Seminars

Title of seminar: Wave turbulence in Bose-Einstein condensates

Abstract:

Wave turbulence refers to a state of a continuous medium that is populated by random, mutually interacting waves spanning a broad range of scales. When these waves are only weakly nonlinear, we can turn to WT theory, a powerful mathematical framework that statistically characterizes such turbulent wave fields.

In this talk, I will focus on how WT theory has been applied and further developed to describe density waves in Bose-Einstein condensates, covering both 4-wave and 3-wave interaction regimes. This allows us to derive universal scaling solutions, including steady-state spectra known as the Kolmogorov–Zakharov (KZ) spectra and self-similar evolutions. All these theoretical predictions have been tested and confirmed by direct numerical simulations of the nonlinear Schrödinger equation (the mean-field description of a BEC) as well as by simulations of the ascociated wave-kinetic equations derived by WT thoery.

I will also discuss the current state of experiments relevant to turbulent BECs. In particular, I will share insights from our latest results on the “equation of state” for steady turbulent cascades in Bose-Einstein condensates—recently reported in [Dora et al. Nature 620, 521 (2023)].

Title of seminar: Metrology with hot atomic vapors

Abstract:

Despite being more than 50 years old, the field of metrology with hot atomic vapors is still receiving a lot of attention. Vapor cells remain pivotal in numerous fundamental precision physics experiments. Over the past 15 years, the advent of miniaturized cells and lasers have enabled the development of quantum sensors boasting excep;onal precision and minimal power consumption. This innovation has led to a myriad of practical applications. The rapid progress in laser technologies, integrated photonics, and material science has further enhanced these sensors while also paving the way for new technologies essential to quantum optics. The experimental simplicity, robustness, and fundamental properties of hot atomic systems hold significant promise for realizing real-world quantum devices. In this seminar, I will present my contributions to this expansive field. These include the spectroscopy of hot vapors at the nanoscale, the development of optically pumped magnetometers and their
applications to the detection of dark matter and rotation sensing, as well as the development of miniaturized optical clocks. The presentation will culminate in a discussion on the emerging applications of hot vapors in the realm of quantum optics.

Title of seminar: Shear flow and vortex array instabilities in annular strongly-correlated atomic superfluids

Title of seminar: Onset of continuous cavity-mediated collective phenomena in an atomic beam

Abstract:

An atom-cavity system is a simple yet rich open quantum system.

By adjusting the characteristic rate of each individual component, the nature of the interplay can be manipulated. Here, we present the case where the dissipation rate of a single mode of an optical cavity is larger than the decay rate of the gain medium. In this scenario, photons do not remain in the cavity mode for long, so the cavity field is primarily determined by the photons emitted and absorbed by the atomic gain medium. Contributions to the atomic decay rate include spontaneous decay into vacuum modes as well as inhomogeneous and homogeneous broadenings. This regime is often referred to as the ‘bad-cavity regime.

We demonstrate, theoretically and experimentally, that a large ensemble of atoms can drive this coupled system into the ‘strong-coupling regime’, characterized by a non-linear response to an external drive. We argue that ­­this regime arises not from strong single atom-cavity coupling, but from the collective weak coupling of many atoms to a single cavity mode. For an inverted sample, we will show that this regime results in optical amplification of the external drive.

The experimental system consists of a beam of Sr atoms. The beam has a large vertical velocity, resulting in a short transit time for atoms passing through the cavity mode, while being cooled in the other two directions. The velocity along the cavity axis is particularly important as it determines the atom-cavity interaction phase. We show how to prepare an ensemble of either ground or excited state atoms that exhibit collective effects.

Title of seminar: Probing supersolidity through excitations in a spin-orbit-coupled Bose-Einstein condensate

Abstract:

Spin-orbit-coupled Bose-Einstein condensates are a flexible experimental platform to engineer synthetic quantum many-body systems. In particular, they host the so-called stripe phase, an instance of a supersolid state of matter. The peculiar excitation spectrum of the stripe phase, a definite footprint of its supersolidity, has so far remained out of experimental reach. We achieve in situ imaging of the stripes and directly observe both superfluid and crystal excitations. We investigate superfluid hydrodynamics and reveal a stripe compression mode, thus demonstrating that the system possesses a compressible crystalline structure. Through the frequency softening of this mode, we locate the supersolid transition point. Our results establish spin-orbit-coupled supersolids as a platform of choice to investigate supersolidity and its rich dynamics.

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.