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New principle may help explain why nature is quantum

May 14, 2013
Courtesy of National University of Singapore
and World Science staff

Like chil­dren, sci­en­tists are al­ways ask­ing “why?” One ques­tion they’ve yet to an­swer is why na­ture pick­ed quan­tum phys­ics, in all its weird glo­ry, as a sen­si­ble way to be­have. 

Re­search­ers Corsin Pfis­ter and Steph­a­nie Wehner at the Cen­tre for Quan­tum Tech­nolo­gies at the Na­t­ional Uni­vers­ity of Sin­ga­pore tack­le this ques­tion in a pa­per pub­lished May 14 in the joural Na­ture Com­mu­nica­t­ions.

Things that fol­low quan­tum rules, such as atoms, elec­trons or the pho­tons that make up light, are full of sur­prises. They can ex­ist in more than one place at once, for in­stance, or ex­ist in a shared state where the prop­er­ties of two par­t­i­cles in­ter­act in what Ein­stein called “spooky ac­tion at a dis­tance,” no mat­ter the dis­tance be­tween them. Be­cause ex­pe­ri­ments have con­firmed such things, re­search­ers are con­fi­dent the the­o­ry is right. But it would still be eas­i­er to swal­low if they could show quan­tum phys­ics sprang from un­der­ly­ing prin­ci­ples that seem sen­si­ble.

One way to ap­proach the prob­lem is to im­ag­ine all the the­o­ries one could pos­sibly come up with to de­scribe na­ture, and then work out what prin­ci­ples help to sin­gle out quan­tum phys­ics. 

A good start is to as­sume in­forma­t­ion can’t trav­el faster than light, as es­tab­lished by Ein­stein’s the­o­ry of rel­a­ti­vity, but this is­n’t enough to de­fine quan­tum phys­ics as the only way na­ture might be­have, Pfis­ter and Wehner say.

They think they have come across a use­ful new prin­ci­ple, which “is very good at rul­ing out oth­er the­o­ries,” said Pfis­ter. In short, the prin­ci­ple is that if a meas­ure­ment yields no in­forma­t­ion, then the sys­tem be­ing meas­ured has not been dis­turbed. Quan­tum phys­i­cists ac­cept that gain­ing in­forma­t­ion from quan­tum sys­tems causes dis­turb­ance. Pfis­ter and Wehner sug­gest that in a sen­si­ble world the re­verse should be true, too. If you learn noth­ing from meas­ur­ing a sys­tem, then you can’t have dis­turbed it.

Con­sid­er the fa­mous Schrodinger’s cat par­a­dox, they say, a thought ex­pe­ri­ment in which a cat in a box sim­ul­ta­ne­ously ex­ists in two states (this is known as a “quan­tum su­per­po­si­tion.”) Ac­cord­ing to quan­tum the­o­ry it is pos­sible that the cat is both dead and alive – un­til, that is, the cat’s state of health is “meas­ured” by open­ing the box. When the box is opened, al­low­ing the health of the cat to be meas­ured, the su­per­po­si­tion “col­laps­es” and the cat ends up de­fin­i­tively dead or alive. The meas­ure­ment has dis­turbed the cat.

This is a prop­er­ty of quan­tum sys­tems in gen­er­al. Per­form a meas­ure­ment for which you can’t know the out­come in ad­vance, and the sys­tem changes to match the out­come you get. What hap­pens if you look a sec­ond time? The re­search­ers as­sume the sys­tem is not evolv­ing in time or af­fect­ed by any out­side in­flu­ence, which means the quan­tum state stays col­lapsed. You would then ex­pect the sec­ond meas­ure­ment to yield the same re­sult as the first. Af­ter all, “If you look in­to the box and find a dead cat, you don’t ex­pect to look again lat­er and find the cat has been res­ur­rect­ed,” said Steph­a­nie. “You could say we’ve for­mal­ized the prin­ci­ple of ac­cepting the facts.”

Pfis­ter and Wehner ar­gue that this prin­ci­ple rules out var­i­ous the­o­ries of na­ture. They note par­tic­u­larly that a class of the­o­ries they call “dis­crete” are in­com­pat­ible with the prin­ci­ple. These the­o­ries hold that quan­tum par­t­i­cles can take up only a fi­nite num­ber of states, rath­er than choose from an infi­nite, con­tin­u­ous range of pos­sibil­i­ties. The pos­sibil­ity of such a dis­crete “s­tate space” has been linked to quan­tum gravita­t­ional the­o­ries pro­pos­ing si­m­i­lar dis­creteness in space­time, where the fab­ric of the uni­verse is made up of ti­ny brick-like el­e­ments rath­er than be­ing a smooth, con­tin­u­ous sheet. 

