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

https://arxiv.org/abs/1905.12383

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.

Paper on Quantum Optical Networks via Polariton Exchange Interactions

https://arxiv.org/abs/1903.12442

Mohammadsadegh Khazali, Callum R. Murray, and Thomas Pohl

We examine the dynamics of Rydberg polaritons with dipolar interactions that propagate in multiple spatial modes. The dipolar excitation-exchange between different Rydberg states mediates an effective exchange between polaritons that enables photons to hop across different spatial channels. Remarkably, the efficiency of this photon exchange process can increase with the channel distance and becomes optimal at a finite rail separation. Based on this mechanism, we design a simple photonic network that realises a two photon quantum gate with a robust pi-phase, protected by the symmetries of the underlying photon interaction and the geometry of the network. These capabilities expand the scope of Rydberg-EIT towards multidimensional geometries for nonlinear optical networks and explorations of photonic many-body physics.

Paper on prospects for strongly coupled atom-photon quantum nodes

https://arxiv.org/abs/1812.06020

N. Cooper, C. Briddon, E. Da Ros, V. Naniyil, M. Greenaway, L. Hackermueller

We discuss the trapping of cold atoms within microscopic voids drilled perpendicularly through the axis of an optical waveguide. The dimensions of the voids considered are between 1 and 40 optical wavelengths. By simulating light transmission across the voids, we find that appropriate shaping of the voids can substantially reduce the associated loss of optical power. Our results demonstrate that the formation of an optical cavity around such a void could produce strong coupling between the atoms and the guided light. By bringing multiple atoms into a single void and exploiting collective enhancement, cooperativities ~400 or more should be achievable. The simulations are carried out using a finite difference time domain method. Methods for the production of such a void and the trapping of cold atoms within it are also discussed.

Paper on dynamical creation and detection of dark entangled phases in a chiral atom chain on the arXiv

https://arxiv.org/abs/1902.08525

Giuseppe Buonaiuto, Ryan Jones, Beatriz Olmos, Igor Lesanovsky

Open quantum systems with chiral interactions can be realized by coupling atoms to guided radiation modes in waveguides or optical fibres. In their steady state these systems can feature intricate many-body phases such as entangled dark states, but their detection and characterization remains a challenge. Here we show how such collective phenomena can be uncovered through monitoring the record of photons emitted into the guided modes. This permits the identification of dark entangled states but furthermore offers novel capabilities for probing complex dynamical behavior, such as the coexistence of a dark entangled and a mixed phase. Our results are of direct relevance for current experiments, as they provide a framework for probing, characterizing and classifying dynamical features of chiral light-matter systems.

Paper on 3-photon correlations mediated by a Rydberg superatom

Nontrivial three-photon correlations can be imprinted onto initially uncorrelated photons through an interaction with a single Rydberg superatom, something the SDU group has recently observed experimentally:

https://doi.org/10.1103/PhysRevLett.121.103601

By exploiting the Rydberg blockade mechanism, they turn a cold atomic cloud into a single effective emitter with collectively enhanced coupling to a focused photonic mode which gives rise to clear signatures in the connected part of the three-body correlation function of the outgoing photons. The results are in good agreement with a quantitative model for a single, strongly coupled Rydberg superatom. Furthermore, they developed an idealized but exactly solvable model of a single two-level system coupled to a photonic mode, which allows for an interpretation of the experimental observations in terms of bound states and scattering states.

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.