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Bizarre “quantum” behavior noted in device large enough to see

March 22, 2010
Courtesy of Nature Publishing Group
and World Science staff

A de­vice large enough to be seen with the un­aided eye has been made to show “quan­tum” be­hav­ior, a pos­si­bil­ity that pre­vi­ously ap­plied only to much smaller ob­jects, phys­i­cists say.

Quan­tum be­hav­ior is a set of seem­ingly non­sen­si­cal rules that sci­en­tists have found to apply most clearly at atom­ic and smaller size scales. He­re, ob­jects be­have—as they pre­s­ent them­selves in ex­pe­ri­ments—in par­a­dox­i­cal ways, for in­stance, as though they were in two places at once, or pos­sessed of oth­er mu­tu­ally con­tra­dic­to­ry prop­er­ties. 

Be­cause no phys­i­cist has yet found clear and widely agreed-upon ways to avoid such con­clu­sions, this strange pic­ture of real­ity must be tak­en more or less as a giv­en in ex­pe­ri­ments at the quan­tum realm.

The bi­zarre ef­fects fade away at larg­er sizes be­cause as more par­t­i­cles are added in­to a sys­tem, the seem­ingly ab­surd in­di­vid­ual be­hav­iors blend to­geth­er and ef­fec­tively can­cel each oth­er out. 

Now, though, a group of phys­i­cists re­port that ob­servably “quan­tum” be­hav­ior can be re­stored to a thing large enough to see with the na­ked eye, un­der spe­cial cir­cum­stances. 

The ob­ject must be dras­tic­ally cooled un­til it reaches a so-called “quan­tum ground state,” where all vibra­t­ions due to heat are elim­i­nat­ed, ac­cord­ing to the re­search­ers, An­drew Cle­land of the Uni­vers­ity of Cal­i­for­nia, San­ta Bar­ba­ra, and col­leagues.

The group made a vi­brat­ing de­vice akin to a ti­ny drum, with a very high os­cilla­t­ion speed — over 6 bil­lion os­cilla­t­ions per sec­ond. They then cooled the “quan­tum drum” to one-fortieth of a de­gree above the the­o­ret­ic­ally min­i­mum tempe­rature at­tain­a­ble any­where, called ab­so­lute ze­ro. The res­o­na­tor, about a twen­ty-fifth of a mil­li­me­ter wide, was al­so linked elec­tric­ally to a well-un­der­stood quan­tum de­vice, called a su­pe­rcon­duct­ing quan­tum bit or “qu­bit.”

The sci­en­tists then used the qubit to stim­u­late the res­o­na­tor to pro­duce the small­est pos­si­ble un­it of vi­b­ra­t­ional en­er­gy, called a pho­non, pro­ducible by the de­vice. They fur­ther trans­ferred this en­er­gy re­peat­edly be­tween the res­o­na­tor and qu­bit. 

Be­cause a pho­non be­haves in cer­tain ways as both a wave and a par­t­i­cle, it ex­hibits a prope­rty called wave-par­t­i­cle du­al­ity si­m­i­lar to that found in oth­er ob­jects at the quan­tum realm. Ex­am­ples in­clude par­t­i­cles of light, called pho­tons.

The set­up im­plies that the re­search­ers have achieved “quan­tum con­trol” over the ap­pa­rat­us, they said.

Phys­i­cist Markus As­pel­meyer of the Uni­vers­ity of Vi­en­na, who was not in­volved in the re­search, agreed. Cle­land’s group has achieved “con­trol [of] mac­ro­scop­ic [vis­i­ble] me­chan­i­cal ob­jects in their small­est pos­si­ble state of mo­tion,” he ex­plained in an ar­ti­cle in the March 18 is­sue of the re­search jour­nal Na­ture. “Suc­cess in achiev­ing that goal her­alds a new genera­t­ion of quan­tum ex­pe­ri­ments.”

Cle­land and col­leagues, whose own find­ings are pub­lished in the same is­sue of the jour­nal, said they al­so dem­on­strat­ed in their de­vice the pos­si­bil­ity of some of the par­a­dox­i­cal be­hav­ior here­te­fore seen only in the sub-microscopic quan­tum realm.

They man­aged to place the res­o­na­tor in a state called supe­rposition, in which the sys­tem is “both ex­cit­ed and not ex­cit­ed at the same time,” that is, both mov­ing and sta­t­ionary, said Cle­land in an e­mail. “This is not the same as half of an ex­cita­t­ion, as the ex­cita­t­ions are indivis­i­ble.”

* * *

A device large enough to be seen with the unaided eye has been made to show “quantum” behavior, a possibility that previously applied only to much smaller objects, physicists say. Quantum behavior is a set of seemingly nonsensical rules that scientists have found to apply most clearly at atomic and smaller size scales. Here, objects behave—as they present themselves in experiments—in paradoxical ways. For instance, they may act as though they are in two places at once, or possess other mutually contradictory properties. Because no physicist has yet found clear and widely agreed-upon ways to avoid such conclusions, this paradoxical picture of reality must be taken more or less as a given in experiments in the quantum realm. The bizarre effects fade away at larger sizes because as more particles are added into a system, the seemingly absurd individual behaviors blend together and effectively cancel each other out. Now, though, a group of physicists report that observably “quantum” behavior can be restored to a thing large enough to see with the naked eye, under special circumstances. The object must be drastically cooled until it reaches a so-called “quantum ground state,” where all vibrations due to heat are eliminated, according to the researchers, Andrew Cleland of the University of California, Santa Barbara, and colleagues. The group made a vibrating device akin to a tiny drum, with a very high oscillation speed — over 6 billion oscillations per second. They then cooled the “quantum drum” to one-fortieth of a degree above the theoretically minimum temperature attainable anywhere, called absolute zero. The resonator, about a twenty-fifth of a millimeter wide, was also linked electrically to a well-understood quantum device, called a superconducting quantum bit or “qubit.” The scientists then used the qubit to stimulate the resonator to produce the smallest possible unit of vibrational energy, called a phonon, producible by the device. They further transferred this energy repeatedly between the resonator and qubit. Because a phonon behaves in certain ways as both a wave and a particle, it exhibits a property called wave-particle duality similar to that found in other objects at the quantum realm. Examples include particles of light, called photons. The setup implies that the researchers have achieved “quantum control” over the apparatus, they said. Physicist Markus Aspelmeyer of the University of Vienna, who was not involved in the research, agreed. Cleland’s group has achieved “control [of] macroscopic [visible] mechanical objects in their smallest possible state of motion,” he explained in an article in the March 18 issue of the research journal Nature. “Success in achieving that goal heralds a new generation of quantum experiments.” Cleland and colleagues, whose own findings are published in the same issue of the journal, said they also demonstrated in their device the possibility of some of the paradoxical behavior heretefore seen only in the sub-microscopic quantum realm. They managed to place the resonator in a state called superposition, in which the system is “both excited and not excited at the same time,” that is, both moving and stationary, said Cleland in an email. “This is not the same as half of an excitation, as the excitations are indivisible.”