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Room temperature quantum networks with Rydberg atoms in microcells: Tuning the Rydberg atom-surface interaction

Final Report Summary - RYDSURF (Room temperature quantum networks with Rydberg atoms in microcells: Tuning the Rydberg atom-surface interaction)

The control of quantum correlations is a key step of emerging technologies for quantum information processing and communication. Among them, atomic systems based on ultracold single atoms or ions have been proved very successful in this task. However, mostly due to the requirement of ultralow temperatures, the scaling to a large number of interconnected devices is difficult. In contrast, mesoscopic ensembles of atoms which can be well controlled in their geometry and which provide coherent long range interactions promise a significant simplification for quantum devices and networks. They do not need to operate in well defined motional quantum states and finite temperatures up to even above room temperature operation of the resulting quantum devices might be possible. In case of a successful demonstration of the basic units the upscaling to quantum networks with millions of nodes seems within reach.

The central goal of this project is to explore to what extend such quantum devices and networks based on finite temperature ensembles with switchable long range interaction atoms are possible and competitive. We plan to study Rydberg interacting mesoscopic ensembles at room temperature and above. As the range of interaction can be on the order of micrometers, standard techniques in lithography can be used to produce mesoscopic ensembles confined in glass cells. Because the preservation of quantum coherence in such systems represents one of the greatest challenges, the evaluation of the effect of the confining surfaces on the coherence properties of the quantum ensemble is crucial. To study the effect of the coupling of the surfaces of different materials and coatings with the atomic ensemble, we designed and built a UHV apparatus which allows the exchange of samples on a weekly basis and offers the possibility to study surfaces in-situ (Figure 1). The distance between the two surfaces can be stabilized in the 50 nm regime and can operate at high vapor pressures. Apart from the possibility of fast testing of different materials and coatings and clarify the role of surface polaritons as a source of decoherence, this new setup will allow for the measurement Casimir-Polder forces at finite temperature for dielectrics, which is a hot topic in surface science. Some collaboration with theoretical groups has already been established.

In the second phase of the project, the aim is to investigate the concept of an integratable single photon source based on alkali vapor cells. For that, the first milestone is to demonstrate that the blockade effect, present in ultracold ensembles and which is of pivotal importance to operate quantum gates, can be exploited in thermal cells. This is a challenging task, mainly because of the lack of available tools compared to cold atom experiments. In this respect, we found that it is possible to measure the current produced by the ionization of Rydberg atoms in gas collisions.
For that we designed and built a new cell equipped with guarded field plates and showed that currents as low as tenths of pA can be detected following an approach that could be easily integrated in microcells. This new method is very sensitive and outperforms purely optical techniques (like electromagnetically induced transparency, EIT) where both spectroscopies can be applied (Figure 2). Moreover, we successfully applied this technique to a new generation of electrically contacted, addressable cells and demonstrated that the detected ionization current is a measure of Rydberg state populations. This unique technique provides a promising way to study interaction effects between Rydberg atoms and represents a breakthrough towards the demonstration of the Rydberg blockade mechanism in thermal vapor cells. Evidence for van der Waals interactions, which are responsible for the blockade effect in cold atoms, has also been shown in thermal vapors in our group very recently, supporting the idea to exploit a blockade effect in a thermal gas.

Experiments for the proof of principle demonstration of the first single photon source based the Rydberg blockade are ongoing. Such source will feature deterministic and directed emission of photons in a narrow bandwidth. Its implementation promises a number of advantages over other quantum technologies, such as semiconductor quantum dots, concerning scalability, efficiency and compatibility with quantum memories. Rydberg atoms confined in vapor microcells could therefore arise as the building blocks for quantum networks that can operate at room temperature in a robust, micrometer scaled quantum information devices.

More information about this project can be found at:
http://www.pi5.uni-stuttgart.de/