Physicists seek to put one thing in two places

By Aussiegirl

To quote the English astronomer, Sir Arthur Eddington: Not only is the universe stranger than we imagine, it is stranger than we can imagine . And what could be stranger than something being in two places at once? If they can ever manage to accomplish this with macroscopic objects, I foresee problems: it's bad enough when my TV remote is in only one place and I can't find it -- but if it were in two different places at once...oy!

Physicists seek to put one thing in two places

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 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.