Chiral Quantum Optics: Recent Developments and Future Directions

By Samudrapom DamReviewed by Louis CastelAug 6 2025

A paper recently published in the journal PRX Quantum reviewed platforms for chiral light-matter interfaces and highlighted the challenges to harness their full potential.

Image Credit: luchschenF/Shutterstock.com

Introduction to Chiral Quantum Optics

Chiral quantum optics is an emerging field of research focused on matter-light interactions that depend asymmetrically on the spin and momentum of particles. This asymmetry opens up new possibilities for controlling electronic and photonic degrees of freedom. Two key concepts are essential for designing and understanding chiral effects: time-reversal symmetry (TRS) breaking and photonic spin-momentum locking.

The electric field divergence is zero in the absence of charge according to Maxwell’s equations, which imposes limitations on the polarization of a propagating optical field in the +z direction. These limitations lead to photonic spin-momentum locking, where the light's direction of travel is directly tied to its transverse spin, or polarization.

Maxwell's equations are typically time-reversal invariant, which implies that if an electromagnetic field is a valid solution, then its time-reversed counterpart is also a solution. However, this symmetry can be broken for an isolated local system by factors like an external magnetic field or an external drive. TRS breaking and chirality are closely linked to the rise of nonreciprocal behavior.

The chiral light-matter interaction investigating platforms have recently expanded from quantum dots (QDs) and laser-cooled atoms to different solid-state systems like two-dimensional (2D) layered materials and microcavity polaritons. They are often integrated into photonic structures like cavities, ring resonators, and waveguides. These platforms can be categorized into active materials and photonic devices.

Active Materials

Active materials are interfaced with diverse photonic architectures to realize chiral light-matter interaction.

Microcavity Polaritons: These serve as powerful tools in chiral physics, where magnetic fields are used to break time-reversal symmetry (TRS), enabling chiral light-matter coupling. This has led to the development of a polaritonic topological insulator and a system with spin-selective coupling. Chiral properties can originate from the matter component of the polaritons, or from the photonic component through the optical spin Hall effect.

However, two main challenges must be overcome for broader applications, including the need for ultralow operating temperatures and strong magnetic fields. Researchers are exploring novel materials and using magnetic substrates to address these issues. Although the small topological band gaps in these structures limit practical use, improvements in fabrication techniques can enhance cavity quality and light-matter coupling, leading to more accessible chiral edge modes.

Transition-metal Dichalcogenides (TMDs): The strong magnetic properties of excitons in TMDs make them excellent for developing devices with chiral features. Their large magnetic factor allows for spin-selective light-matter coupling, enabling the creation of chiral nanocavities and the generation of quantum light. This chirality arises from a magnetic field that breaks TRS, lifting the energy degeneracy between opposite-spin transitions.

However, the chiral response is highly sensitive to the emitter’s precise position within the photonic structure, highlighting the need for more carefully engineered helical modes. The coupling between TMD excitons and photonic waveguides is also weak due to the limited electric fields. Other downsides include small high-quality areas, inhomogeneous broadening, and the dependence of excitonic resonances on the surrounding materials.

QDs: QDs are well-suited for chiral light-matter interactions thanks to their compact size, atom-like energy levels, and ability to emit high-purity single photons. They integrate easily into photonic devices, and their chiral coupling has been demonstrated across several platforms, including standard photonic crystal (PhC) waveguides as well as topological spin and valley Hall PhCs. However, the efficiency of chiral coupling is highly sensitive to the emitter's position due to spatial variations in the guided modes' polarization. Additionally, coupling efficiency drops off near the waveguide’s band edge, precisely where desirable effects like slow light and Purcell enhancement are typically strongest.

Photonic Devices

0D Open Cavities: Zero-dimensional (0D) optical nanocavities like open Bragg mirror cavities with tunable curved mirrors enable chiral light-matter interaction using TMDs/QDs. Future directions include leveraging the magnetic proximity effect to eliminate large magnetic fields and improving cavity quality factors and mode-emitter overlap for enhanced chiral emission control.

Photonic Waveguides: Photonic waveguides are a popular platform for studying chiral light-matter interactions. These waveguides, including conventional line-defect PhCs, optical fibers, and nanobeam ridge waveguides, guide light in two opposite directions. Researchers are actively working to integrate materials like TMDs, QDs, and polaritons into these waveguides to create one-dimensional (1D) chiral systems.

Despite their potential, these 1D waveguides face challenges. The efficiency of chiral coupling is highly dependent on the emitter's position, making it difficult to scale up the systems for many-body applications. Additionally, topological PhC waveguides face an extra hurdle in designing structures with distinct, non-degenerate chiral modes.

Ring Resonators: Photonic ring resonators like race-track rings and disks are a common platform for chiral quantum optics. They support clockwise and counterclockwise light propagation. By applying an external magnetic field to lift the degeneracy of the optical transitions, the coupling becomes chiral, allowing emitters to selectively couple to either the clockwise or counterclockwise modes. However, position-dependent coupling efficiency remains a big challenge, while limited Purcell enhancement currently hinders the study of many-body chiral phenomena.

2D Cavities: 2D cavities formed by parallel mirrors like distributed Bragg reflectors (DBRs)/metallic surfaces have recently been developed for chiral light-matter interaction. In this platform, 2D confinement is enabled by a pair of parallel mirrors. These cavities can also be formed using highly reflective 2D materials like TMD monolayers. By applying an external magnetic field, the TRS is broken for embedded emitters like TMDs, QDs, or quantum wells, leading to the degeneracy lifting between circularly polarized optical transitions. This results in a spin-selective light-matter interaction between cavity modes and the emitters.

Future challenges for extensive application include increasing the area of high-quality chiral modes in TMD cavities and reducing the linewidth of TMD excitons to mitigate the need for strong magnetic fields. Ongoing improvements in fabrication techniques like nanosqueegee and chemical vapor deposition are addressing these issues.

Conclusion

To summarize, chiral quantum optics has advanced rapidly with the emergence of novel experimental platforms, enabling exploration of new interaction regimes and coupling with quantum degrees of freedom. Future outlook includes subwavelength arrays and extending concepts to other frequency regimes like microwaves to overcome current limitations of optical systems.

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