Paper on Nanofiber-based high-Q microresonator for cryogenic applications published

Johanna Hütner, Thomas Hoinkes, Martin Becker, Manfred Rothhardt, Arno Rauschenbeutel, and Sarah M. Skoff

We demonstrate a cryo-compatible, fully fiber-integrated, alignment-free optical microresonator. The compatibility with low temperatures expands its possible applications to the wide field of solid-state quantum optics, where a cryogenic environment is often a requirement. At a temperature of 4.6 K we obtain a quality factor of (9.9 ± 0.7) × 106. In conjunction with the small mode volume provided by the nanofiber, this cavity can be either used in the coherent dynamics or the fast cavity regime, where it can provide a Purcell factor of up to 15. Our resonator is therefore suitable for significantly enhancing the coupling between light and a large variety of different quantum emitters and due to its proven performance over a wide temperature range, also lends itself for the implementation of quantum hybrid systems.

Paper on Interaction signatures and non-Gaussian photon states published

B. Olmos, G. Buonaiuto, P. Schneeweiss, and I. Lesanovsky

We study theoretically a laser-driven one-dimensional chain of atoms interfaced with the guided optical modes of a nanophotonic waveguide. The period of the chain and the orientation of the laser field can be chosen such that emission occurs predominantly into a single guided mode. We find that the fluorescence excitation line shape changes as the number of atoms is increased, eventually undergoing a splitting that provides evidence for the waveguide-mediated all-to-all interactions. Remarkably, in the regime of strong driving the light emitted into the waveguide is nonclassical with a significant negativity of the associated Wigner function. We show that both the emission properties and the non-Gaussian character of the light are robust against voids in the atom chain, enabling the experimental study of these effects with present-day technology. Our results offer a route towards novel types of fiber-coupled quantum light sources and an interesting perspective for probing the physics of interacting atomic ensembles through light.

Paper on Imaging and Localizing Individual Atoms Interfaced with a Nanophotonic Waveguide published

Y. Meng, C. Liedl, S. Pucher, A. Rauschenbeutel, and P. Schneeweiss

Single particle-resolved fluorescence imaging is an enabling technology in cold-atom physics. However, so far, this technique has not been available for nanophotonic atom-light interfaces. Here, we image single atoms that are trapped and optically interfaced using an optical nanofiber. Near-resonant light is scattered off the atoms and imaged while counteracting heating mechanisms via degenerate Raman cooling. We detect trapped atoms within 150 ms and record image sequences of given atoms. Building on our technique, we perform two experiments which are conditioned on the number and position of the nanofiber-trapped atoms. We measure the transmission of nanofiber-guided resonant light and verify its exponential scaling in the few-atom limit, in accordance with Beer-Lambert’s law. Moreover, depending on the interatomic distance, we observe interference of the fields that two simultaneously trapped atoms emit into the nanofiber. The demonstrated technique enables postselection and possible feedback schemes and thereby opens the road toward a new generation of experiments in quantum nanophotonics.

Individual atoms (yellow) in the vicinity of a nano glass fiber are photographed with a sensitive camera. A laser beam is used to illuminate the atoms.

Paper on Correlating photons using weakly atoms published

Adarsh S. Prasad, Jakob Hinney, Sahand Mahmoodian, Klemens Hammerer, Samuel Rind, Philipp Schneeweiss, Anders S. Sørensen, Jürgen Volz & Arno Rauschenbeutel

Photons in a nonlinear medium can repel or attract each other, resulting in strongly correlated quantum many-body states1,2. Typically, such correlated states of light arise from the extreme nonlinearity granted by quantum emitters that are strongly coupled to a photonic mode2,3. However, unavoidable dissipation (such as photon loss) blurs nonlinear quantum effects when such approaches are used. Here, we generate strongly correlated photon states using only weak coupling and taking advantage of dissipation. An ensemble of non-interacting waveguide-coupled atoms induces correlations between simultaneously arriving photons through collectively enhanced nonlinear interactions. These correlated photons experience less dissipation than the uncorrelated ones. Depending on the number of atoms, we experimentally observe strong photon bunching or antibunching of the transmitted light. This realization of a collectively enhanced nonlinearity may turn out to be transformational for quantum information science and opens new avenues for generating non-classical light, covering frequencies from the microwave to the X-ray regime.

