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Physicists seek to put one thing in two places

Sept. 25, 2006
Special to World Science  

Physi­cists say they have made an ob­ject move just by watch­ing it. This is in­spir­ing them to a still bold­er proj­ect: put­ting a small, or­di­nary thing in­to two places at once.

It may be a “fan­ta­sy,” ad­mits Keith Schwab of Cor­nell Uni­ver­si­ty in Ith­a­ca, N.Y., one of the re­search­ers. Then again, the first ef­fect seemed that way not long ago, and the sec­ond is re­lat­ed.

The gray sliv­er reach­ing from top to bot­tom, slanted in the im­age, is a na­no­me­chan­i­cal re­s­o­na­tor, a sub-mi­c­ro­s­co­pic de­vice that can vi­brate like a pia­no string. The im­age was tak­en with a scan­ning el­ec­tron mi­cro­scope and col­or­ized. (Cour­te­sy Cor­nell Uni­ver­si­ty)

The re­search comes from the edge of quan­tum me­chan­ics, the sub­mi­cro­sco­pic realm of fun­da­men­tal par­t­i­cles. There, things be­have with to­tal dis­re­gard for our com­mon sense.

They can show signs of be­ing in two places at once; of be­ing both waves and par­ti­cles; of tak­ing on some cha­r­ac­ter­is­t­ics on­ly at the mo­ment these are meas­ured; and of act­ing syn­chro­nous­ly while far apart, with no ap­par­ent way to com­mu­ni­cate.

Al­though these ti­ny build­ing blocks of our uni­verse do this, the re­l­a­tively huge things we see eve­ry day don’t. The un­can­ny be­hav­ior fades the big­ger a thing be­comes.

This is be­cause when quan­tum en­t­i­ties are com­bined to make or­di­na­ry ob­jects, the rules go­vern­ing each com­po­nen­t’s be­ha­v­ior add up to pro­duce new rules. These in­c­rea­s­ing­ly re­sem­ble the laws of our fa­mi­l­iar re­a­li­ty as more ad­di­tions take place.

But just how big can some­thing be and still show signs of slip­ping back in­to its quan­tum-me­chan­i­cal na­ture? 

Schwab and his col­leagues de­cid­ed to find out. In work de­s­cribed in the Sept. 14 is­sue of the re­search jour­nal Na­ture, they built a de­vice co­los­sal by quan­tum stan­dards: about nine thou­sandths of a mil­li­me­ter long, con­tain­ing some 10 tril­lion atoms. 

The ob­ject was a sliv­er of alu­mi­num and a type of ce­ram­ic, fixed at both ends but free to vi­brate like a gui­tar string in be­tween. To meas­ure its move­ments, the sci­en­tists set near­by a ti­ny de­tec­tor called a su­per­con­duct­ing sin­gle elec­tron tran­sis­tor.

They found that ran­dom mo­tions of charge-carrying par­ti­cles, elec­trons, in the de­tec­tor em­a­nat­ed forc­es that af­fect­ed the me­tal­lic sliv­er. When the de­tec­tor was tuned for max­i­mum sen­si­tiv­i­ty, these forc­es slowed down the sliv­er’s shak­ing, cool­ing it as a re­sult. This ef­fect, Schwab said, is a ba­si­cal­ly quan­tum-me­chan­i­cal phe­nom­e­non called back-action, in which the act of ob­serv­ing some­thing ac­tu­al­ly gives it a nudge. 

Back-action in quan­tum me­chan­ics al­so makes it im­pos­si­ble to know a par­ti­cle’s ex­act lo­ca­tion and speed si­mul­ta­ne­ous­ly. This lim­i­ta­tion is called the un­cer­tain­ty prin­ci­ple. A com­mon ex­am­ple: meas­ur­ing place and speed re­quires some de­tec­tor that can “see” the par­ti­cle. But this in­volves bounc­ing a light wave off it, which gives it a ran­dom push.

“We made meas­urements of po­si­tion that are so in­tense—so strongly cou­pled—that by look­ing at it we can make it move,” said Schwab. Nor­mal­ly, such mo­tion would­n’t cool an ob­ject. But the mo­tion can be such as to op­pose on­go­ing move­ments and slow them down. This re­duces an ob­ject’s heat, which is just the jig­gling of par­ti­cles in it.

If back-action ap­plies such a large item, Schwab rea­sons, may­be that can al­so be true of oth­er quan­tum-me­chan­i­cal rules. Particularly in­tri­guing, he said, is the superpo­si­tion prin­ci­ple, which holds that a par­ti­cle can be in two places at once.

A classic ex­am­ple is the shoot­ing of light par­ti­cles, called pho­tons, through two slits in a bar­rier. Past the slits, they will be­have as if they were waves. This alone is no sur­prise: it’s a well-known quan­tum me­chan­i­cal phe­nom­e­non that par­ti­cles can par­a­dox­i­cal­ly act like waves in some sit­u­a­tions. The pho­tons’ wav­i­ness then makes them “in­ter­fere” with each oth­er. In oth­er words, they make pat­terns like those seen when you toss two peb­bles in a pond, and the rip­ples they make overlap. 

When the waves passing the two slits mu­tu­al­ly in­ter­fere, the pat­tern be­comes vi­si­ble if you set up anoth­er wall where the pho­tons can land. There, al­ter­nat­ing bright and dark stripes ap­pear.

