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Tiny “cannon” shoots single light particles

Aug. 29, 2014
Courtesy of University of Copenhagen - 
Niels Bohr Institute 
and World Science staff

Re­search­ers have cre­at­ed a ti­ny “can­non” that shoots out par­t­i­cles of light, called pho­tons, one at a time.

They’re claim­ing the in­ven­tion is a “break­through” in long­stand­ing at­tempts to de­vel­op pho­ton­ics, or su­per-fast com­put­ers us­ing pho­tons. Nor­mal com­put­ers use elec­trons, which are un­its of elec­tri­city rath­er than of light.

To make the change, one has to cre­ate a stream of sin­gle pho­tons and con­trol their di­rec­tion. Re­search­ers around the world have tried to do this, but now sci­en­tists at the Uni­vers­ity of Co­pen­ha­gen say they have done it. The find­ings are pub­lished in the sci­en­tif­ic jour­nal Phys­i­cal Re­view Let­ters.

Pho­tons, which make up light, and elec­trons, which make up elec­tric cur­rents, act very dif­fer­ent­ly. Elec­trons are classed as part of a family of par­t­i­cles called fermions that can easily flow in­di­vid­u­ally. Pho­tons, in­stead, are so-called “bosons” that pre­fer to clump to­geth­er. But be­cause in­forma­t­ion for quan­tum com­mu­nica­t­ion based on pho­ton­ics lies in the in­di­vid­ual pho­ton, one has to be able to send them one at a time.

“We do this by cre­at­ing an ex­tremely strong in­ter­ac­tion be­tween light and mat­ter,” said Pe­ter Lo­dahl, head of the Quan­tum Pho­ton­ics re­search group at the uni­vers­ity’s Niels Bohr In­sti­tute.

The re­search­ers de­vel­oped their “can­non” as part of a ti­ny op­ti­cal chip, or piece of com­put­er cir­cuit­ry. This con­sists of a ti­ny crys­tal with a light source, called a quan­tum dot, at its cen­ter. Next they “shine la­ser light on the quan­tum dot, where there are atoms with elec­trons in or­bit around the nu­cle­us,” said Søren Sto­bbe, an­oth­er mem­ber of the re­search group at the in­sti­tute.

The la­ser light stim­u­lates the elec­trons, which then jump to dif­fer­ent or­bits and emit in­di­vid­ual pho­tons in the pro­cess, Sto­bbe added. “Nor­mally, light is scat­tered in all di­rec­tions, but we have de­signed the pho­tonic chip so that all of the pho­tons are sent through only one chan­nel.” 

Lo­dahl and Sto­bbe said they can con­trol the pho­tons and send them in the de­sired di­rec­tion with a 98.4 per­cent suc­cess rate, open­ing up “fas­ci­nat­ing” new op­por­tun­i­ties for fun­da­men­tal ex­pe­ri­ments and new tech­nolo­gies. They’re in the pro­cess of patent­ing sev­er­al parts of their work, aim­ing to de­vel­op a pro­to­type sin­gle-pho­ton source usa­ble for en­cryp­tion or com­plex cal­cula­t­ions.

It’s al­so hoped the work will pave the way for quan­tum com­put­ers, which would ex­ploit the some­times ap­par­ently non­sen­si­cal laws of quan­tum phys­ics, or na­ture at the sub­a­tom­ic scale, to achieve un­prec­e­dent­ed pow­er and speed.


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Researchers have created a tiny “cannon” that shoots out particles of light, called photons, one at a time. They’re claiming the invention is a “breakthrough” in longstanding attempts to develop photonics, or superfast computers using photons. Normal computers use electrons, which are units of electricity rather than of light. To make the change, one has to create a stream of single photons and control their direction. Researchers around the world have tried to do this, but now scientists at the University of Copenhagen say they have done it. The findings are published in the scientific journal Physical Review Letters. Photons, which make up light, and electrons, which make up electric currents, act very differently. Electrons are classed as part of a family of particles called fermions that can easily flow individually. Photons, instead, are so-called “bosons” that prefer to clump together. But because information for quantum communication based on photonics lies in the individual photon, one has to be able to send them one at a time. “We do this by creating an extremely strong interaction between light and matter,” said Peter Lodahl, head of the Quantum Photonics research group at the university’s Niels Bohr Institute. The researchers developed their “cannon” as part of a tiny optical chip, or piece of computer circuitry. This consists of a tiny crystal with a light source, called a quantum dot, at its center. Next they “shine laser light on the quantum dot, where there are atoms with electrons in orbit around the nucleus,” said Søren Stobbe, another member of the research group at the institute. The laser light stimulates the electrons, which then jump to different orbits and emit individual photons in the process, Stobbe added. “Normally, light is scattered in all directions, but we have designed the photonic chip so that all of the photons are sent through only one channel.” Lodahl and Stobbe said they can control the photons and send them in the desired direction with a 98.4 percent success rate, opening up “fascinating” new opportunities for fundamental experiments and new technologies. They’re in the process of patenting several parts of their work, aiming to develop a prototype single-photon source usable for encryption or complex calculations. It’s also hoped the work will pave the way for quantum computers, which would exploit the sometimes apparently nonsensical laws of quantum physics, or nature at the subatomic scale, to achieve unprecedented power and speed.