BOULDER, Colo.—Physicists at the NationalInstitute of Standards and Technology(NIST) have for the first time coaxed twoatoms in separate locations to take turnsjiggling back and forth while swapping thesmallest measurable units of energy. Bydirectly linking the motions of two physicallyseparated atoms, the technique has thepotential to simplify information processing infuture quantum computers and simulations.
Described in a paper published Feb.23 by Nature,* the NIST experiments enticed two beryllium ions (electrically chargedatoms) to take turns vibrating in an electromagnetic trap, exchanging units of energy, orquanta, that are a hallmark of quantum mechanics. As little as one quantum was tradedback and forth in these exchanges, signifying that the ions are "coupled" or linked together.These ions also behave like objects in the larger, everyday world in that they are"harmonic oscillators" similar to pendulums and tuning forks, making repetitive, back-and-forthmotions.
"First one ion is jiggling a little and the other is not moving at all; then the jigglingmotion switches to the other ion. The smallest amount of energy you could possibly see ismoving between the ions," explains first author Kenton Brown, a NIST post-doctoralresearcher. "We can also tune the coupling, which affects how fast they exchange energyand to what degree. We can turn the interaction on and off."
The experiments were made possible by a novel, one-layer ion trap cooled to minus269 C (minus 452 F) with a liquid helium bath. The ions, 40 micrometers apart, float abovethe surface of the trap. In contrast to a conventional two-layer trap, the surface trapfeatures smaller electrodes and can position ions closer together, enabling strongercoupling. Chilling to cryogenic temperatures suppresses unwanted heat that can distort ionbehavior.
The energy swapping demonstrations begin by cooling both ions with a laser toslow their motion. Then one ion is cooled further to a motionless state with two opposingultraviolet laser beams. Next the coupling interaction is turned on by tuning the voltages ofthe trap electrodes. In separate experiments reported in Nature, NIST researchersmeasured the ions swapping energy at levels of several quanta every 155 microsecondsand at the single quantum level somewhat less frequently, every 218 microseconds.Theoretically, the ions could swap energy indefinitely until the process is disrupted byheating. NIST scientists observed two round-trip exchanges at the single quantum level.
NIST physicists used this apparatus to coax two beryllium ions (electrically charged atoms) into swapping the smallest measurable units of energy back and forth, a technique that may simplify information processing in a quantum computer. The ions are trapped about 40 micrometers apart above the square gold chip in the center. The chip is surrounded by a copper enclosure and gold wire mesh to prevent buildup of static charge.
(Photo Credit: Y. Colombe/NIST)
To detect and measure the ions' activity, NIST scientists apply an oscillating pulseto the trap at different frequencies while illuminating both ions with an ultraviolet laser andanalyzing the scattered light. Each ion has its own characteristic vibration frequency; whenexcited, the motion reduces the amount of laser light absorbed. Dimming of the scatteredlight tells scientists an ion is vibrating at a particular pulse frequency.
To turn on the coupling interaction, scientists use electrode voltages to tune thefrequencies of the two ions, nudging them closer together. The coupling is strongest whenthe frequencies are closest. The motions become linked due to the electrostaticinteractions of the positively charged ions, which tend to repel each other. Couplingassociates each ion with both characteristic frequencies.
The new experiments are similar to the same NIST research group's 2009demonstration of entanglement—a quantum phenomenon linking properties of separatedparticles—in a mechanical system of two separated pairs of vibrating ions (seehttp://www.nist.gov/pml/div688/jost_060309.cfm). However, the new experiments coupledthe oscillators' motions more directly than before and, therefore, may simplify informationprocessing. In this case the researchers observed quantum behavior but did not verifyentanglement.
The new technique could be useful in a future quantum computer, which would usequantum systems such as ions to solve problems that are intractable today. For example,quantum computers could break today's most widely used data encryption codes. Directcoupling of ions in separate locations could simplify logic operations and help correctprocessing errors. The technique is also a feature of proposals for quantum simulations,which may help explain the mechanisms of complex quantum systems such as high-temperaturesuperconductors.
In addition, the demonstration also suggests that similar interactions could be usedto connect different types of quantum systems, such as a trapped ion and a particle of light(photon), to transfer information in a future quantum network. For example, a trapped ioncould act as a "quantum transformer" between a superconducting quantum bit (qubit) anda qubit made of photons.
Source: National Institute of Standards and Technology (NIST)