But bi­zarre­ly, this works even if you fire just one pho­ton at a time through the slits. You can see the ef­fect then by put­ting pho­to­graph­ic film on the land­ing wall, so each pho­ton leaves a last­ing mark. Keep fir­ing pho­tons, and the marks grad­u­al­ly add up to make the stripes again.

It’s as if each pho­ton is in­ter­fer­ing with it­self—that is, go­ing through both slits si­mul­ta­ne­ous­ly. This al­so works for big­ger par­ti­cles, up to a point. But what point? Schwab wants to know. “We’re try­ing to make a me­chan­i­cal de­vice be in two places at one time. What’s real­ly neat is it looks like we should be able to do it,” he said. “The hope, the dream, the fan­ta­sy is that we get that superpo­si­tion and start mak­ing big­ger de­vices and find the break­down.”

In a com­men­tary in the same is­sue of Na­ture, Mi­chael Roukes of the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy in Pas­a­de­na, Calif., wrote that Schwab’s work with the cool­ing is part of an emerg­ing field, quan­tum electrome­chan­ics. This, he added, fo­cus­es on sub­mi­cro­scop­ic de­vices called nanome­chan­i­cal sys­tems, “poised mid­way be­tween two seem­ingly an­ti­thet­i­c do­mains” of size: fun­da­men­tal par­ti­cles at one end, the ob­jects of eve­ryday life at the oth­er.

* * *

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Physicists say they have made an object move just by watching it, and that this is leading them to a new project: trying to put a small, ordinary object into two places simultaneously. It may be a “fantasy,” admits Keith Schwab of Cornell University in Ithaca, N.Y. one of the researchers. Then again, the first effect also seemed that way not long ago, and the two are related. The research comes from the edge of quantum mechanics, the submicroscopic realm of elementary particles, where things act in seemingly nonsensical ways. Elementary particles can show signs of being in two places at once; of being both waves and particles; of taking on some characteristics only at the moment these are measured; and of acting synchronously with each other over great distances, with no apparent way to communicate. Although these minuscule building blocks of our universe do this, the relatively huge things that we see every day don’t. The uncanny behavior fades away the bigger a thing becomes, because when quantum entities are combined to make an ordinary-sized object, the rules governing each component’s behavior add up. The additions produce new rules that become closer to those of our familiar world the more additions take place, and the bigger the object. But just how large can an object be and still show signs of slipping into its fundamental quantum-mechanical nature? Schwab and his colleagues decided to find out. In work described in the Sept. 14 issue of the research journal Nature, they built a device colossal by quantum standards: about nine thousandths of a millimeter long, containing some 10 trillion atoms. The object was a sliver of aluminum on a type of ceramic, fixed at both ends but free to vibrate like a guitar string in between. To measure its movements, the scientists set nearby a tiny detector known as a superconducting single electron transistor. They found that random motions of charge-carrying particles, electrons, in the detector emanated forces that affected the metallic sliver. When the detector was tuned for maximum sensitivity, these forces actually slowed down the sliver’s shaking, cooling it as a result. This effect, Schwab said, is a basically quantum-mechanical phenomenon called back-action, in which the act of observing something actually gives it a nudge. In quantum mechanics, back-action also makes it impossible to know a particle’s exact location and speed simultaneously. This limitation is called the uncertainty principle. A common example: one way to measure place and speed is with some detector that can “see” the particle. But this involves bouncing a light wave off it, which gives it a random push. “We made measurements of position that are so intense—so strongly coupled—that by looking at it we can make it move,” said Schwab. Normally, such motion wouldn’t cool an object. But if the motion is such as to oppose ongoing movements and slow them down, it can reduce an object’s heat, which is nothing more than the tiny movements and vibrations of the particles in it. If back-action applies such a large object, Schwab reasoned, maybe that can also be true of other quantum-mechanical rules. Particularly intriguing, he said, is the superposition principle, which holds that a particle can be in two places at once. A textbook example is the shooting of light particles, called photons, through two slits in a wall. Past the slits, they will behave as if they were waves, following a well-known quantum mechanical phenomenon that particles can paradoxically act like waves in some situations. The photons’ waviness then causes them to “interfere” with each other. In other words, they make patterns like those seen when you drop two pebbles near each other in a pond, and the waves start to overlap. When the waves coming through the two slits mutually interfere, the pattern shows up if you set up another wall where the particles can land. There, alternating bright and dark stripes appear. Bizarrely, this works even if you fire just one photon at a time through the slits. You can see the effect then by putting photographic film on the landing wall, so each photon leaves a lasting mark. Keep firing photons, and the marks will gradually add up to make the same stripes as before. It’s as if each photon is interfering with itself—that is, going through both slits simultaneously. This also works for bigger particles. But how large can a thing be and still do this? Schwab wants to know. “We’re trying to make a mechanical device be in two places at one time. What’s really neat is it looks like we should be able to do it,” he said. “The hope, the dream, the fantasy is that we get that superposition and start making bigger devices and find the breakdown.” In a commentary that appeared in the same issue of Nature, Michael Roukes of the California Institute of Technology in Pasadena, Calif., wrote that Schwab’s work with the cooling is part of an emerging field, quantum electromechanics. This focuses on submicroscopic devices called nanomechanical systems, he added, which are “poised midway between two seemingly antithetical domains” of size: fundamental particles at one end, the objects of everyday life at the other.