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“Spooky” atomic links could lead to “quantum internet”

Sept. 5, 2007
Courtesy University of Michigan
and World Science staff

Phys­i­cists say they’ve coaxed at­oms to seem­ingly com­mu­ni­cate across open spaces, in a phe­nom­e­non Al­bert Ein­stein re­ferred to as “spooky action-at-a-dis­tance.” The re­search­ers call it a step to­ward a type of super-fast com­put­er known as a quan­tum com­put­er, or even to­ward a “quan­tum in­ter­net” that would link up such machines.

In the mys­te­ri­ous phe­nom­e­non of quan­tum en­tan­gle­ment, a meas­ure­ment of a prop­er­ty of one par­t­i­cle guar­an­tees that a par­t­i­cle "en­tangled" with it will have the same prop­er­ty, even if the two are far apart, in space. (Il­lus­tra­tion cour­te­sy NA­SA)
Or­di­nary com­put­ers pro­cess un­its of in­forma­t­ion called bits, which ex­ist in one of two states: 1 or 0. Quan­tum com­put­ers would in­stead use un­its called “qu­bits” that can ex­ist in ei­ther state, but al­so in both si­mul­ta­ne­ous­ly. 

The machines would do this by ex­ploit­ing the fact that ex­tremely small par­t­i­cles, typ­ic­ally at­om-sized or smaller, can ex­ist in two states at once, such as two dif­fer­ent spin align­ments. This ca­pa­bil­ity is due to the bi­zarre laws of quan­tum me­chan­ics, the phys­ics of the small­est ob­jects.

Qubits’ abil­ity to ex­ist in com­bina­t­ions, or “su­per­po­si­tions,” of dif­fer­ent states the­o­ret­ic­ally would speed up cal­cula­t­ions ex­po­nen­tial­ly, as a com­put­er could act on all the states at once. 

An ad­di­tion­al fea­ture re­quired by prac­ti­cal quan­tum com­put­ers is an­oth­er quan­tum-mechanical phe­nom­e­non called en­tan­gle­ment—the pro­cess that spooked Ein­stein. In en­tan­gle­ment, two par­t­i­cles un­der cer­tain con­di­tions can de­ter­mine each oth­er’s prop­er­ties al­though they have no known way to com­mu­ni­cate. It’s like con­trol­ling the out­come of one coin flip with the out­come of an­oth­er coin flip. En­tan­gle­ment pro­vides a way to “wire to­geth­er” qubits in a com­put­er, re­search­ers say.

In the new stu­dy, the phys­i­cists at the Un­ivers­ity of Mich­i­gan in Ann Ar­bor, Mich. en­tan­gled two at­oms trapped in en­clo­sures a me­ter (39 inches) apart. This link­age “could be the fun­da­men­tal piece of a radic­ally new quan­tum com­put­er ar­chi­tec­ture,” said Chris­to­pher Mon­roe, who worked as the pro­ject’s prin­ci­pal in­ves­ti­ga­tor while a pro­fes­sor at Mich­i­gan, though he’s now at the Un­ivers­ity of Mar­y­land. A pa­per on the find­ings ap­pears in the Sept. 6 edi­tion of the re­search jour­nal Na­ture.

The dis­tance be­tween the at­oms is the ex­pe­ri­men­t’s most im­por­tant fea­ture, said Da­vid Moeh­ring, lead au­thor of the pa­per. He re­cently grad­u­at­ed from Mich­i­gan and now works at the Max Planck In­sti­tute for Quan­tum Op­tics in Gar­ch­ing, Ger­ma­ny. 

Shorter-range en­tan­gle­ment has been per­formed be­fore, but the new study used a meth­od of en­tan­gle­ment that in prin­ci­ple could be ex­tend­ed to any range, he went on. This “re­mote” en­tan­gle­ment is nec­es­sary for net­works of quan­tum com­put­ers, he added, which would con­sti­tute a “quan­tum in­ter­net.” Very small-scale quan­tum com­put­ers have been claimed to work be­fore, but phys­i­cists say large-scale ones that could ef­fec­tively re­place tra­di­tion­al com­put­ers are years away.

The at­oms in the new ex­pe­ri­ment were ac­tu­ally ions, or charged at­oms, of the rare-earth el­e­ment yt­ter­bi­um. These can emit two pho­tons, or light waves, of dif­fer­ent en­er­gies. The type of pho­ton re­leased re­veals each at­om’s spe­cif­ic state, so the pho­ton it­self is en­tan­gled with its at­om. 

By ma­ni­pu­lat­ing the pho­tons emitted from each of the two at­oms and guid­ing them to in­ter­act along a fi­ber op­tic thread, the re­search­ers were able to en­tan­gle the at­oms, Mon­roe said. While the thread was needed to es­tab­lish en­tan­gle­ment, he added, the fi­ber could be sev­ered and the at­oms would stay en­tan­gled, even if one were moved—care­ful­ly—“to Jupiter.”

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Physicists report having coaxed two atoms to communicate across space in a phenomenon that Albert Einstein referred to as “spooky action-at-a-distance.” The researchers call it a step toward a type of super-fast computer known as a quantum computer, or even toward a “quantum internet” that would link up such machines. Ordinary computers process information using units of information, called bits, that exist in one of two states: 1 or 0. Quantum computers would use units, called “qubits,” that can exist in either state, but also in both simultaneously. The machines would do this by exploiting the fact that extremely small particles, typically atom-sized or smaller, can exist in two states at once, such as two different spin alignments. This capability is due to the bizarre laws of quantum mechanics, the physics of the smallest objects. Qubits’ ability to exist in combinations, or “superpositions,” of different states theoretically would speed up calculations exponentially, as a computer could act on all the states at once. An additional feature required by practical quantum computers is another quantum-mechanical phenomenon called entanglement—the process that spooked Einstein. In entanglement, two particles under certain conditions can determine each other’s properties although they have no known way to communicate. It’s like controlling the outcome of one coin flip with the outcome of another coin flip. Entanglement provides a way to “wire together” qubits in a computer, researchers say. In the new study, the physicists at the University of Michigan in Ann Arbor, Mich. entangled two atoms trapped in enclosures a meter (39 inches) apart. This linkage “could be the fundamental piece of a radically new quantum computer architecture,” said Christopher Monroe, who worked as the project’s principal investigator while a professor at Michigan though he’s now at the University of Maryland. A paper on the findings appears in the Sept. 6 edition of the research journal Nature. The distance between the atoms is the experiment’s most important feature, said David Moehring, lead author of the paper, who recently graduated from the University of Michigan and now works at the Max Planck Institute for Quantum Optics in Garching, Germany. Shorter-range entanglement has been performed before, but the new study used a method of entanglement that in principle could be extended to any range, he went on. He added that this “remote” entanglement is necessary for networks of quantum computers, which could constitute a “quantum internet.” Very small-scale quantum computers have been claimed to work before, but physicists say large-scale ones that could effectively replace traditional computers are years away. The atoms in the new experiment were actually ions, or charged atoms, of the rare-earth element ytterbium. These can emit two different types of photons, or light waves, of different energies. The type of photon released reveals each atom’s specific state, so the photon itself is entangled with its atom. By manipulating the photons emitted from each of the two atoms and guiding them to interact along a fiber optic thread, the researchers were able to entangle the atoms, Monroe said. While the thread was needed to establish entanglement of the atoms, he added the fiber could be severed and the atoms would stay entangled, even if one were—carefully—”taken to Jupiter.”