# Mainz 2017 – wissenschaftliches Programm

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# Q: Fachverband Quantenoptik und Photonik

## Q 29: Quantum Gases: Bosons IV

### Q 29.9: Vortrag

### Dienstag, 7. März 2017, 16:30–16:45, P 204

**Observing the Goldstone and the amplitude mode in a supersolid quantum gas** — •Julian Léonard, Andrea Morales, Philip Zupancic, Tilman Esslinger, and Tobias Donner — Institute for Quantum Electronics, ETH Zurich, Switzerland

A phase transition with continuous symmetry breaking of a scalar quantum field can give rise to excitations of its phase and amplitude. A supersolid presents an intriguing example for such an excitation spectrum, as both continuous gauge symmetry of the superfluid phase and the translational symmetry of the lattice modulation are broken simultaneously.

We investigate the interplay of these symmetries by studying the elementary excitations across the superfluid-supersolid phase transition. The supersolid phase is realized by coupling a Bose-Einstein condensate with the fields of two crossed cavity modes. We perform cavity-enhanced Bragg spectroscopy on the quantum gas to measure its excitation spectrum. In the superfluid phase, we observe two independent modes that soften upon approaching the critical point. Inside the supersolid phase the modes split into a low-energetic Goldstone mode and a high-energy amplitude mode.