"Long before it's in the papers"
January 27, 2015


Quantum physics may have just gotten simpler

Dec. 20, 2014
Courtesy of the Centre for Quantum Technologies 
at the National University of Singapore
and World Science staff

Here’s a nice sur­prise: quan­tum phys­ics is less com­pli­cat­ed than we thought, ac­cord­ing to new re­search. The work links two strange fea­tures of the quan­tum world—or na­ture at the small­est scales, such as that of sub­a­tom­ic par­t­i­cles—call­ing them dif­fer­ent man­i­festa­t­ions of the same thing. 

Quan­tum phys­ics says that par­t­i­cles can be­have like waves, and vi­ce versa. Re­search­ers have now con­clud­ed that this 'wave-particle du­al­i­ty' is simp­ly the quan­tum un­cer­tain­ty prin­ci­ple in dis­guise. (Cred­it: Tim­o­thy Yeo / CQT, Na­tion­al U. of Sin­ga­pore )

These fea­tures go by the names “wave-par­t­i­cle du­al­ity” and the “uncer­tainty prin­ci­ple.” In work pub­lished Dec. 19 in the jour­nal Na­ture Com­mu­nica­t­ions, the re­search­ers, who did the work at the Na­t­ional Uni­vers­ity of Sin­ga­pore, say the first is just the sec­ond in dis­guise.

The con­nec­tion “comes out very nat­u­rally when you con­sid­er them as ques­tions about what in­forma­t­ion you can gain about a sys­tem,” said one of the sci­en­tists, Steph­a­nie Weh­ner, who is now at the Delft Uni­vers­ity of Tech­nol­o­gy in the Neth­er­lands.

Wave-par­t­i­cle du­al­ity is the idea that a quan­tum ob­ject can be­have like a wave, but that the wave be­hav­ior stops if you try to lo­cate the ob­ject. 

The duality is seen in ex­pe­ri­ments in which sub­a­tom­ic par­t­i­cles, such as elec­trons, are fired one by one at a screen with two thin slits. The par­t­i­cles pile up be­hind the slits not in two heaps, but in a striped pat­tern as you’d ex­pect for waves that “in­ter­fere” with each oth­er. An ever­yday ex­am­ple of wave in­ter­fer­ence oc­curs when you toss two peb­bles in a pond at once a small dis­tance away from each oth­er: when the two sets of rip­ples meet, they form char­ac­ter­is­tic pat­terns as their effects add up.

How­ev­er, in the quan­tum case, the pat­tern van­ishes if you sneak a look at which slit a par­t­i­cle goes through—at which point the par­t­i­cles start to act like par­t­i­cles and not waves.

The quan­tum un­cer­tain­ty prin­ci­ple is the idea that it’s im­pos­si­ble to know cer­tain pairs of things about a quan­tum par­t­i­cle at once. For ex­am­ple, the more pre­cisely you know the po­si­tion of an at­om, the less pre­cisely you can know its speed. It’s a lim­it on the fun­da­men­tal know­a­bil­ity of na­ture, not a state­ment on meas­ure­ment skill. The new work finds that there is an ident­ical sort of limit on how much you can learn about a sys­tem’s wave ver­sus the par­t­i­cle be­hav­ior.

Wave-par­t­i­cle du­al­ity and un­cer­tain­ty have been fun­da­men­tal con­cepts in quan­tum phys­ics since the early 1900s. “We were guid­ed by a gut feel­ing, and only a gut feel­ing, that there should be a con­nec­tion,” said co-re­searcher Pat­rick Coles, who is now at the In­sti­tute for Quan­tum Com­put­ing in Wa­ter­loo, Can­a­da.

One can write equa­t­ions that cap­ture how much can be learn­ed about pairs of prop­er­ties subject to the un­cer­tain­ty prin­ci­ple. Coles, Weh­ner and co-author Je­drzej Kan­iew­ski work with a form of such equa­t­ions known as “en­tropic un­cer­tain­ty rela­t­ions,” and they found that all the maths pre­vi­ously used to de­scribe wave-par­t­i­cle du­al­ity could be re­for­mu­lat­ed in terms of these rela­t­ions.

