Session U4: Polarons and Quantum Optics with Ultracold Atoms

Interferometric measurement of many-body topological invariants using polarons

We present a scheme for the direct detection of many-body topological invariants in ultra cold quantum gases in optical lattices. We generalize single-particle interferometric schemes developed for the detection of topologically non-trivial band structures [Atala et.al., Nature Physics 9, 795 (2013)] by coupling a spin-1/2 impurity to a (topological) excitation of an interacting many-body system. Performing Ramsey interferometry in combination with Bloch oscillations of the resulting polaronic particle allows to directly detect the many body-topological invariant. In particular we consider adiabatic Thouless pumps in the super-lattice Bose-Hubbard model, which transport a quantized amount of particles across a one-dimensional lattice. In the presence of inter-atomic interactions this quantized current is given by a many-body Chern number, which can be measured using our protocol. These systems also support symmetry-protected topological phases, the invariants of which can be obtained from our protocol as well.   

Observation of grand-canonical number statistics in a photon Bose-Einstein

Large (super-Poissonian) statistical fluctuations are a well known for the thermal behavior of bosons, as revealed in Hanbury Brown-Twiss experiments. For a Bose-Einstein condensed sample such large fluctuations usually conflict with particle number conservation, and when e.g. a cold atom cloud condenses to a BEC the fluctuations dampen and the sources acquires second-order coherence. In 2010, we have observed Bose-Einstein condensation of photons in a dye-filled optical microcavity, where photons thermalize with dye molecules by repeated absorption-emission processes. Here we report measurements of photon correlations and the statistical fluctuations of a photon Bose-Einstein condensate realized in the dye microcavity system. The dye molecules act both as a heat bath and a particle reservoir to realize grand-canonical conditions for the photon gas. We observe a regime with condensate number fluctuations of order of the total particle number, which demonstrates Bose-Einstein condensation under grand-canonical statistical conditions. In this regime, condensation and extremely large statistical fluctuations coexist.   

Crossover between a laser-like state and a photon Bose-Einstein condensate

Bose-Einstein condensation has been observed with cold atomic gases, quasiparticles in solid state systems as polaritons, and more recently also with photons in a dye-filled microcavity. Here we examine in detail the thermalization dynamics of the photon gas in the dye-filled microcavity system, which proceeds by absorption and re-emission processes to the rovibrational temperature of the dye molecules. We use pulsed excitation of the dye-filled microcavity. When the thermalization by absorption and emission is faster than the photon loss rate in the cavity, the photons accumulate at lower energy states above the cavity low-frequency cutoff, and the system finally thermalizes to a Bose-Einstein condensate of photons. On the other hand, for a small reabsorption with respect to the photon loss, the state remains laser-like. We observe a crossover between a laser-like state and a photon condensate, which can be controlled by adjusting the ratio of the dye reabsorption versus the cavity loss rate.   

Dissipative preparation of squeezed states with ultracold atomic gases

We present a dissipative quantum state preparation scheme for the creation of phase- and number-squeezed states [1,2]. It utilizes ultracold atoms in a double-well configuration immersed in a background BEC acting as a dissipative quantum reservoir [2]. We derive a master equation starting from microscopic physics, and show that squeezing develops on a time scale proportional to $1/N$, where $N$ is the number of particles in the double well. This scaling, caused by bosonic enhancement, allows us to make the time scale for the creation of squeezed states very short. Effects of the dephasing which limits the lifetime of the squeezed states can be avoided by stroboscopically switching the driving off and on. We show that this approach leads to robust stationary squeezed states. We also provide the necessary ingredients for a potential experimental implementation.\\[4pt] [1] G. Watanabe and H. M\"akel\"a, Phys.\ Rev.\ A {\bf 85}, 023604 (2012).\\[0pt] [2] R.~C.~F. Caballar, S. Diehl, H. M\"akel\"a, M. Oberthaler, and G. Watanabe, Phys.\ Rev.\ A {\bf 89}, 013620 (2014).   

Generation of Planar squeezing in a cold atomic ensemble

We report on an experiment designed to squeeze simultaneously two components of the collective atomic spin of an atomic ensemble via stroboscopic quantum non-demolition (QND) measurements [G. Puentes, G. Colangelo, R. J. Sewell, M. W. Mitchell, New J, Physics 15 103031 (2013)]. We work with an ensemble of one million 87 Rb atoms, cooled in the F $=$ 1 ground state and held in a weakly focused single beam optical dipole trap. We probe the atoms with us pulses of linearly polarised off-resonant light on the D2 line, detected by a shot-noise limited polarimeter. To produce a PQS, we apply a magnetic field By to coherently rotate an initially Fx polarized coherent spin state in the x; z plane, and semi-continously probe the spins. This allows us to successively measure and squeeze the Fz and Fx components of the atomic spin, while maintaining a large spin polarization in the Fx - Fz plane.   

Polaron Properties of a Fermi Impurity in a Dipolar Condensate

Motivated by recent experimental advancement in achieving dipolar quantum gases, we consider a polaronic model in which impurity fermions interact with background bosons in a dipolar Bose-Einstein condensate (BEC). The polaron phenomenon in such a model arises from the coupling between impurities and phonons of the dipolar condensate, which, due to the competition between the attractive and repulsive part of the dipole-dipole interaction, obey an anisotropic dispersion spectrum. We use an effective self-energy on the mass shell to investigate how this anisotropy affects the Cerenkov radiation of Bogolubov phonon modes, which can be directly verified by experiments in which a dipolar BEC moves against an obstacle. We characterize the polarons using Fermi liquid theory and study the spectral function and radio-frequency (rf) spectroscopy of the impurity fermions, which are directly accessible to the rf spectroscopy experiments in cold atoms.   

Radio frequency spectroscopy of polarons in ultracold Bose gases

Recent experimental advances enabled the realization of mobile impurities immersed in a Bose-Einstein condensate (BEC) of ultracold atoms. We consider impurities with two or more internal hyperfine states, and study their radio-frequency (RF) absorption spectra, which correspond to transitions between two different hyperfine states. We calculate RF spectra for the case when one of the hyperfine states involved interacts with the BEC, while the other state is non-interacting, by performing a non-perturbative resummation of the probabilities of exciting different numbers of phonon modes. We discuss both the direct RF measurement, in which the impurity is initially in an interacting state, and the inverse RF measurement, in which the impurity is initially in a non-interacting state. In the latter case, in order to calculate the RF spectrum, we solve the problem of polaron formation: a mobile impurity dynamically gets dressed by Bogoliubov phonons, using a time-dependent variational ansatz of coherent states.