Spectroscopy of Rydberg Excitons for Quantum Information Science

Converting quantum information between distinct frequency regimes of the electromagnetic spectrum, for example, between radio/microwave frequencies and optical frequencies, is known as quantum transduction. It is a key functionality for quantum networking. One approach to implementing quantum transduction uses quantum systems with levels whose energetic spacing matches both frequency regimes. Highly excited atomic states, known as Rydberg states, can be closely spaced, with transition frequencies in the radio/microwave regime, while transitions to low-lying states occur in the optical regime. This project will focus on characterizing Rydberg states in excitons rather than atoms for use in quantum transduction. An exciton is a bound state between an electron and a hole, the lack of an electron, in a semiconductor. Excitons have the advantage of being hosted in solid-state materials and thus more easily integrated with solid-state quantum platforms such as superconducting circuits. However, they also experience decoherence processes, which are deleterious to quantum information applications, due to interactions with the host crystal. Characterizing these processes will be a key outcome of the project. The primary materials to be studied will be cuprous oxide, which has been shown to host Rydberg excitons with principal quantum numbers exceeding 25. Both bulk crystals and thin films will be studied. The primary method will be multidimensional coherent spectroscopy (MDCS), which uses a sequence of ultrafast optical pulses to coherently drive multiple excited Rydberg states. MDCS can separate homogeneous decoherence from inhomogeneous dephasing and characterize the coupling and decoherence of states within a manifold. MDCS pulse sequences can simulate the pathways used in quantum transduction. Both non-colinear and colinear MDCS geometries will be used. In addition to cuprous oxide, excitons in transition metal dichalcogenides will be explored, which have much higher dipole moments, but the observation of Rydberg excitons is limited to a principal quantum number of approximately 5 and much broader linewidths. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.