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** KOÇ UNIVERSITY PHYSICS SEMINAR**

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Speaker: |
Aydın Cem Keser, University of New South Wales |

Title: |
Analogue Stochastic Gravity in Strongly-Interacting Bose-Einstein Condensates |

Date: |
December 21, 2018 (Friday) |

Time: |
10:30 A.M. |

Cookie & Tea: |
SCI 129 10:15 A.M. |

Place: |
SCI 129 (not the usual classroom) |

**Abstract:
**Collective modes propagating in a moving superfluid are known to satisfy wave equations in a curved space time, with a metric determined by the underlying superflow. We use the Keldysh technique in a curved space-time to develop a quantum geometric theory of fluctuations in superfluid hydrodynamics. This theory relies on a ‘quantized’ generalization of the two-fluid description of Landau and Khalatnikov, where the superfluid component is viewed as a quasi-classical field coupled to a normal component — the collective modes/phonons representing a quantum bath. This relates the problem in the hydrodynamic limit to the ‘quantum friction’ problem of Caldeira-Leggett type. By integrating out the phonons, we derive stochastic Langevin equations describing a coupling between the superfluid component and phonons. These equations have the form of Euler equations with additional source terms expressed through a fluctuating stress-energy tensor of phonons. Conceptually, this result is similar to stochastic Einstein equations that arise in the theory of stochastic gravity. We formulate the fluctuation-dissipation theorem in this geometric language and discuss possible physical consequences of this theory.

**Short Bio: **Aydın Cem Keser is a theoretical condensed matter physicist and electronics engineer and is currently a Post-Doctoral Research Associate at the Centre for Future Low Energy Electronics Technologies and University of New South Wales in Australia. His research interests include transport in disordered and stochastic systems, quantum hydrodynamics, condensed matter analogs of cosmological phenomena. He completed his PhD in the University of Maryland Joint Quantum Institute. He is the author of the thesis titled “Classical Analogies in the Solution of Quantum Many Body Problems” that won the Springer thesis award. Dr. Keser is a Fulbright Fellow.