Research Directions
Our experimental and theoretical research in quantum optics spans a wide spectrum, from fundamental quantum photonics to applied optical engineering. We investigate novel regimes of nonlinear quantum optics, including strong photon-photon interactions, nonlinear and collective atom-light interactions, open quantum system dynamics, and the application of optical neural networks for AI. Explore this page to learn more about each of our projects!
Rydberg Excitons
Creating isolated, two-level quantum systems (qubits) that can be coupled to perform gate operations or study many-body quantum systems is one of the outstanding goals in quantum information science. A highly excited atom whose electron has moved to a large principal quantum number is known as Rydberg atom. In cold-atom systems, Rydberg states have been used to create and coherently control non-classical states of light and atoms. Due to the large extent of the electron wavefunction hence the giant size of the Rydberg atoms, their dipole-dipole, and van-der-Waals interactions are very strong. Despite the tremendous success of cold Rydberg atoms in exploring various aspects of fundamental as well as quantum-information science, their integration and scalability have remained elusive so far, due to the demand for bulky laser cooling schemes. Semiconductor and solid-state systems on the other hand are ideal candidates for scalable quantum technologies. Silicon, the essential ingredient of transistors and electronic logic gates, was the backbone of the computer technology revolution half a century ago. So we are asking ourselves; " Is there a way of combining the strength of Rydberg atoms with the benefits of solid-state system scalability?"
In our lab, we are studying Rydberg physics in solid-state platforms. Exciton, aka the Hydrogen atom of a condensed matter physicist, is an electron-hole pair bounded by the Coulomb attraction. We are interested in studying the quantum optical properties of excitons excited to a state with a high principal quantum number (n>10). The size of a Rydberg exciton wavefunction grows as n2, leading to a very large dipole moment and exciton-exciton interaction energy. If the interaction is strong enough, one cannot have more than one excitation within a finite volume around each Rydberg exciton. This phenomenon known as " Rydberg blockade", depicted in the figure, is a unique powerful feature of Rydberg atoms.
Our lab at Purdue University not only strives to explore the fundamentals of quantum optics in these highly-interacting photon ensembles but also is determined to employ these findings towards scalable, chip-scale quantum technologies. Rydberg exciton-polaritons offer a promising combination of such new capabilities afforded by the strong interactions between Rydberg states with the technological advantages of semiconductor photonics.
Single-photon Generation: Materials, Techniques, and the Rydberg Exciton Frontier, A. Keni, K. Barua, K. Heshami, A. Javadi, and H. Alaeian [https://arxiv.org/abs/2412.01573]
Bottom-up Fabrication of 2D Rydberg Exciton Arrays in Cuprous Oxide, K. Barua, S. Peana, A. Deepak Keni, V. Mkhitaryan, V. Shalaev, Y. P. Chen, A. Boltasseva, and H. Alaeian [https://arxiv.org/abs/2408.03880]
Collective Quantum Effects
The radiative environment of an atom is profoundly affected by the presence and states of nearby atoms, which collectively interact with light. In such systems, the internal states of neighboring atoms evolve dynamically, altering the ensemble's radiative properties. For a fully inverted ensemble of atoms confined to a single spatial location, these interactions give rise to a brief, intense burst of light. Unlike the typical exponential decay observed in independent atoms, this emission displays an initial increase in intensity. This phenomenon, known as Dicke superradiance, emerges from the synchronization of atomic states during decay, resulting in a coherent phase-locking mechanism that accelerates the emission rate. Superradiant bursts have been experimentally observed in dense, disordered atomic systems and within optical cavities, where the confined light field effectively replicates the spatial overlap of atoms in a single location.
In our group, we explore the emergence and control of collective quantum effects in ensembles of quantum emitters through both theoretical and experimental approaches. Our research focuses on understanding how dimensionality, Markovianity, and nonlinearity influence these phenomena.
Dissiptive Phase Transitions in the Two-Photon Dicke Model, A. J. Shah, P. Kirton, S. felicetti, and H. Alaeian [https://arxiv.org/abs/2412.14271]
All-Optical Neural Networks
All-optical implementations of deep neural networks (DNNs) offer significant advantages over traditional electronic architectures across various machine learning (ML) applications. Recent studies suggest that optical implementations could reduce energy consumption by 2–3 orders of magnitude compared to state-of-the-art CMOS technologies. Furthermore, complex-valued DNNs have demonstrated numerous benefits over their real-valued counterparts. Despite this progress, all-optical neural networks (AONNs) face challenges related to scalability and programmability at both device and system levels. Without a well-defined design framework to address these issues synergistically, current efforts fall short of achieving a transformative impact, such as optical "supremacy" over electronic computing.
In our lab, we aim to overcome these challenges by developing energy-efficient, large-scale AONNs leveraging the atomic physics toolbox.