Thursday, September 28, 2006

Physicists seek to put one thing in two places

Sounds Quizzy isn't it.. But seems to be quite intresting invention to watch... and move objects :-)

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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