October 2018

Abstracts of the QSIT Lunch Seminar, Thursday, October 4, 2018

Coupling two order parameters in a quantum gas

Andrea Morales – Quantum Optics Group (Esslinger group), ETH Zurich

Intertwined order is a controversial concept in condensed matter physics. Though its microscopic origin is often unknow, it implies the simultaneous existence of independent order parameters and therefore can allow materials to feature multiple properties. For example, ferroelectric and ferromagnetic orders can coexist in multiferroic materials leading to enhanced functionalities, or in high-temperature superconductors intertwining between charge- and spin-order can form superconducting states at high-transition temperatures.

I will report on our recent experimental realization of an intertwined ordered phase in a quantum gas where we can control the microscopic interaction between the atoms. Our system is realized by a superfluid Bose-Einstein condensate (BEC) that can undergo self-organization phase transition to the modes of two crossed optical cavities. The BEC is illuminated with a transverse pump laser beam whose detuning from atomic resonance can be changed to explore different regimes of interaction.

Far away from the atomic resonance of the D2 line of Rb87 we realize a supersolid phase of matter by symmetry enhancement of the composite order parameter to a U(1) symmetry [1]. Here we observe the simultaneous existence of a Higgs and Goldstone mode [2]. Approaching the atomic resonance this symmetry breaks down to a Z2xZ2, and we observe the emergence of a broad intertwined phase arising from the coupling of the individual order parameters. Intriguingly, we can induce coexistence of the orders even below the critical point of the individual ones. We explain our results with a microscopic Hamiltonian model which is also mapped to a mean-field free energy reproducing the phenomenology of the phases [3].

[1] Nature, 543, 87-90 (2017)
[2] Science, 358, 1415-1418 (2017)
[3] Nature materials, 17, 686-690 (2018)
 

3D Cavity Cooling of a Levitated Nanoparticle

Dominik Windey - Photonics Group (Novotny Group), ETH Zurich

Optomechanics studies the coupling between light and mechanical objects. Well controlled optomechanical systems are promising candidates to observe the quantum to classical transition or could be used as ultrasensitive force sensors due to their high mechanical quality factors Q. The cooling of mechanical systems by optical methods has been demonstrated with various systems such as cantilevers, toroidal cavities, nanobeams or membranes. An advantage of a levitated nanoparticle over many of the above mentioned systems is the nearly perfect decoupling of the nanoparticle from their environment due to the absence of clamping losses.
In our experiment we use a cavity for cooling and for increasing the position detection sensitivity of the levitated nanoparticle. The levitated nanparticle is trapped inside the cavity mode by strongly focussed optical tweezers, which axis is orthogonal to the cavity axis. This configuration allows us -for the first time - to observe three dimensional cavity cooling of the particles center-of-mass motion. Additionally, our small mode volume design of the cavity leads to Purcell enhanced scattering of the particle into the cavity mode which can be used for highly efficient particle position detection and should allow us to further decrease the center-of-mass energy by parametric feedback cooling [1,2].

[1] Gieseler, J. and Deutsch, B. and Quidant, R. and Novotny, L., Subkelvin parametric feedback cooling of a laser-trapped nanoparticle, Phys. Rev. Lett., 2012.
[2] Vijay Jain, Jan Gieseler, Clemens Moritz, Christoph Dellago, Romain Quidant, and Lukas Novotny, Direct Measurement of Photon Recoil from a Levitated Nanoparticle, Phys. Rev. Lett., 2016.