# Mainz 2017 – wissenschaftliches Programm

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# Q: Fachverband Quantenoptik und Photonik

## Q 6: Quantum Gases: Bosons I

### Q 6.2: Vortrag

### Montag, 6. März 2017, 14:45–15:00, P 204

**Lowest-Lying Collective Frequencies of a Photon BEC in Presence of Temporally Retarded Interaction** — •Enrico Stein and Axel Pelster — Fachbereich Physik und Forschungszentrum OPTIMAS, Technische Universität Kaiserslautern, Germany

Usually, collective mode frequencies of BECs are theoretically described for an instantaneous two-particle interaction between the bosons. Recent experiments on photon BECs show, however, that the effective photon-photon interaction is significantly influenced by memory effects which are presumably due to diffusion processes [1,2]. Therefore, we analyse the lowest-lying collective modes of a two-dimensional BEC in presence of a temporally retarded interaction by solving the underlying Gross-Pitaevskii equation with a variational approach [3]. We find that the frequencies for both the breathing and the quadrupole mode are shifted for a few percent with respect to a non-retarded two-particle interaction. Furthermore, we find the same order of magnitude for the violation of the Kohn theorem [4], i.e. the dipole-mode frequency differs from the trap frequency. Finally, we discuss how these retardation effects can be enhanced in an anisotropic harmonic confinement. All these findings are essential for determining the photon-photon interaction strength from measuring the lowest-lying collective frequencies of the photon BEC.

[1] J. Klaers, et al., Appl. Phys. B **105**, 17 (2011)

[2] J. Schmitt, et al., Phys. Rev. A **92**, 011602(R) (2015)

[3] V.M. Perés-Garciá, et al., Phys. Rev. Lett. **77**, 5320 (1996)

[4] A.L. Fetter and D. Rokhsar, Phys. Rev. A **57**, 1191 (1998)