“It was like we had dis­cov­ered the ‘Rosetta Stone’ that con­nect­ed two dif­fer­ent lan­guages,” said Coles. “The lit­er­a­ture on wave-par­t­i­cle du­al­ity was like hi­er­o­glyph­ that we could now trans­late in­to our na­tive tongue.”

Be­cause the en­tropic un­cer­tain­ty rela­t­ions used in their transla­t­ion have al­so been used in de­mon­strat­ing the se­cur­ity of quan­tum cryp­tog­ra­phy—schemes for se­cure com­mu­nica­t­ion us­ing quan­tum par­t­i­cles—the re­search­ers sug­gest the work could help in­spire new cryp­tog­ra­phy methods.

In ear­li­er pa­pers, Wehner and col­la­bo­ra­tors found con­nec­tions be­tween the un­cer­tain­ty prin­ci­ple and oth­er as­pects of phys­ics, namely quan­tum “non-local­ity” and the sec­ond law of ther­mo­dy­nam­ics. The first deals with par­t­i­cles’ abil­ity to act as though they can com­mu­ni­cate in­stan­ta­ne­ously over long dis­tances; the sec­ond states that dis­or­der in the uni­verse can al­ways in­crease but not de­crease. The re­search­ers say their next goal is to think about how all this fits into a big­ger pic­ture of how na­ture works.

* * *

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Here’s a nice surprise: quantum physics is less complicated than we thought, according to new research. The work unites two strange features of the quantum world—nature at the smallest scales, such as that of subatomic particles—calling them different manifestations of the same thing. These features go by the names “wave-particle duality” and the “uncertainty principle.” In work published Dec. 19 in the journal Nature Communications, the researchers at National University of Singapore say the first is just the second in disguise. The connection “comes out very naturally when you consider them as questions about what information you can gain about a system,” said one of the scientists, Stephanie Wehner, who is now at the Delft University of Technology in the Netherlands. Wave-particle duality is the idea that a quantum object can behave like a wave, but that the wave behavior stops if you try to locate the object. This is seen in experiments in which subatomic particles, such as electrons, are fired one by one at a screen with two thin slits. The particles pile up behind the slits not in two heaps, but in a striped pattern as you’d expect for waves that “interfere” with each other. An everyday example of interference occurs when you toss two pebbles in a pond at once a small distance away from each other: when the two sets of ripples meet, they form characteristic patterns, canceling each other out at some spots and reinforcing each other at others. However, in the quantum case, interference pattern vanishes if you sneak a look at which slit a particle goes through—at which point the particles start to act like particles. The quantum uncertainty principle is the idea that it’s impossible to know certain pairs of things about a quantum particle at once. For example, the more precisely you know the position of an atom, the less precisely you can know its speed. It’s a limit on the fundamental knowability of nature, not a statement on measurement skill. The new work shows that how much you can learn about the wave versus the particle behavior of a system is constrained in exactly the same way. Wave-particle duality and uncertainty have been fundamental concepts in quantum physics since the early 1900s. “We were guided by a gut feeling, and only a gut feeling, that there should be a connection,” said co-researcher Patrick Coles, who is now at the Institute for Quantum Computing in Waterloo, Canada. One can write equations that capture how much can be learned about pairs of properties that are affected by the uncertainty principle. Coles, Wehner and co-author Jedrzej Kaniewski are specialists in a form of such equations known as “entropic uncertainty relations,” and they found that all the maths previously used to describe wave-particle duality could be reformulated in terms of these relations. “It was like we had discovered the ‘Rosetta Stone’ that connected two different languages,” said Coles. “The literature on wave-particle duality was like hieroglyphics that we could now translate into our native tongue. We had several eureka moments when we finally understood what people had done,” he said. Because the entropic uncertainty relations used in their translation have also been used in proving the security of quantum cryptography—schemes for secure communication using quantum particles—the researchers suggest the work could help inspire new cryptography protocols. In earlier papers, Wehner and collaborators found connections between the uncertainty principle and other aspects of physics, namely quantum “non-locality” and the second law of thermodynamics. The first deals with particles’ ability to act as though they can communicate instantaneously over long distances; the second states that disorder in the universe can always increase but not decrease. The researchers say their next goal is to think about how these pieces fit together and what bigger picture that paints of how nature is built.