We focus on researching novel quantum technology like narrow-band sources of entanglement, long-distance quantum networks and quantum sensing, as well as on fundamental questions of physics – can there be quantum superposition of gravitational fields, do we need to quantize gravity? Can we entangle objects via their relative gravitational interaction? Some of these questions may have to be addressed in a space environment. To this end, we are working on proof-of-principle experiments on ground and on technology development for future space missions.

Quantum optics, entanglement and quantum communication

Schematic of sharing entanglement globally via two optical space links

Entanglement will be the basic resource for any future quantum network.Global sharing of entanglement will be instrumental for enabling a host of future quantum technologies. Examples are the connecting of distant quantum computers or distributed quantum sensing. To cover truly global distances, it will be imperative to realize quantum repeaters, which in turn rely on the swapping of entanglement between distant nodes and its efficient storage in quantum memories.

Our group works on implementing bright and narrow-band single-photon sources at various wavelengths and on the frequency conversion of single photons to efficiently store the photons in quantum memories. At the same time, we collaborate with research teams in Slovenia and in other parts of Europe to implement the necessary infrastructure for future quantum networks, and to increase the technological readiness of quantum technologies.

Quantum optomechanics with optically trapped particles

Optically trapping dielectric particles (sub-micron to micron size) in ultra-high vacuum in principle allows to control the motion of the trapped particles on the quantum level. Such optomechanical systems promise allowing to isolate the motion of the particles very well from coupling to the environment, e.g., via vibrations. Very recently, several groups have successfully demonstrated cooling the motion of such trapped particles close to the quantum ground state of motion, and researchers are now exploring ways of preparing the center-of-mass motion as well as rotational degrees of freedom of optically trapped particles in non-classical states. Such system promise high-precision sensors, the implementation of quantum transducers, where the optomechanical systems can, e.g., couple to optical fields but also to magnetic fields or microwave fields, and potentially to use optomechanical systems as quantum memories for storing quantum information. Moreover, quantum optomechanics with optically trapped particles are a very promising platform for testing the foundations of quantum physics (see below).

In our lab, we are currently working towards tackling several key challenges we need to face before we can harness the full potential of this architecture. Some examples are: (1) how can we reliably and on demand load individual sub-micron test particles into an optical trap in ultra-high vacuum and, potentially, at cryogenic temperatures? (2) how can we guide and trap such particles, and how can we ensure that they are not charged (charges could introduce excessive coupling to the environment)? (3) How can we keep the particles at low temperatures despite interacting with them via optical fields? In our research group, we currently address the first two points in a research project, where we aim to first optically trap test particles at low vacuum and then to guide them through a hollow-core photonic-crystal fiber into a ultra-high-vacuum chamber. You can find more details on the project page.

Testing the foundations of quantum physics

Quantum optomechanics with optically trapped particles promises excellent decoupling between the center-of-mass motion of the optomechanical system and the environment. Moreover, one can release optically trapped particles from the trapping potential to study the free quantum evolution of test particles to study the evolution of macroscopic superpositions and matter-wave interferometry with massive test particles. As we go to higher-mass test particles, we may eventually see deviations from the predictions of quantum physics as gravitational effects become more significant. As we probe parameter regimes where quantum physics has not been tested yet, such experiments also have the potential to allow testing for deviations from the standard model of particle physics, e.g., due to coupling to exotic matter or dark matter.

In our lab, we aim to develop novel ways to prepare macroscopic superpositions and to test the free quantum evolution of massive test particles. Eventually, we plan to perform such experiments in microgravity, in orbit, and in a deep-space fundamental science mission (the MAQRO proposal – see below).

Experiments in microgravity and in space – towards the MAQRO space mission

Graphical impression of the MAQRO spacecraft facing the sun with the payload shielded to allow passive radiative cooling to achieve extremely high vacuum and cryogenic temperatures.

In 2010, R. Kaltenbaek and others proposed the fundamental-science space mission MAQRO, which would use quantum optomechanics and matter-wave interferometry with massive dielectric test particles to test the predictions of quantum physics with test masses orders of magnitude beyond what was then considered to be possible in Earth-based experiments. Over the years, the mission proposal was investigated in increasing detail, and the concept formed the basis for an in-depth feasibility study by the European Space Agency (ESA) in 2018. This “QPPF” study concluded that the mission concept is feasible in principle, but that we will have to increase the technological readiness of key components, and that we will have to address three critical issues (achieving the necessary vacuum and shielding from radiation in space, implementing a reliable loading and guiding mechanism for the test particles, demonstrating a reliable way for preparing the macroscopic quantum superpositions without introducing excessive decoherence). In order to tackle these challenges, we submitted a white paper for a NASA research campaign in 2021, and we are collaborating with other research teams to develop technology, proof-of-principle experiments on ground and in micro-gravity, and we plan to implement in-orbit demonstrators within the next ten years.