Optical Wells for a Super-Photon

The artist’s rendering shows how potential wells are created for the light in the microresonator through heating with an external laser beam. (Source: D. Dung, U. Bonn)

Many thousands photons can be merged to form a single super-photon if they are suffi­ciently concen­trated and cooled. The indi­vidual particles merge with each other in a photonic Bose-Einstein conden­sate, making them indistin­guishable. It has long been known that normal atoms form such conden­sates. Martin Weitz from the Insti­tute of Applied Physics at the Uni­versity of Bonn attracted atten­tion among experts in 2010 when he produced a Bose-Einstein conden­sate from photons for the first time.

In his latest study, Weitz’ team experi­mented with this kind of super-photon. In the experi­mental setup, a laser beam was rapidly bounced back and forth between two mirrors. In between was a pigment that cooled the laser light to such an extent that a super-photon was created from the indi­vidual light portions. “The special thing is that we have built a kind of optical well in various forms, into which the Bose-Einstein conden­sate was able to flow,” reports Weitz.

The researchers used a trick: It mixed a polymer into the pigment between the mirrors, which changed its refrac­tive index depending on the tempera­ture. The route between the mirrors for the light thus changed so that longer light wave­lengths passed between the mirrors when heated. The extent of the light path between the mirrors could be varied, in that the polymer could be warmed via a very thin heating layer. “With the help of various tempera­ture patterns, we were able to create dif­ferent optical dents,” explains Weitz. The geo­metry of the mirror only appeared to warp, while the refrac­tive index of the polymer changed at certain points – however, this had the same effect as a hollow shape. Part of the super-photon flowed into this appa­rent well. In this way, the researchers were able to use their apparatus to create different, very low-loss patterns that captured the photonic Bose-Einstein conden­sate.

The team of researchers inves­tigated in detail the formation of two neigh­boring wells, controlled via the tempera­ture pattern of the polymer. When the light in both optical hollows remained at a similar energy level, the super-photon flowed from one well into the neigh­boring one. “This was a precursor of optical quantum circuits,” high­lighted the physicist at the Uni­versity of Bonn. “Perhaps even complex arrange­ments, for which quantum ent­anglement occurs in inter­action with a possible photon inter­action in suitable materials, can be produced with this experi­mental setup.”

This would, in turn, be the prere­quisite for a new technique for quantum communi­cation and quantum computers. “But that’s still a long way off,” says Weitz. The findings by the research team could also con­ceivably be used to further develop lasers – for instance for highly precise welding work. (Source: U. Bonn)

Reference: D. Dung et al.: Variable Potentials for Thermalized Light and Coupled Condensates, Nat. Phot., online 14 August 2017; DOI: 10.1038/nphoton.2017.139

Link: Quantum Optics Group (M. Weitz), Inst. for Applied Physics, University Bonn, Bonn, Germany


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