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University of Toronto physicists take quantum leap toward ultra-precise measurement

June 3, 2014

TORONTO, ON – For the first time, physi­cists at the Uni­ver­si­ty of Toron­to (U of T) have over­come a major chal­lenge in the sci­ence of mea­sure­ment using quan­tum mechan­ics. Their work paves the way for great advances in using quan­tum states to enable the next gen­er­a­tion of ultra-pre­cise mea­sure­ment tech­nolo­gies.

“We’ve been able to con­duct mea­sure­ments using pho­tons – indi­vid­ual par­ti­cles of light – at a res­o­lu­tion unat­tain­able accord­ing to clas­si­cal physics,” says Lee Roze­ma, a PhD can­di­date in Pro­fes­sor Aephraim Steinberg’s quan­tum optics research group in U of T’s Depart­ment of Physics, and one of the lead authors along with MSc can­di­date James Bate­man of a report on the dis­cov­ery pub­lished online today in Phys­i­cal Review Let­ters. “This work opens up a path for using entan­gled states of light to car­ry out ultra-pre­cise mea­sure­ments.”

Many of the most sen­si­tive mea­sure­ment tech­niques in exis­tence, from ultra-pre­cise atom­ic clocks to the world’s largest tele­scopes, rely on detect­ing inter­fer­ence between waves – which occurs, for exam­ple, when two or more beams of light col­lide in the same space. Manip­u­lat­ing inter­fer­ence by pro­duc­ing pho­tons in a spe­cial quan­tum state known as an “entan­gled” state – the sort of state famous­ly dis­missed by a skep­ti­cal Albert Ein­stein as imply­ing “spooky action at a dis­tance” – pro­vid­ed the result Roze­ma and his col­leagues were look­ing for. The entan­gled state they used con­tains N pho­tons which are all guar­an­teed to take the same path in an inter­fer­om­e­ter – either all N take the left-hand path or all N take the right-hand path, but no pho­tons leave the pack.

The effects of inter­fer­ence are mea­sured in devices known as “inter­fer­om­e­ters.” It is well known that the res­o­lu­tion of such a device can be improved by send­ing more pho­tons through it – when clas­si­cal light beams are used, increas­ing the num­ber of pho­tons (the inten­si­ty of the light) by a fac­tor of 100 can improve the res­o­lu­tion of an inter­fer­om­e­ter by a fac­tor of 10. How­ev­er, if the pho­tons are pre­pared in a quan­tum-entan­gled state, an increase by a fac­tor of 100 should improve the res­o­lu­tion by that same full fac­tor of 100.

The sci­en­tif­ic com­mu­ni­ty already knew res­o­lu­tion could be improved by using entan­gled pho­tons. Once sci­en­tists fig­ured out how to entan­gle mul­ti­ple pho­tons the the­o­ry was proved cor­rect but only up to a point. As the num­ber of entan­gled pho­tons rose, the odds of all pho­tons reach­ing the same detec­tor and at the same time became astro­nom­i­cal­ly small, ren­der­ing the tech­nique use­less in prac­tice.

So Roze­ma and his col­leagues devel­oped a way to employ mul­ti­ple detec­tors in order to mea­sure pho­tons in entan­gled states. They designed an exper­i­men­tal appa­ra­tus that uses a “fibre rib­bon” to col­lect pho­tons and send them to an array of 11 sin­gle-pho­ton detec­tors.

“This allowed us to cap­ture near­ly all of the mul­ti-pho­tons orig­i­nal­ly sent,” says Roze­ma. “Send­ing sin­gle pho­tons as well as two, three and four entan­gled pho­tons at a time into our device pro­duced dra­mat­i­cal­ly improved res­o­lu­tion.”

The U of T exper­i­ment built on a pro­pos­al by Nation­al Uni­ver­si­ty of Sin­ga­pore physi­cist Mankei Tsang. In 2009, Tsang posit­ed the idea of plac­ing detec­tors at every pos­si­ble posi­tion a pho­ton could reach so that every pos­si­ble event could be record­ed, whether or not mul­ti­ple pho­tons hit the same detec­tor. This would enable the cal­cu­la­tion of the aver­age posi­tion of all the detect­ed pho­tons, and could be done with­out hav­ing to dis­card any of them.  The the­o­ry was quick­ly test­ed with two pho­tons and two detec­tors by Uni­ver­si­ty of Ottawa physi­cist Robert Boyd.

“While two pho­tons are bet­ter than one, we’ve shown that 11 detec­tors are far bet­ter than two,” says Stein­berg, sum­maris­ing their advance­ment on Boyd’s results. “As tech­nol­o­gy pro­gress­es, using high-effi­cien­cy detec­tor arrays and on-demand entan­gled-pho­tons sources, our tech­niques could be used to mea­sure increas­ing­ly high­er num­bers of pho­tons with high­er res­o­lu­tion.”

The dis­cov­ery is report­ed in a study titled “Scal­able spa­tial super­res­o­lu­tion using entan­gled pho­tons” pub­lished in the June 6 issue of Phys­i­cal Review Let­ters. It is rec­om­mend­ed as an Editor’s Sug­ges­tion, and is accom­pa­nied by a com­men­tary in the jour­nal Physics which describes the work as a viable approach to effi­cient­ly observ­ing super­re­solved spa­tial inter­fer­ence fringes that could improve the pre­ci­sion of imag­ing and lith­o­g­ra­phy sys­tems.

In addi­tion to Stein­berg, Roze­ma and Bateman’s col­lab­o­ra­tors on the research includ­ed Dylan Mahler, Ryo Okamo­to of Hokkai­do and Osa­ka Uni­ver­si­ties, Amir Feizpour, and Alex Hay­at, now at the Tech­nion — Israel Insti­tute of Tech­nol­o­gy. Sup­port for the research was pro­vid­ed by the Nat­ur­al Sci­ences and Engi­neer­ing Research Coun­cil of Cana­da and the Cana­di­an Insti­tute for Advanced Research, as well as the Yama­da Sci­ence Foun­da­tion.

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Note to media: Image(s) and access to research paper described here avail­able at



Lee Roze­ma
Depart­ment of Physics
Uni­ver­si­ty of Toron­to
Tel: 416–946-3162

Aephraim Stein­berg
Depart­ment of Physics
Uni­ver­si­ty of Toron­to
Tel: 416–978-0713

Sean Bet­tam
Com­mu­ni­ca­tions, Fac­ul­ty of Arts & Sci­ence
Uni­ver­si­ty of Toron­to
Tel: 416–946-7950