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Teaser, summary, work performed and final results

Periodic Reporting for period 1 - CoMiC (Cold Molecules in Cavities)


Quantum technologies are a recent hot topic in the EU and worldwide. The control of matter on the quantum level offers completely new perspectives for computation, communication and simulation. One of those simulation tasks is chemistry, since quantum physics plays a crucial...


Quantum technologies are a recent hot topic in the EU and worldwide. The control of matter on the quantum level offers completely new perspectives for computation, communication and simulation. One of those simulation tasks is chemistry, since quantum physics plays a crucial role in chemical reactions and molecular dynamics, which are relevant for many new developments, ranging from new fuel to new pharma. Those are problems which can quickly become very complex when simulated on classical computers. Recent methods in experimental quantum physics allow to cool matter particles, such as atoms or molecules, to very low temperatures, where quantum effects can be studied in a very clean and isolated environment. By methods of laser cooling, the particles are slowed down by orders of magnitude compared to their motion at room temperature, taking away almost their entire kinetic energy. This opened up the new field of ultracold chemistry, where precision studies of chemical reactions between those particles became possible. Now, their reaction or collision energies can be very precisely controlled, e.g., by laser light, which fuses together two or more atoms, forming a molecule (so-called photoassociation). Those molecules are usually not stable, as they will quickly decay into states of lower energy by releasing the excess energy as a photon (spontaneous emission). This process is, however, not well controlled - the molecules can randomly end up in one of many possible quantum states. Here, our project sets in: We want to control spontaneous emission to strongly enhance the production of a desired final quantum state. For this, we make use of an optical cavity, which is placed around the atoms/molecules and which strongly modifies the electromagnetic environment of the particles. By this, certain photon energies in the spontaneous emission process can be preferred, and it will be possible to produce an almost pure sample of cold molecules in a certain state, which would be an important starting point for further studies in ultracold chemistry. At the same time, we will observe the emitted photons with high efficiency, giving real-time insight into reaction dynamics (Objective 1). Furthermore, the detection of single particles in their quantum states, without destroying them, is another very important quantum technology. In this project, we want to make use of the cavity to detect single cold molecules. The presence of a molecule in a certain quantum state inside the cavity will modify the optical properties of the cavity, such as transmission, which can be measured (Objective 2).

Work performed

An important task was to develop a suitable experimental scheme for the demonstration of cavity-assisted molecule formation in ultracold rubidium atoms (Objective 1). This means, to find suitable molecular transitions for coupling them to an optical cavity, and find a cavity design that offers highly-efficient molecule production while respecting the constraints of an ultracold atom vacuum setup. The theoretical basis of the scheme has been investigated both analytically and numerically and feasible cavity parameters have been found for which efficient molecule formation should be possible. Those parameter studies are not only relevant for this project but can be considered as general guidelines also for other experimental systems, using, e.g., other species than rubidium. Our work has been published in New J. Phys. 20, 123015 (2018). For the experimental realisation of the scheme, an optical cavity had to be integrated into an existing ultracold atom setup. For this, a suitable cavity mirror design had to be found. Early attempts with conventional fibre cavity mirrors failed, which led to the development of two new types of compact cavity mirrors, consisting of short fibre pieces and gradient index lenses. Those designs promise strong molecule-cavity coupling (strong light confinement) while still having good coupling of light in and out of the cavity. A process for producing those mirrors has been developed, including polishing, alignment, gluing and characterisation. However, the fabrication has not been completed before the end of the project. Concerning the ultrahigh vacuum setup, a new chamber has been designed, build and tested, which respects the special constraints of the setup (limited space) while offering good vibration damping for the cavity and optical access from several sides. The laser system for the photoassociation of the molecules and the control of the cavity has also been built.

Final results

A new tool for quantum technologies in the sub-area of ultracold chemistry has been developed and advanced. In a theoretical study, the feasibility of cavity-assisted molecule formation in a realistic ultracold atom setup has been demonstrated and concrete parameters and efficiencies have been provided. This could trigger further development of ultracold chemistry as a platform for quantum simulation. On the technical side, new compact cavity designs have been designed that provide strong light-matter coupling in small setups, while having good optical coupling to outside light (laser) modes. Since cavities are of high interest also in other fields of quantum physics, such as cold-atom, cold-ion or even solid state qubits, a technical publication of those cavity designs could be of high relevance for a broader audience.