Seminars

Title of seminar: Fast compression of a Bose-Einstein condensate and the new experiment for 2D Bose gases

Abstract:

In this seminar, I will first present the latest results on the evolution of a Bose-Einstein condensate after an abrupt compression where our preliminary analysis indicates an enhanced three-body losses.

Next, I will focus on describing a new experimental setup which aims to produce a 2D Bose gas of potassium-39 and explore the dynamics of quantum vortices in two dimensions.

Title of seminar: Mid- and Long-Wave Infrared Optoelectronics for Free-Space Optical Communications

Abstract:

Free-space optics offer an attractive alternative for broadband data transmission when fiber optics are neither practical nor feasible. This technology has emerged as a strong candidate with a wide range of potential applications from everyday broadband internet to satellite links. Today, the availability of high-quality transmitters and detectors operating in the near-infrared window makes the 1.55-micron optical wavelength the natural choice for free-space optical systems. However, the atmospheric transmission spectrum has two other transparency windows that can be considered namely one between 3 and 5 microns and the other between 8 and 11 microns. In this context, midinfrared quantum cascade devices have become prime candidates for data transmission applications, especially in fog or haze, where they significantly outperform near-infrared and visible light sources. In addition, atmospheric turbulence in the propagation path severely degrades the optical signal by causing beam spreading and wandering, scintillation or loss of spatial coherence. In this seminar, I will review our recent results on free-space optical communications, which have shown that quantum cascade lasers can be used as unpredictable light sources for data security, using complex photonic chaos for message encryption and synchronized chaos for message transmission. Between 8 and 11 microns, this technology is even more interesting because the random thermal background radiation of the blackbody further complicates the task of deciphering a potential spy, meaning that communication is even more secure. My presentation will also cover our recent advances in multi-gigabit-per-second room temperature data transfer based on midinfrared quantum optoelectronics. While these results can profoundly revolutionize the future of free-space laser communications, the goal, however, will be to implement this new optoelectronic technology in an actual outdoor testbed, especially to prove the superiority of the mid and long wave infrared domains.

Quantum hydrodynamics and turbulence in atomic BECs

Quantum hydrodynamics and turbulence [1] have been long studied in superfluid helium since 1950’s, and in atomic Bose-Einstein condensates (BECs) too. In this talk, I would discuss the important topics of atomic BECs.

Introduction

We will overview briefly characteristics of quantum hydrodynamics and turbulence compared with those of classical turbulence.

Single-component BECs

Turbulence likes isotropy [2] and rotation causes anisotropy. Such competition of two contradictory effects has been studied in classical turbulence [3]. Rotation in a quantum fluid forms a vortex lattice along the rotational axis and gives other interesting effects. There are not so many experimental works [4-6] on rotating quantum turbulence, and theoretical numerical studies are also limited [7]. I will discuss rotating quantum turbulence of BECs [8].

Two new scenarios for quantum friction: chiral media and rotating molecules

In the first scenario, we study the quantum friction experienced by a polarizable charged particle moving with parallel motion to a chiral medium-vacuum interface. We employ macroscopic quantum electrodynamics ^1,2 to obtain the Casimir–Polder frequency shift and decay rate. These results are a generalization of the respective quantities to matter being insensitive to parity². By examining the non-retarded and retarded limits we find that the an effective optical rotatory strength is relevant for a non-vanishing frequency shift and decay rate. For the second scenario, we investigate the rotational motion of diatomic molecules in free space interacting with the quantum electromagnetic field³. Using macroscopic quantum electrodynamics² we obtain the rotation-dependent decay rates of the molecule. By analyzing the behavior of the resulting rates at zero and finite temperature, we find a connection between the decelerating rotational dynamics and quantum friction.

¹ Stefan Yoshi Buhmann, David T. Butcher and Stefan Scheel. New Journal of Physics 14, 083034 (2012).
² S. Y. Buhmann. Dispersion Forces II. Many-Body Effects, Excited Atoms, Finite Temperature and Quantum Friction. (Springer, Berlin Heidelberg, 2012).
³ Stefan Yoshi Buhmann, M. R. Tarbutt, Stefan Scheel, and E. A. Hinds, Phys. Rev. A 78, 052901 (2008).

Non-stationary quantum many-body dynamics

Rydberg atoms in the vicinity of an optical nanofiber

Connecting atomic quantum registers via photonic channels into a quantum network is a promising way to achieve scalability, one of the crucial challenges in quantum technologies. Free-space scenarios considered so far are relatively easy to implement but suffer the drawback of strong losses. Optical nanowaveguides constitute an interesting alternative which offer strong transverse confinement of the field and hence strong coupling to atomic nodes in their vicinity. In particular optical nanofibers (ONFs) received much attention within the past two decades.

On the other hand, within the past two decades, the giant van der Waals interactions between Rydberg atoms and the associated blockade phenomenon have been key in many quantum information proposals, including atomic quantum registers and repeaters. In free-space, the energy shifts induced by the van der Waals interaction between two atoms prepared in levels of principal quantum numbers n > 50 and a few µm apart can be of the order of tens of GHz and effectively forbid their simultaneous resonant laser excitation. This blockade mechanism was employed to generate entanglement and perform gates in systems of individual atoms and atomic ensembles.

In this seminar, I shall present the theoretical results my collaborators and I obtained on the spontaneous emission and van der Waals interaction of rubidium atoms in the vicinity of a silica ONF [1,2] in view of building a Rydberg-atom-ONF interface. In particular, on the specific cases of S and P states, I shall analyze how the presence of the nanofiber and the symmetry breaking it induces modify the blockade and the anisotropy of the Rydberg-Rydberg interaction.

References

[1] E. Stourm, M. Lepers, J. Robert, S. Nic Chormaic, K. Mølmer, E. Brion, Spontaneous emission and energy shifts of a Rydberg rubidium atom close to an optical nanofiber, Phys. Rev. A 101, 052508 (2020).
[2] E. Stourm, M. Lepers, J. Robert, S. Nic Chormaic, K. Mølmer, E. Brion, Interaction of two Rydberg atoms in the vicinity of an optical nanofibre, New J. Phys. 25, 023022 (2023)