Paper and Open Data on the Lifetime Measurement of the Cesium 5²D₅⸝₂ State published

We measure the lifetime of the cesium 5²D₅⸝₂ state using a time-resolved single-photon-counting method. We excite atoms in a hot vapor cell via an electric quadrupole transition at a wavelength of 685 nm and record the fluorescence of a cascade decay at a wavelength of 852 nm. We extract a lifetime of 1353(5) ns for the 5²D₅⸝₂ state, in agreement with a recent theoretical prediction. In particular, the observed lifetime is consistent with the literature values of the polarizabilities of the cesium 6P states. Our measurement contributes to resolving a long-standing disagreement between a number of experimental and theoretical results.

In order to improve the traceability of the presented measurements and analysis, the raw experimental data used in this paper has been made available in an open-access repository. Examples of source codes used for the analysis of the data are provided as well.

Welcome to Humboldt University, Berlin

The largest part of the research group around Prof. A. Rauschenbeutel, ErBeStA consortium member and coordinator, moved this summer to Humboldt University of Berlin. Still, TU Wien will remain in the ErBeStA consortium as a project partner. The corresponding efforts there will be led by Dr. Sarah Bayer-Skoff. Here are a few photos of the move to Berlin.

Paper on Slow light-enhanced optical imaging

M. Scheucher, K. Kassem, A. Rauschenbeutel, P. Schneeweiss, and J. Volz

Optical fibers play a key role in many different fields of science and technology. For many of these applications it is of outmost importance to precisely know and control their radius. In this manuscript, we demonstrate a novel technique to determine the local radius variation of a 30 micrometer diameter silica fiber with sub-Angström precision over more than half a millimeter in a single shot, by imaging the mode structure of the fiber’s whispering gallery modes (WGMs). We show that in these WGMs the speed of light propagating along the fiber axis is strongly reduced, which enables us to determine the fiber radius with significantly enhanced precision, far beyond the diffraction limit. By exciting several different axial modes at different probing fiber positions, we verify the precision and reproducibility of our method and demonstrate that we can achieve a precision better than 0.3 Angström. The demonstrated method can be generalized to other experimental situations where slow light occurs and, thus, has a large range of potential applications in the realms of precision metrology and optical sensing.

Quantum Technology: New EU Research Network measures Bell-States

With a 3 million euro endowment, comes a new European research network headed by TU Wien. The goal: a device that can analyze specific entangled quantum states.

The goal is ambitious; develop a novel device that can reliably measure special quantum states known as “Bell-States”. A total of seven research institutes have joined forces to pursue the goal of an optical “Bell-State Analyzer”. The new project is coordinated at Atominstitut of TU Wien, and the European Union is providing a total of 3 million euros of funding.

Quantum Entanglement

An astonishing phenomenon plays a central role in practically all modern quantum technology; quantum entanglement, in which individual particles can no longer be viewed as separate from another. When the state of one particle is measured, this inevitably changes the state of the other particle, meaning the pair can only be described as a single entity.

The degree of entanglement of particles can vary. Two-particle states with the maximum possible degree of entanglement are known as “Bell-States”, named after the Quantum Theoretician, John Bell. In modern quantum technology, Bell-states play an important role and often consist of pairs of photons. Their use spans the fields of quantum teleportation, quantum cryptology, and even quantum sensor technology.

Unfortunately, as of yet, it has not been possible to reliably detect optical Bell-states. Only the state of individual photons can be measured, which however is not the same as measuring entangled quantum states, which simultaneously describe both photons.

New Capabilities through Novel Ideas

New discoveries in recent years have shown that the detection of Bell-states should indeed be possible. With help of a single atom, photon pairs can be manipulated such that their shared state becomes measurable. Thanks to novel methods of nano- fabrication, optical chips may as well allow for the miniaturization of this novel technology.

Still, there remain important scientific and technical problems that need solving, but the path is clear: The project consortium is therefore confident that in the next few years its combined efforts will bring an optical Bell-state analyzer to fruition. The network comprises TU Wien, the Universität Rostock, the University of Nottingham, the Universität Wien, Syddansk Universitet, Aarhaus Universitet, and the Ferdinand-Braun-Institut in Berlin. The project officially started on the July 1st, 2018 and will continue for the next 3 years.