As is of­ten the case in re­search, Pfis­ter and Wehner reached this point hav­ing set out to solve an en­tirely dif­fer­ent prob­lem.

Pfis­ter was try­ing to find a gen­er­al way to de­scribe the ef­fects of meas­ure­ments on states, a prob­lem that he found impos­sible to solve. In an at­tempt to make prog­ress, he wrote down fea­tures a “sen­si­ble” an­swer should have. The prop­er­ty of in­forma­t­ion gain ver­sus dis­turb­ance was on the list. 

He then no­ticed that if he im­posed the prop­er­ty as a prin­ci­ple, some the­o­ries would fail. Pfis­ter and Wehner are keen to point out it’s still not the whole an­swer to the big “why”: the­o­ries oth­er than quan­tum phys­ics, in­clud­ing clas­si­cal phys­ics, are com­pat­ible with the prin­ci­ple. But as re­search­ers com­pile lists of prin­ci­ples that each rule out some the­o­ries to reach a set that sin­gles out quan­tum phys­ics, they say, the prin­ci­ple of in­forma­t­ion gain ver­sus dis­turb­ance seems like a good one to in­clude.


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Like small children, scientists are always asking”why?” One question they’ve yet to answer is why nature picked quantum physics, in all its weird glory, as a sensible way to behave. Researchers Corsin Pfister and Stephanie Wehner at the Centre for Quantum Technologies at the National University of Singapore tackle this question in a paper published May 14 in the joural Nature Communications. Things that follow quantum rules, such as atoms, electrons or the photons that make up light, are full of surprises. They can exist in more than one place at once, for instance, or exist in a shared state where the properties of two particles interact in what Einstein called “spooky action at a distance,” no matter the distance between them. Because experiments have confirmed such things, researchers are confident the theory is right. But it would still be easier to swallow if they could show quantum physics sprang from underlying principles that seem sensible. One way to approach the problem is to imagine all the theories one could possibly come up with to describe nature, and then work out what principles help to single out quantum physics. A good start is to assume information can’t travel faster than light, as established by Einstein’s theory of relativity, but this isn’t enough to define quantum physics as the only way nature might behave, Corsin and Stephanie say. They think they have come across a useful new principle. This principle “is very good at ruling out other theories,” said Corsin. In short, the principle is that if a measurement yields no information, then the system being measured has not been disturbed. Quantum physicists accept that gaining information from quantum systems causes disturbance. Corsin and Stephanie suggest that in a sensible world the reverse should be true, too. If you learn nothing from measuring a system, then you can’t have disturbed it. Consider the famous Schrodinger’s cat paradox, they say, a thought experiment in which a cat in a box simultaneously exists in two states (this is known as a “quantum superposition.”) According to quantum theory it is possible that the cat is both dead and alive – until, that is, the cat’s state of health is “measured” by opening the box. When the box is opened, allowing the health of the cat to be measured, the superposition “collapses” and the cat ends up definitively dead or alive. The measurement has disturbed the cat. This is a property of quantum systems in general. Perform a measurement for which you can’t know the outcome in advance, and the system changes to match the outcome you get. What happens if you look a second time? The researchers assume the system is not evolving in time or affected by any outside influence, which means the quantum state stays collapsed. You would then expect the second measurement to yield the same result as the first. After all, “If you look into the box and find a dead cat, you don’t expect to look again later and find the cat has been resurrected,” said Stephanie. “You could say we’ve formalised the principle of accepting the facts.” Corsin and Stephanie argue that this principle rules out various theories of nature. They note particularly that a class of theories they call “discrete” are incompatible with the principle. These theories hold that quantum particles can take up only a finite number of states, rather than choose from an infinite, continuous range of possibilities. The possibility of such a discrete “state space” has been linked to quantum gravitational theories proposing similar discreteness in spacetime, where the fabric of the universe is made up of tiny brick-like elements rather than being a smooth, continuous sheet. As is often the case in research, Corsin and Stephanie reached this point having set out to solve an entirely different problem. Corsin was trying to find a general way to describe the effects of measurements on states, a problem that he found impossible to solve. In an attempt to make progress, he wrote down features a “sensible” answer should have. The property of information gain versus disturbance was on the list. He then noticed that if he imposed the property as a principle, some theories would fail. Corsin and Stephanie are keen to point out it’s still not the whole answer to the big “why”: theories other than quantum physics, including classical physics, are compatible with the principle. But as researchers compile lists of principles that each rule out some theories to reach a set that singles out quantum physics, they say, the principle of information gain versus disturbance seems like a good one to include.