The long term goal of our group is to develop new methods to manipulate many-body states in a regime where the quantum mechanical aspects dominate their behavior and their properties. On the one hand, this should lead to new tools that allow one to probe physical laws and to measure fundamental constants with increasing precision. On the other hand, the progress of experimental methods also drives the advances in our understanding of the ever mysterious, beautiful, accurate, yet deeply dissatisfying structure of quantum mechanics. This interplay between theoretical concepts and experimental realizations promises to be very fertile in fields such as quantum control, quantum feedback and its limits, many-particle quantum systems, and many-particle entanglement (quantum computing).We use various methods, but most include laser cooled atoms (to be able to keep atoms localized, and attain long coherence time) and laser-light interaction to manipulate the atoms, the photons, or both, at the quantum level. Using internal states of atoms in combination with laser light, that has essentially zero entropy, allows us to reduce thermal noise without having to cool the atoms to very low (sub-microkelvin) temperatures.
Cavity quantum electrodynamics with atomic ensembles
In cavity electrodynamics, an atom interacts so strongly with the vacuum field of a resonator that even a single excitation is coherently exchanged back and forth between the atom (atom in the excited state, cavity in the vacuum state) and the cavity (atom in the ground state, cavity contains one photon). This process can be used not only for quantum communication using individual photons, but also to generate entanglement between atoms via the cavity field. Here the photons act as messengers between the atoms, building up quantum correlations (entanglement) between them. Such states can be used to improve precision measurements through entanglement.
Spin squeezing for atomic clocks beyond the standard quantum limit
The precision of a standard atomic clock improves with the square root of the number of particles (standard quantum limit). This limit arises from the projection noise in the final readout of the atomic phase. It is possible to overcome the standard quantum limit by means of quantum correlations (entanglement) between particles, such that the quantum noise is redistributed away from the quantity of interest (squeezing of spin noise, or short spin squeezing). Such correlation can only be induced via an interaction between the atoms. Interestingly, it is possible to replace the direct interaction between the atoms (collisions, typically not desirable in a precision experiment) with an effective long-distance interaction between atoms mediated by a light field. We have demonstrated spin squeezing in proof-of-principle experiments, and are working towards applying spin squeezing in a state-of-the-art ytterbium atomic clock operating on an optical transition.
Laser cooling methods to reach quantum degeneracy
Laser cooling of atoms has not only supplied the basis for the control and manipulation of matter at the quantum limit, e.g. in form of Bose-Einstein condensation, but has also resulted in a number of important applications and devices, many of which are tied to precision measurements and atomic clocks. However, laser cooling had previously stopped at high atomic densities, and another, much slower and less efficient cooling method, evaporative cooling, was necessary to achieve Bose-Einstein condensation. We have recently succeeded in reaching Bose-Einstein condensation by laser cooling alone, and are working towards applying similar techniques to Fermi gases and to molecules.
Strong interactions between individual photons
In vacuum, the interaction between individual photons is completely negligible: laser or light beams simply pass through one another. However, we have recently created a new optical medium with an exquisitely strong nonlinearity, so strong in fact, that two photons bind together and form a bound state, a molecule consisting of two photons. We are working towards expanding this effect towards larger clusters of photons, and hope to be able to create matter analogues with light, e.g., a crystal consisting of photons.
Search for dark matter with atomic precision measurements
Precision measurements in atomic physics have reaches such precision that even minute effects associated with high-energy particles or dark matter can be potentially detected with precision experiments at low frequencies. We are currently performing experiments where hypothetical dark-matter particles can be detected or excluded based on a tiny frequency shift on an optical transition in an atomic clock.