Physicists at the National Institute of Standards and Technology (NIST) have, for the first time, linked the quantum properties of two separated ions (electrically charged atoms) by manipulating them with microwaves instead of the usual laser beams. The feat raises the possibility of replacing today's complex, room-sized quantum computing "laser parks" with miniaturized, commercial microwave technology similar to that used in smart phones.
Microwaves have been used in past experiments to manipulate single ions, but the NIST group is the first to position microwaves sources close enough to the ions—just 30 micrometers away—and create the conditions enabling entanglement, a quantum phenomenon expected to be crucial for transporting information and correcting errors in quantum computers.
Described in the August 11, 2011, issue of Nature,* the 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 of the size previously required. Low-power ultraviolet lasers still are needed to cool the ions and observe experimental results but might eventually be made as small as those in portable DVD players. Compared to complex, expensive laser sources, microwave components could be expanded and upgraded more easily to build practical systems of thousands of ions for quantum computing and simulations.
"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 NIST physicist Dietrich Leibfried, a co-author of the new paper.
Quantum computers would harness the unusual rules of quantum physics to solve certain problems—such as breaking today's most widely used data encryption codes—that are currently intractable even with supercomputers. A nearer-term goal is to design quantum simulations of important scientific problems, to explore quantum mysteries such as high-temperature superconductivity, the disappearance of electrical resistance in certain materials when sufficiently chilled.
Ions are a leading candidate for use as quantum bits (qubits) to hold information in a quantum computer. Although other promising candidates for qubits—notably superconducting circuits, or "artificial atoms"—are manipulated on chips with microwaves, ion qubits are at a more advanced stage experimentally in that more ions can be controlled with better accuracy and less loss of information.
The use of microwaves reduces errors introduced by instabilities in laser beam pointing and power as well as laser-induced spontaneous emissions by the ions. However, microwave operations need to be improved to enable practical quantum computations or simulations. The NIST researchers achieved entanglement 76 percent of the time, well above the minimum threshold of 50 percent defining the onset of quantum properties but not yet competitive with the best laser-controlled operations at 99.3 percent.
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 details, see the NIST Aug. 11 news announcement "NIST Physicists 'Entangle' Two Atoms Using Microwaves for the First Time" at www.nist.gov/pml/div688/microwave-quantum-081011.cfm.