i n s i d e - Perimeter Institute

i n s i d e t h e p e r i m e t e rp i n e w sWhere the Rules Come FromThe rules of quantum mechanicsare powerful: they are the bottomrung of a vast ladder of science,and the fundamental tools that let usbuild everything from microchips topowerplants. But where do they comefrom? Traditionally, physicists wouldanswer, “That’s just the way the worldis.” But is that all we can say? Or is itpossible to derive quantum theory frommore fundamental principles?PI Postdoctoral Researcher GiulioChiribella and his colleagues GiacomoMauro D’Ariano and Paolo Perinottifrom the University of Pavia, Italy, havedevised a framework for grapplingwith such deep questions. In theirpaper “Informational Derivation ofQuantum Theory,” recently publishedin Physical Review A and highlightedby the American Physical Society, theresearchers show that quantum theorycan be derived from six fundamentalassumptions about how information isprocessed.This work is in the spirit of JohnWheeler’s belief that “it” — physicalreality — comes from “bit” —information. That, as Wheeler wrote,“every item of the physical world has atbottom — a very deep bottom, in mostinstances — an immaterial source andexplanation; that which we call realityarises in the last analysis from the posingof yes-no questions.” The universe is acomputational machine, using the ruleswe call physics to derive its next statefrom its current one. Information sits atthe core of this computer, just as it sits atthe core of an ordinary computer.The paper by Chiribella, D’Ariano andPerinotti explores the ways in whichthe universe-computer handles suchinformation. The researchers assumefive elementary axioms: causality, perfectdistinguishability, ideal compression,local distinguishability, and pureconditioning. These axioms definefive ordinary features of information,and are fulfilled both by classical andquantum theory. The real breakthroughis the definition of a sixth axiom, called“purification” — an axiom so importantthat Chiribella et al elevated it to thelevel of postulate.Purification states that a system withuncertain properties — what theauthors call a “mixed state” — can bemodelled as part of a larger system withdefinite properties — a “pure state.”For instance, consider a spin-zero piondecaying into two spinning photons.Each photon is in a mixed state — itsspin could be up or down, and thereis no way to predict which. The pair ofphotons together, though, comprise apure state in which they must always spinin opposite directions. If a system is in apure state, then it is impossible to knowanything more about it.In other words, the purification postulateimplies entanglement. Pushed a littlefurther, purification also explains whyquantum information can’t be copiedwithout destroying it but can be“teleported” — that is, replicated at adistant location after being destroyed atits point of origin.By combining the purification postulatewith the other five axioms, Chiribellaet al were able to derive the entiremathematical formalism behindquantum mechanics, and presentquantum theory in simple physicalterms, without the need for abstractmathematical jargon. The rules comefrom somewhere after all.Having the deep principles from whichone can reconstruct existing theoryis exciting, but does this work opennew avenues? Maybe. In the way newgeometries were born when Euclid’s08 Fall 2011