Session D10: Jin Award for Best Ph.D. Thesis

Photon Thermalization in Driven Open Quantum Systems

Photons are the most accessible massless particles known. However, their lack of mass and extremely weak interactions mean that typically the thermal description of light is that of blackbody radiation. While lasers provide a severe example of a nonequilibrium problem of light, recent interests in the near-equilibrium physics of so-called photon gases, such as in Bose condensation of light or in attempts to make photonic quantum simulators, suggest one re-examine near-equilibrium cases. Here we consider peculiar driven open quantum system scenarios where near-equilibrium dynamics can lead to equilibration of photons with a finite number, following a thermal description closer to that of an ideal gas than to blackbody radiation. Specifically, we show how laser cooling of a well-isolated mechanical mode or atomic motion can provide an effective bath which enables control of both the chemical potential and the temperature of light. We then theoretically demonstrate that Bose condensation of photons can be realized by cooling an ensemble of two-level atoms inside a cavity. This engineered chemical potential for light admits future applications in many-body quantum simulations and enables an analogous voltage bias for photonic circuit elements.   

Laser Cooling and Inelastic Collisions of the Polyatomic Radical SrOH

Qualitatively new vibrational and rotational motions present in complex molecules with three and more atoms offer unique opportunities for advancing various research areas within physics, chemistry and quantum technology. We have identified a large class of polyatomic molecules, alkaline earth monoalkoxides (MOR), with diverse constituents and geometries that have strong optical transitions and diagonal Franck-Condon factors, enabling photon cycling. I will describe our experimental results on direct Doppler and Sisyphus laser cooling of the linear triatomic radical strontium monohydroxide (SrOH). Furthermore, using the bichromatic standing-wave laser field, we achieved significant force enhancement compared to radiation pressure, paving the way for rapid coherent deceleration of molecular beams. In addition to controlling the external motion, laser light was used for preparing SrOH molecules in a specific quantum state in order to study state-selective inelastic collisions with cryogenic helium gas. Finally, I will outline the prospects of using laser-cooled polyatomic molecules to probe physics beyond the Standard Model via permanent electric dipole moment search or spectroscopic effects arising from interactions with ultralight bosonic dark matter.   

Indistinguishability, coherence and entanglement of atom-like systems in nanophotonic devices

Atom-like systems strongly interacting with single photons form the basis for quantum networks and enable optical nonlinearities at the single-photon level. Realizing such systems in solid-state materials is attractive for scalability and high-bandwidth operation, but is complicated by environmental dynamics that degrade optical coherence. In this talk, I will describe a platform for addressing this challenge using silicon-vacancy (SiV) color centers in diamond nanophotonic devices. The inversion symmetry of the SiV center leads to a first-order insensitivity of its optical transitions to dominant environmental dephasing mechanisms. This property results in narrow optical transitions and enables high-cooperativity atom-photon interactions for a single SiV coupled to a nanocavity. Using this platform, we demonstrate quantum nonlinear cavity transmission controlled by a single color center. Detuning the cavity from a pair of SiV centers leads to photon-mediated interactions and spectrally resolved superradiant and subradiant states. Finally, we show that the detection of an indistinguishable photon from a pair of spatially separated SiV centers in a waveguide leads to superradiant emission of two entangled SiV centers. These results open up new opportunities for realizing quantum optical experiments in new regimes using nanoscale devices.   

Highly excited states of small molecules and negative atomic ions

The interactions of electrons with other quantum particles, despite over a century of intense study, still yield rich physical surprises today. In this talk I will discuss some of these surprises as revealed by electron-atom and electron-electron interactions in two differentsystems: Long-range Rydberg molecules and atomic negative ions. A Rydberg atom bonds to a ground state atom to form a molecule colloquially called a ``trilobite.'' I will show how the intricate details of the electron-atom interaction, including the diverse spin couplings present in alkali atoms, manifest themselves in the molecular properties probed in spectroscopy. Another line of research focuses on the formation of polymers, in both symmetric and disordered atomic environments, and on the strong dipolar interaction between Rydberg dimers. In atoms with two active electrons the Rydberg electron may interact strongly and become correlated with the other valence electron. We exploit this to form molecules with favorable properties. This correlation is also critical in explaining doubly excited resonances observed in a different two-active-electron process: negative ion photodetachment. We show that the unexpected threshold laws observed in recent experiments stem from the correlated electronic behavior.* *This work was supervised by and performed in collaboration with Prof. Chris H. Greene at Purdue University, West Lafayette, IN, USA.