Media Releases

U of T scientist leads international team in quantum physics first

June 2, 2011

Vital contribution to ongoing debate

TORONTO, ON — Quan­tum mechan­ics is famous for say­ing that a tree falling in a for­est when there’s no one there does­n’t make a sound. Quan­tum mechan­ics also says that if any­one is lis­ten­ing, it inter­feres with and changes the tree.  And so the famous para­dox: how can we know real­i­ty if we can­not mea­sure it with­out dis­tort­ing it?

An inter­na­tion­al team of researchers, led by Uni­ver­si­ty of Toron­to physi­cist Aephraim Stein­berg of the Cen­tre for Quan­tum Infor­ma­tion and Quan­tum Con­trol, have found a way to do just that by apply­ing a mod­ern mea­sure­ment tech­nique to the his­toric two-slit inter­fer­om­e­ter exper­i­ment in which a beam of light shone through two slits results in an inter­fer­ence pat­tern on a screen behind.

That famous exper­i­ment, and the 1927 Neils Bohr and Albert Ein­stein debates, seemed to estab­lish that you could not watch a par­ti­cle go through one of two slits with­out destroy­ing the inter­fer­ence effect: you had to choose which phe­nom­e­non to look for.

“Quan­tum mea­sure­ment has been the philo­soph­i­cal ele­phant in the room of quan­tum mechan­ics for the past cen­tu­ry,” says Stein­berg, who is lead author of Observ­ing the Aver­age Tra­jec­to­ries of Sin­gle Pho­tons in a Two-Slit Inter­fer­om­e­ter, to be pub­lished in Sci­ence on June 2.  “How­ev­er, in the past 10 to 15 years, tech­nol­o­gy has reached the point where detailed exper­i­ments on indi­vid­ual quan­tum sys­tems real­ly can be done, with poten­tial appli­ca­tions such as quan­tum cryp­tog­ra­phy and com­pu­ta­tion.”

With this new exper­i­ment, the researchers have suc­ceed­ed for the first time in exper­i­men­tal­ly recon­struct­ing full tra­jec­to­ries which pro­vide a descrip­tion of how light par­ti­cles move through the two slits and form an inter­fer­ence pat­tern. Their tech­nique builds on a new the­o­ry of weak mea­sure­ment that was devel­oped by Yakir Aharonov’s group at Tel Aviv Uni­ver­si­ty. Howard Wise­man of Grif­fith Uni­ver­si­ty pro­posed that it might be pos­si­ble to mea­sure the direc­tion a pho­ton (par­ti­cle of light) was mov­ing, condi­tioned upon where the pho­ton is found. By com­bin­ing infor­ma­tion about the pho­ton’s direc­tion at many dif­fer­ent points, one could con­struct its entire flow pat­tern ie. the tra­jec­to­ries it takes to a screen.

“In our exper­i­ment, a new sin­gle-pho­ton source devel­oped at the Nation­al Insti­tute of Stan­dards and Tech­nol­o­gy in Col­orado was used to send pho­tons one by one into an inter­fer­om­e­ter con­struct­ed at Toron­to.  We then used a quartz cal­cite, which has an effect on light that depends on the direc­tion the light is prop­a­gat­ing, to mea­sure the direc­tion as a func­tion of posi­tion. Our mea­sured tra­jec­to­ries are con­sis­tent, as Wise­man had pre­dict­ed, with the real­is­tic but uncon­ven­tion­al inter­pre­ta­tion of quan­tum mechan­ics of such influ­en­tial thinkers as David Bohm and Louis de Broglie,” said Stein­berg.

The orig­i­nal dou­ble-slit exper­i­ment played a cen­tral role in the ear­ly devel­op­ment of quan­tum mechan­ics, lead­ing direct­ly to Bohr’s for­mu­la­tion of the prin­ci­ple of com­ple­men­tar­i­ty.  Com­ple­men­tar­i­ty states that observ­ing par­ti­cle-like or wave-like behav­iour in the dou­ble-slit exper­i­ment depends on the type of mea­sure­ment made: the sys­tem can­not behave as both a par­ti­cle and wave simul­ta­ne­ous­ly.  Stein­berg’s recent exper­i­ment sug­gests this does­n’t have to be the case: the sys­tem can behave as both.

“By apply­ing a mod­ern mea­sure­ment tech­nique to the his­toric dou­ble-slit exper­i­ment, we were able to observe the aver­age par­ti­cle tra­jec­to­ries under­go­ing wave-like inter­fer­ence, which is the first obser­va­tion of its kind.  This result should con­tribute to the ongo­ing debate over the var­i­ous inter­pre­ta­tions of quan­tum the­o­ry,” said Stein­berg. “It shows that long-neglect­ed ques­tions about the dif­fer­ent types of mea­sure­ment pos­si­ble in quan­tum mechan­ics can final­ly be addressed in the lab, and weak mea­sure­ments such as the sort we use in this work may prove cru­cial in study­ing all sorts of new phe­nom­e­na.

“But most­ly, we are all just thrilled to be able to see, in some sense, what a pho­ton does as it goes through an inter­fer­om­e­ter, some­thing all of our text­books and pro­fes­sors had always told us was impos­si­ble.”

Research part­ners include the Uni­ver­si­ty of Toron­to’s Cen­tre for Quan­tum Infor­ma­tion and Quan­tum Con­trol, Depart­ment of Physics and Insti­tute for Opti­cal Sci­ences, the Nation­al Insti­tute of Stan­dards and Tech­nol­o­gy in Boul­der, Col­orado, the Insti­tute for Quan­tum Com­put­ing at the Uni­ver­si­ty of Water­loo, Grif­fith Uni­ver­si­ty, Aus­tralia, and the Lab­o­ra­toire Charles Fab­ry in Orsay, France.  Research was fund­ed by the Nat­ur­al Sci­ences and Engi­neer­ing Research Coun­cil of Cana­da, the Cana­di­an Insti­tute for Advanced Research, and Quan­tum Works.

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For more infor­ma­tion, please con­tact:

Aephraim Stein­berg*
U of T Cen­tre for Quan­tum Infor­ma­tion and Quan­tum Con­trol
* cur­rent­ly in Japan, con­tact by email to arrange phone or skype inter­view

Kris­ter Shalm
(519) 888‑4567 ext. 38861
(519) 725‑0576

Kim Luke, Com­mu­ni­ca­tions
U of T Fac­ul­ty of Arts & Sci­ence
416 978 4352