Chiral quantum optics: recent developments, and future directions

TL;DR

Chiral quantum optics investigates directional light-matter interactions arising from spin-momentum locking and TRS breaking, with non-reciprocity enabling novel quantum dynamics. The paper reviews foundational concepts, surveys solid-state platforms (2D cavities, ring resonators, waveguides, and 0D open cavities) and active media (QDs, TMDs, polaritons), and discusses experimental challenges and strategies for scaling toward many-body regimes. It highlights advances in novel quantum light sources, chiral quantum gates, and emergent polariton and dissipative many-body phases, as well as beyond-dipole interactions via orbital angular momentum. The outlook identifies opportunities for topological, nonlinear, and higher-dimensional chiral photonics, and stresses the need for improved materials, fabrication, and theoretical frameworks to realize truly nonlinear many-body chiral quantum systems.

Abstract

Chiral quantum optics is a growing field of research where light-matter interactions become asymmetrically dependent on momentum and spin, offering novel control over photonic and electronic degrees of freedom. Recently, the platforms for investigating chiral light-matter interactions have expanded from laser-cooled atoms and quantum dots to various solid-state systems, such as microcavity polaritons and two-dimensional layered materials, integrated into photonic structures like waveguides, cavities, and ring resonators. In this perspective, we begin by establishing the foundation for understanding and engineering these chiral light-matter regimes. We review the cutting-edge platforms that have enabled their successful realization in recent years, focusing on solid-state platforms, and discuss the most relevant experimental challenges to fully harness their potential. Finally, we explore the vast opportunities these chiral light-matter interfaces present, particularly their ability to reveal exotic quantum many-body phenomena, such as chiral many-body superradiance and fractional quantum Hall physics.

Figures (6)

• Figure 1: Basic concepts in chiral quantum optics systems. Upper panel: Spin-momentum locking. An electromagnetic mode propagating in the +z direction of a dielectric waveguide (panel a) has a distribution of electric field intensity shown in panel b. For concreteness, the waveguide's height and width are chosen to be $140$ nm and $280$ nm, respectively, and the electric field propagation of wavelength $\lambda\!=\!940$ nm is considered. The polarization of such a mode is determined by a position-dependent superposition of its transversal and longitudinal field components with a relative phase that varies from $0$ to $2\pi$. This leads to regions where the direction of propagation and the circular degree of polarization are locked, as shown in panel c. $S_3\!=\!2\rm{Im}\{E_xE_{y}^*\}/|E|^2$ is the degree of circular polarization for the mode shown in b. In the presence of an external magnetic field that is perpendicular to the propagation direction, the circularly polarized transitions of the emitter are split. In this scenario, spin-momentum locking manifests as directional emission for each transition depending on the emitter's position. (d) For a $\sigma^+$-polarized point source positioned in the upper (lower) part of the waveguide indicated by the dashed circle, the emission is preferentially in the right (left) direction of the waveguide. The emission direction will be reversed for $\sigma^-$-polarized point source (not shown).
• Figure 2: A toy model for achieving non-reciprocal interactions. A pair of modes must simultaneously interact via both coherent tunneling (described by the Hamiltonian in blue) and collective decay (encoded in the dissipator in orange). If the tunneling (or jump operator) is complex, TRS is broken, leading to non-reciprocal dynamics.
• Figure 3: Emerging platforms for chiral light-matter interaction. Rows are associated with photonic architectures harnessed to induce this effect, while the active materials used for the matter component are compiled in the columns. Reprinted from Refs. lobanov2015polarizationlyons2022giantantoniadis2022chirallukin2023twomehrabad2023chiralma2022chipcoles2016chiralityHallett2022Barik2018lodahl2017chiralyang2019chiralshreiner2022electricallyKlembt2018lyons2022giantantoniadis2022chiral
• Figure 4: Regimes of interest in chiral quantum optics. Different recently explored phenomena in chiral quantum optics classified in terms of the operators involved and the number of excitations in the system. The vertical axis corresponds to the operators involved, namely if they are harmonic oscillator, ladder spin, or more generic forms of non-quadratic operators leading to non-Gaussian physics. The horizontal axis corresponds to the number of excitations, discerning between single-, few- and many-body phenomena.
• Figure 5: Potential applications of chiral quantum light-matter interfaces to obtain novel photonic states and interactions. Promising avenues include (a) topological lasing in a microresonator array bandres18a and (b) generation of topological photon pairs through spontaneous four-wave mixing mittal2018topological. Non-Gaussian correlations can be achieved with (c) multi-photon bound states in a platform with two-level systems chirally coupled to a photonic waveguide mahmoodian2020dynamics, (d) spontaneous emission of biexciton states ostfeldt2022 or (e) single-photon states with time-bin entanglement between topological edge modes Vega2023TopologicalQED. (f) Cluster state generation using chiral waveguide coupling to $V$-level emitters pichler17a and sequential driving. (g) Passive photonic phase gate enabled by chiral light-matter couplings to $V$-level emitters schrinski2022passive.