Séminaires

Title of seminar: Quantum jumps in a single atom: application in photon counting and role in open quantum systems

Abstract:

The notion of quantum jumps was first introduced by Niels Bohr in 1913 to describe the abrupt change of state of electrons in atoms. They were experimentally observed for the first time in trapped ions in 1986 using the concept of electron shelving which was used to precisely measure the lifetime of metastable states in ions. In this talk, I will first describe a photon counting technique based on quantum jump detection in a single Rubidium atom which heralds a photon absorption [Phys. Rev. Research 6, 033338 (2024)]. Such a technique is interesting for a growing number of applications that require high sensitivity and strong background rejection through frequency discrimination, for example free space quantum communication in daylight or space classical communications. The quantum efficiency, dark counts and background rejection of this single-atom photodetector were characterized using single-photon level laser photons. With this technique, I will also report on the detection of spontaneous parametric down conversion photons, which enable studies of atom-light interaction at the single quanta level. Finally, I will present a proposal to experimentally distinguish different unravelings of the so-called Lindblad master equation appealing to stochastic conditional dynamics via quantum trajectories [Phys. Rev. Research 6, L032057 (2024)]. Each unraveling of the master equation is known to be linked to a photodetection scheme. We show that it can be unraveled using nonlinear quantum trajectories that correspond to different types of quantum jumps. We focus on the simple case of resonance fluorescence of a single atom in two popular photodetection schemes: the Poisson-type, corresponding to direct detection of the photons scattered from the two-level emitter, and the Wiener-type, revealing complementary attributes of the scattered field such as the wave amplitude and the spectrum.

Title of seminar: An optical lattice clock with a bosonic isotope of mercury

Abstract:

Among other neutral species, mercury has interesting properties for an optical lattice clock such as a low sensitivity to blackbody radiation (16 times lower than Yb, 30 times lower than Sr) and a
high vapour pressure at room temperature. So far, the 199Hg fermionic isotope was the only isotope used in a mercury clock, but its limited lifetime in the excited state will prevent to fully exploit the
new generation of ultrastable lasers to come. Using bosonic isotopes instead is a way to bypass this limit thanks to a potentially unlimited lifetime.

We report the first observation of the ¹⁹⁸Hg bosonic transition in an optical lattice clock, which results from several key experimental developments and a challenging search of a narrow transition in a wide uncertainty span.

The bosonic clock transition is forbidden and needs to be induced thanks to a high magnetic field. This is the so called quenching method¹. It allows longer probing times, adjustable to the laser properties. Hence, a first key step was developing a setup to produce a large enough magnetic field to induce the bosonic transition with the highest coupling achievable. Another challenge was implementing a widely tunable and flexible probe laser while maintaining the ultra-low noise properties, in order to probe any of the mercury isotopes with no additional noise. The coupling is also increasing with the probe power, so a major step was to significantly increase our deep UV ultrastable light power.

Given all these experimental advances, we calculated that we still had a relatively weak coupling and hence a narrow line transition to be found in a large frequency span. We performed various measurements and checks thanks to the ¹⁹⁹Hg isotope, especially a substantial alignment work, to optimize our chances to find the transition. Thanks to cumulated efforts, the search of the 198Hg transition was a success, making it the first observation of a mercury bosonic isotope transition.

We further obtained an operational optical lattice clock with the bosonic ¹⁹⁸Hg, already reaching a stability of 10-15 at 1 second. We have undertaken several studies of this new transition, in particular we measured the quadratic Zeeman shift coefficient with an uncertainty suitable to control this shift to 10^-17 or better. We also started studying other systematic effects such as the probe light shift and measuring the magic wavelength. Finally, we are working towards measuring, for the first time, the ¹⁹⁸Hg/⁸⁷Sr optical frequency ratio.

• ¹ A. V. Taichenachev et al., “Magnetic Field-Induced Spectroscopy of Forbidden Optical Transitions with Application
  to Lattice-Based Optical Atomic Clocks”, Phys. Rev. Lett., vol. 96(8), p.083001, 2006.

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).