Controlled Coupling of Light and Matter

Artistic representation of a plasmonic nanoresonator realized by a narrow slit in a gold layer. Upon approaching the quantum dot (red) to the slit opening the coupling strength increases. (Source: H. Groß)

Physicists from the Julius-Maxi­milians-Univer­sität Würz­burg JMU in Germany and Imperial College London in the UK succeeded to control the coupling of light and matter at room tempera­ture. This achieve­ment is parti­cularly signi­ficant as it builds the founda­tions for a realization of practical photonic quantum techno­logies. Indeed, while many demon­strations of optical quantum processes require cryogenic tempera­tures to protect the quantum states, the present work elevates the quantum processes to room tempera­ture and introduces control­lability – both vital elements of quantum techno­logies such as quantum computers, which to a certain extent calculate with light and are con­ceivably many times more powerful than existing computers.

If a photon-emitter is inti­mately coupled to an optical resonator, then the emitted photon remains in the vicinity of the emitter for a suffi­ciently long period of time, consi­derably boosting its chance to be reabsorbed. “Such a reversal of spon­taneous emission is of highest impor­tance for quantum techno­logies and infor­mation processing, not least as it faci­litates exchange of quantum infor­mation between matter and light while preserving the quantum proper­ties of both,” says Ortwin Hess of Imperial College. Such an exchange of quantum information is, however, usually only possible at very low tempera­tures, which renders spectral lines of emitters spectrally very sharp and there­fore increases the prob­ability of absorption. The teams of Bert Hecht and Ortwin Hess are now among the pio­neering groups in the world who have succeeded in achieving the state of strong coupling of light and a single quantum emitter at room tempera­ture.

To achieve the re-absorption of a photon even at room tempera­ture, the researchers use a plasmonic nano­resonator, which has the form of an extremely narrow slit in a thin gold layer. “This resonator allows us to spatially concen­trate the electro­magnetic energy of a stored photon to an area which is not much larger than the quantum dot itself,” explains co-worker Heiko Groß. As a result, the stored photon is re-absorbed with high proba­bility by the emitter.

While similar ideas have already been implemented by other researchers in systems such as single molecules, the researchers have managed to also control the coupling between the resonator and the quantum emitter by imple­menting a method that allows them to conti­nuously change the coupling and, in particular, to switch it on and off in a precise manner. The team achieved this by attaching the nano­resonator to the tip of an atomic force micro­scope. This way they are able to move it with nano­meter precision within the imme­diate vicinity of the emitter – in this case a quantum dot.

Building on their accom­plishment, the researchers now hope to be able to control­lably mani­pulate the coupling of the quantum dot and the reso­nator not only by changing their distance but also through external stimuli – possibly even by single photons. This would result in unpre­cedented new possi­bilities in the challen­ging route towards a realization of optical quantum computers. “It is clearly a most useful feature that the exchange of energy between the quantum dot and the resonator here happens extremely fast,” says Groß. This solves a challenge of a low-tem­perature set-up: At very low tempera­tures, the oscil­lation of energy between light and matter is signi­ficantly slowed down by the long storage times of the resonator. (Source: JMU)

Reference: H. Groß et al.: Near-field strong coupling of single quantum dots, Sci. Adv. 4: eaar4906 (2018); DOI: 10.1126/sciadv.aar4906

Link: Nano-Optics and Biophotonics Group, University Wuerzburg JMU, Würzburg, Germany

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