to gain deeper insights into its mechanisms.

To build practical systems of thousands of ions for quantum computing and simulations, microwave components could be expanded and upgraded more easily and cheaply than complex, expensive laser sources. “Although quantum computers are not thought of as convenience devices that everybody wants to carry around,” says Leibfried, “they could use microwave electronics similar to what [has been] developed for a mass market to support innovation and reduce costs. The prospect excites us.”

The research was supported by the Intelligence Advanced Research Projects Activity, Office of Naval Research, Defense Advanced Research Projects Agency, National Security Agency, and Sandia National Laboratories. For more information, contact
, 303-497-4880.

Richard Comerford