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Journal of the Optical Society of America B

Rydberg excitons in Cu 2 O: exaggerated quantum optical properties and emerging quantum phenomena [Invited]

Na Young Kim

Published Jan 29, 2026• 2 citations
Publisher

Abstract

Over the past decade, Rydberg excitons in bulk Cu 2 O have emerged as a solid-state analog to atomic Rydberg systems, exhibiting exaggerated quantum optical properties including large dipole moments, strong dipole–dipole interactions, and enhanced nonlinear susceptibilities. High-resolution absorption spectroscopy has revealed elegant scaling laws with principal quantum number across temperatures from tens of mK to above 100 K. While many features follow a hydrogen-like model, non-hydrogenic effects arise from complex bandstructures and many-body interactions. This review summarizes experimental advances in accessing high- n Rydberg states and quantum phenomena arising from diverse exciton interaction mechanisms. It also discusses prospective quantum and nonlinear optical applications.

DOI
10.1364/josab.583541

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Over the past decade, Rydberg excitons in bulk Cu 2 O have emerged as a solid-state analog to atomic Rydberg systems, exhibiting exaggerated quantum optical properties including large dipole moments, strong dipole–dipole interactions, and enhanced nonlinear susceptibilities. High-resolution absorption spectroscopy has revealed elegant scaling laws with principal quantum number across temperatures from tens of mK to above 100 K. While many features follow a hydrogen-like model, non-hydrogenic effects arise from complex bandstructures and many-body interactions. This review summarizes experimental advances in accessing high- n Rydberg states and quantum phenomena arising from diverse exciton interaction mechanisms. It also discusses prospective quantum and nonlinear optical applications.
Abstract Rydberg excitons (analogues of Rydberg atoms in condensed matter systems) are highly excited bound electron-hole states with large Bohr radii. The interaction between them as well as exciton coupling to light may lead to strong optical nonlinearity, with applications in sensing and quantum information processing. Here, we achieve strong effective photon–photon interactions (Kerr-like optical nonlinearity) via the Rydberg blockade phenomenon and the hybridisation of excitons and photons forming polaritons in a Cu2O-filled microresonator. Under pulsed resonant excitation polariton resonance frequencies are renormalised due to the reduction of the photon-exciton coupling with increasing exciton density. Theoretical analysis shows that the Rydberg blockade plays a major role in the experimentally observed scaling of the polariton nonlinearity coefficient as ∝ n4.4±1.8 for principal quantum numbers up to n = 7. Such high principal quantum numbers studied in a polariton system for the first time are essential for realisation of high Rydberg optical nonlinearities, which paves the way towards quantum optical applications and fundamental studies of strongly correlated photonic (polaritonic) states in a solid state system.
Rydberg excitons are semiconductor analogues to Rydberg atoms, where one electron is promoted to an energy level of large principal quantum number η and which behave in a manner similar to hydrogen. Their huge spatial extent results in giant dipole moments and interaction effects, which can be used to create nonlinearities at the single excitation level. In contrast to hydrogen, the effective masses and Rydberg energies involved are moderately small, so that in contrast to Rydberg atoms the high field limit of Rydberg physics can be studied using fields strengths that can be realized in the lab. Here we investigate the effects of external magnetic fields of up to 7T on Rydberg excitons both in Faraday and Voigt geometry. In both cases complicated splitting patterns emerge. We investigate the differences between the two geometries and highlight spectroscopic features that are especially easy to access using them. We show that the large number of resonances in the spectrum renders a microscopic treatment of each individual resonance implausible. We instead demonstrate general effects introduced by the field like avoided crossings and discuss alternative approaches to the level structure in terms of collective descriptions.
Abstract Within the last few years, Rydberg excitons, bound electron–hole pairs in highly excited states, have emerged as a promising technology platform for quantum nonlinear optics, quantum information processing, and quantum sensing. The advanced device designs and sensing concepts in these fields require strong nonlinearities at the few-photon or few-carrier level. Rydberg states offer the required strong nonlinearities as the relevant physical quantities at the heart of such nonlinear effects scale strongly with the principal quantum number, n, of the excited state: For example, their polarizability scales as n7n^7n7 n 7 , resulting in an enormous sensitivity to external fields. We review recent experimental and theoretical results that pave the way toward quantum sensing of the electric fields originating from static charge carriers and strongly diluted electron–hole plasmas. We also discuss the strong nonlinear optical properties of Rydberg excitons and how they could be utilized in terms of sensing. Graphical abstract
Rydberg excitons are, with their ultrastrong mutual interactions, giant optical nonlinearities, and very high sensitivity to external fields, promising for applications in quantum sensing and nonlinear optics at the single-photon level. To design quantum applications it is necessary to know how Rydberg excitons and other excited states relax to lower-lying exciton states. Here, we present photoluminescence excitation spectroscopy as a method to probe transition probabilities from various excitonic states in cuprous oxide. We show giant Rydberg excitons at T=38T=38T=38 mK with principal quantum numbers up to n=30n=30n=30, corresponding to a calculated diameter of $3\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$.
Microwave–optical conversion is key to future networks of quantum devices, such as those based on superconducting technology. Conversion at the single quantum level requires strong nonlinearity, high bandwidth, and compatibility with a millikelvin environment. A large nonlinearity is observed in Rydberg atoms, but combining atomic gases with dilution refrigerators is technically challenging. Here, we demonstrate a strong microwave–optical nonlinearity in a cryogenic, solid-state system by exploiting Rydberg states of excitons in Cu2O. We measure a microwave–optical cross-Kerr coefficient of B0 = 0.022 ± 0.008 m V−2 at 4 K, which is several orders of magnitude larger than other solid-state systems. The results are in quantitative agreement with a nonlinear susceptibility model based on the giant microwave dipole moment between nearby excitonic states. Our results highlight the potential of Rydberg excitons for nonlinear optics and form the basis for a microwave–optical frequency converter based on Cu2O.
Rydberg atoms have emerged as a versatile platform for quantum optics due to their exaggerated properties, particularly their strong long-range interactions, which enable a new regime of light-matter interaction. By mapping the interactions between Rydberg atoms onto photons, effective photon-photon interactions can be realized at the single-photon level, thereby overcoming the intrinsic weakness of conventional optical nonlinearities. In this review, we first introduce the fundamental physical principles of Rydberg-mediated quantum optics, and then discuss some key developments, including single-photon engineering, photonic quantum gates, contactless nonlinear optics, and quantum entanglement, providing a comprehensive overview of the current state and prospects of this rapidly developing field.