Rafael Mottl

Exploring Symmetry Breaking at the Dicke Quantum Phase Transition

We study symmetry breaking at the Dicke quantum phase transition by coupling a motional degree of freedom of a Bose-Einstein condensate to the field of an optical cavity. Using an optical heterodyne detection scheme, we observe symmetry breaking in real time and distinguish the two superradiant phases. We explore the process of symmetry breaking in the presence of a small symmetry-breaking field and study its dependence on the rate at which the critical point is crossed. Coherent switching between the two ordered phases is demonstrated.

Quantum phases from competing short- and long-range interactions in an optical lattice

Insights into complex phenomena in quantum matter can be gained from simulation experiments with ultracold atoms, especially in cases where theoretical characterization is challenging. However, these experiments are mostly limited to short-range collisional interactions; recently observed perturbative effects of long-range interactions were too weak to reach new quantum phases. Here we experimentally realize a bosonic lattice model with competing short- and long-range interactions, and observe the appearance of four distinct quantum phases—a superfluid, a supersolid, a Mott insulator and a charge density wave. Our system is based on an atomic quantum gas trapped in an optical lattice inside a high-finesse optical cavity. The strength of the short-range on-site interactions is controlled by means of the optical lattice depth. The long (infinite)-range interaction potential is mediated by a vacuum mode of the cavity and is independently controlled by tuning the cavity resonance. When probing the phase transition between the Mott insulator and the charge density wave in real time, we observed a behaviour characteristic of a first-order phase transition. Our measurements have accessed a regime for quantum simulation of many-body systems where the physics is determined by the intricate competition between two different types of interactions and the zero point motion of the particles.

Roton-type mode softening in a quantum gas with cavity-mediated long-range interactions.

Cavity-Induced Minimum Tuning the strength and range of interactions in cold atomic gases is crucial to their role as quantum simulators. Most atom-atom interactions are short-ranged. One way to extend the range is to couple the gas to an optical cavity, which can propagate interactions between atoms, making the interactions effectively long-ranged. This system has been used to observe a transition to a “supersolid” phase characterized by a checkerboard atomic density order. Mottl et al. (p. 1570, published online 17 May) used Bragg spectroscopy to measure the excitation spectrum of an ultracold gas of Rb-87 atoms as the interaction strength was varied. Consistent with theoretical predictions, a minimum was observed in the excitation energy, similar to that observed in roton excitations of the superfluid helium. Low-energy excitations of the type present in superfluid helium are observed in a cold gas of rubidium atoms. Long-range interactions in quantum gases are predicted to give rise to an excitation spectrum of roton character, similar to that observed in superfluid helium. We investigated the excitation spectrum of a Bose-Einstein condensate with cavity-mediated long-range interactions, which couple all particles to each other. Increasing the strength of the interaction leads to a softening of an excitation mode at a finite momentum, preceding a superfluid-to-supersolid phase transition. We used a variant of Bragg spectroscopy to study the mode softening across the phase transition. The measured spectrum was in very good agreement with ab initio calculations and, at the phase transition, a diverging susceptibility was observed. The work paves the way toward quantum simulation of long-range interacting many-body systems.

Real-time observation of fluctuations at the driven-dissipative Dicke phase transition

We experimentally study the influence of dissipation on the driven Dicke quantum phase transition, realized by coupling external degrees of freedom of a Bose–Einstein condensate to the light field of a high-finesse optical cavity. The cavity provides a natural dissipation channel, which gives rise to vacuum-induced fluctuations and allows us to observe density fluctuations of the gas in real-time. We monitor the divergence of these fluctuations over two orders of magnitude while approaching the phase transition, and observe a behavior that deviates significantly from that expected for a closed system. A correlation analysis of the fluctuations reveals the diverging time scale of the atomic dynamics and allows us to extract a damping rate for the external degree of freedom of the atoms. We find good agreement with our theoretical model including dissipation via both the cavity field and the atomic field. Using a dissipation channel to nondestructively gain information about a quantum many-body system provides a unique path to study the physics of driven-dissipative systems.

Measuring the dynamic structure factor of a quantum gas undergoing a structural phase transition

The dynamic structure factor is a central quantity describing the physics of quantum many-body systems, capturing structure and collective excitations of a material. In condensed matter, it can be measured via inelastic neutron scattering, which is an energy-resolving probe for the density fluctuations. In ultracold atoms, a similar approach could so far not be applied because of the diluteness of the system. Here we report on a direct, real-time and nondestructive measurement of the dynamic structure factor of a quantum gas exhibiting cavity-mediated long-range interactions. The technique relies on inelastic scattering of photons, stimulated by the enhanced vacuum field inside a high finesse optical cavity. We extract the density fluctuations, their energy and lifetime while the system undergoes a structural phase transition. We observe an occupation of the relevant quasi-particle mode on the level of a few excitations, and provide a theoretical description of this dissipative quantum many-body system.

Exploring cavity-mediated long-range interactions in a quantum gas

Creating quantum gases with long-range atom-atom interactions is a vibrant area of current research, possibly leading to the observation of novel quantum phases and phase transitions. In our approach, we couple the external degree of freedom of a Bose-Einstein condensate to the vacuum mode of a high-finesse optical cavity using a non-resonant standing-wave transverse pump beam. This gives rise to a driven-dissipative system with cavity-mediated atom-atom interactions of global range, which are tunable both in magnitude and sign.