DIALON · From Dicke States to Anderson Localisation of Light in Optical Nanofibres

From Dicke States to Anderson Localisation of Light in Optical Nanofibres

During the DiALON project, Dr. Clément Sayrin has led an experimental work on a recently developed light—matter interface: laser-cooled atoms trapped in the vicinity of an optical nanofibre. The nanofibre is a standard optical fibre with a diameter smaller than the wavelengths of all the light fields used in the experiment. In this situation, light that propagates along the nanofibre exhibits an intense evanescent part that propagates outside the nanofibre. By bringing and maintaining cold atoms very close to the nanofibre surface, it is then possible to couple extremely well nanofibre-guided light to the atomic ensemble. The aim of the project was, originally, to investigate the modification of the emission properties of the atoms into the optical waveguide induced by the linear and possibly regular arrangement of the atoms along the nanofibre. The preliminary experimental work, however, revealed long-overlooked and significantly new effects: when light is tightly confined, as is the case in an optical nanofibre, it acquires an unusual character, namely it becomes chiral: The propagation direction of the light field and the sign of the spin it carries are inherently linked. This is in strong contrast to, e.g., collimated light fields in free-space for which the spin of the photons can take any sign, whatever their propagation direction. In this preliminary experiment, Dr. Sayrin and his team measured how these unique polarisation characteristics qualitatively and quantitatively modify the backscattering properties of the nanofibre-trapped atomic ensemble [1]. They were in particular able to develop a model that takes into account these effects and that reproduces very well their experimental observations. Because these chiral properties were unexpected and opened a new avenue of applications for this system, it was decided to reorient the DiALON project towards their more thorough study and the development of novel tools or devices that would exploit them. The researcher and his colleagues were first able to achieve sub-micrometre resolution internal state preparation and manipulation of the trapped atoms [2]. By directly exploiting the unique polarisation properties of fibre-guided light, they showed that the atoms can be prepared in quantum spin states that depend on the position of the atoms around the nanofibre. They also showed that atoms located on opposite sides of the optical nanofibre, less than a micron away, could be addressed individually via microwave radiation. Beyond the great potential offered by tightly-confined light that it revealed, this work has provided to the team powerful new tools for the control of both the quantum spin state and the position of the trapped atoms. They were rapidly exploited to demonstrate quantum-state controlled directional emission of photons into the nanofibre [3]. Here, the researcher and his team were able to control the propagation direction of photons scattered into the nanofibre by the atoms by tuning the quantum state of the latter. Such an effect could later on be used for the realisation of a photonic router, where the propagation direction of a photon is directly controlled by the quantum state of a single emitter. As such, this work already hinted towards the new capabilities that are offered by the chiral character of nanofibre-guided photons, especially for the development of new nanophotonic integrated devices. The rest of the project has therefore been dedicated to the development of such devices. Firstly, the team used its advanced controlled over the quantum state of the trapped atoms and the polarisation of the nanofibre-guided light fields to realise an all-fibre-based optical memory [4]. To this end, they used so-called electromagnetically induced transparency: the nanofibre-trapped atomic ensemble, otherwise opaque for a probe light field, is rendered transparent when another control light field, with ad hoc polarisation and frequency, is sent through the nanofibre. Under such condition, the velocity of a probe light pulse is significantly reduced: the researcher and his colleagues measured for instance light velocities as small as 50m/s. They were eventually able to fully stop a probe light pulse, i.e., to store it in the atomic ensemble, before recovering it shortly after: the nanofibre-trapped ensemble then constitutes an optical memory, one of the building blocks of future optical quantum information networks. Finally, Dr. Clément Sayrin and his colleagues focused on the demonstration of an integrated non-reciprocal device, namely an optical isolator [5]. Such devices act as one-way streets of light: They allow light to pass in one direction, but they block light coming from the opposite direction. They are crucial elements for the control of light propagation in future (quantum) optical processors, where electric circuits are replaced by nanophotonic waveguides. The team demonstrated a conceptually new type of optical isolator that is not only integrated but that is also the first that simultaneously features good isolation and low losses and operates with single photons. In contrast to, e.g., Faraday isolators that rely on a magnetic field to break the left/right symmetry, the researcher and his colleagues make use of an atomic spin: the direction of the optical isolator is then directly controlled by the quantum state of the trapped atoms. This should open the route to the development of novel integrated optical devices for fibre-based classical and quantum networks. The work achieved by Dr. Clément Sayrin and his colleagues during the DiALON project focused on the unique properties of tightly confined light. The team was not only able to experimentally demonstrate how these effects significantly alter the behaviour of the atom—light interface under study, but could also show how they can be exploited to increase the control over quantum emitters and to develop new integrated nanophotonic devices. The concepts of such devices are universal in that sense that they can also be implemented with other strongly confined optical fields and other quantum emitters. They are, for example, compatible with integrated photonic waveguides that are coupled to solid state emitters or plasmonic structures. All this suggest that this work is likely to find practical applications in the future, whether for the development of optical computation or for building up future quantum information networks. Publications: [1] Backscattering properties of a waveguide-coupled array of atoms in the strongly nonparaxial regime D. Reitz, C. Sayrin, B. Albrecht, I. Mazets, R. Mitsch, P. Schneeweiss, and A. Rauschenbeutel Phys. Rev. A 89, 031804(R) (2014) [2] Exploiting the local polarization of strongly confined light for sub-micrometer-resolution internal state preparation and manipulation of cold atoms R. Mitsch, C. Sayrin, B. Albrecht, P. Schneeweiss, and A. Rauschenbeutel Phys. Rev. A 89, 063829 (2014) [3] Quantum state-controlled directional spontaneous emission of photons into a nanophotonic waveguide R. Mitsch, C. Sayrin, B. Albrecht, P. Schneeweiss, A. Rauschenbeutel Nat. Commun. 5, 5713 (2014) [4] Storage of fiber-guided light in a nanofiber-trapped ensemble of cold atoms C. Sayrin, C. Clausen, B. Albrecht, P. Schneeweiss and A. Rauschenbeutel Optica 2, 353-356 (2015) [5] Nanophotonic optical isolator controlled by the internal state of cold atoms C. Sayrin, C. Junge, R. Mitsch, B. Albrecht, D. O’Shea, P. Schneeweiss, J. Volz, and A. Rauschenbeutel Phys. Rev. X 5, 041036 (2015) Contact details: Project webpage: http://ati.tuwien.ac.at/forschungsbereiche/aqp Dr. Clément Sayrin Laboratoire Kastler Brossel, Collège de France 11 place Marcelin Berthelot 75005 Paris, France clement.sayrin@lkb.ens.fr Prof. Arno Rauschenbeutel Atominstitut, TU Wien Stadionallee 2 1020 Wien, Austria arno.rauschenbeutel@ati.ac.at

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