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A tunable chiral light–matter interface with on-chip spin control

This paper demonstrates a tunable, on-chip chiral light–matter interface where an external magnetic field controls the coupling between a quantum emitter and a photonic-crystal waveguide to achieve near-unity directional emission and coherent spin control, thereby enabling a robust protocol for generating high-fidelity remote spin–spin entanglement without requiring specialized waveguide polarization engineering.

Original authors: Shikai Liu, Joan Alba, Bálint Sárközi, Nikolai Bart, Arne Ludwig, Ming Lai Chan, Peter Lodahl, Anders Sørensen

Published 2026-09-09

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CC BY 4.0

Original authors: Shikai Liu, Joan Alba, Bálint Sárközi, Nikolai Bart, Arne Ludwig, Ming Lai Chan, Peter Lodahl, Anders Sørensen

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

In the quest to build a future internet powered by quantum mechanics, scientists are learning to harness the strange behavior of light and matter at the smallest scales. A key idea in this field is "chirality," a property where an object or interaction has a distinct handedness, much like a left hand cannot be perfectly superimposed on a right hand. In the world of light traveling through tiny channels, chirality means that the direction a photon moves is locked to its spin, or the way it twists as it travels. If a photon is spinning one way, it can only move forward; if it spins the other way, it can only move backward. This one-way traffic is incredibly valuable for building quantum networks, as it allows information to be routed without getting lost or bouncing back to interfere with itself. However, creating a perfect one-way street for light inside a computer chip has been difficult because the materials used often let light twist in imperfect ways, causing some photons to leak in the wrong direction.

A team of researchers has now found a way to fix this problem without needing to redesign the physical chip itself. Instead of trying to carve a perfect path into the material, they used an external magnetic field to tune the behavior of the light-emitting atoms inside the chip. By carefully adjusting the angle and strength of this magnetic field, they were able to force the light to flow in a single, chosen direction with near-perfect efficiency. This breakthrough turns a standard, imperfect chip into a highly precise tool for controlling quantum information, opening a flexible path toward building large-scale quantum networks that can connect distant computers.

The researchers worked with a tiny device known as a photonic-crystal waveguide, which is essentially a microscopic highway for light etched into a semiconductor. Inside this highway sits a single quantum dot, a nanoscale speck of material that acts like an artificial atom. This quantum dot contains a negatively charged exciton, a state where an electron and a "hole" (the absence of an electron) are bound together, forming a four-level system that can interact with light. In a typical setup, the light traveling through the waveguide is not perfectly circularly polarized; it is slightly elliptical, meaning it twists in a way that isn't perfectly round. This imperfection usually causes the light to leak in both directions, ruining the one-way effect.

To solve this, the team applied a magnetic field to the chip. They discovered that by tilting the magnetic field at a specific oblique angle, they could change the way the quantum dot's internal energy levels interact with the light. This adjustment effectively rotated the orientation of the light-emitting dipoles within the dot. Imagine the light-emitting parts of the atom as tiny antennas; the magnetic field allowed the scientists to turn these antennas until they were perfectly aligned to send light only down the waveguide in one direction, while canceling out any emission in the opposite direction. Even though the waveguide itself was not perfectly designed for this task, the magnetic tuning compensated for the flaw. The result was a directional emission efficiency of 99 percent, with only a tiny fraction of light going the wrong way.

Beyond simply directing the light, the researchers showed that this magnetic control could also tune how the system behaves in more complex ways. They observed that the magnetic field could create a strong preference for one type of light emission over another, a property they call a "chiral branching ratio." In one specific configuration, they achieved a ratio where one type of transition was 134 times more likely to happen than another. This is significant because it allows scientists to control not just where the light goes, but also which internal state of the electron is involved in the process. This level of control is essential for creating "cycling" systems, where an electron can be excited and emit light repeatedly without changing its fundamental state, a requirement for reading and writing quantum information reliably.

The team also demonstrated that this setup could be used to control the spin of an electron directly using light traveling through the waveguide. By using a process called Raman scattering, where two photons interact to flip the spin of an electron, they were able to rotate the electron's state with high precision. This is a crucial step for quantum computing, as it proves that information can be manipulated on the chip without needing complex external wiring for every single operation. The magnetic field allowed them to optimize this process, ensuring that the light drove the spin changes efficiently while minimizing unwanted noise or errors.

Finally, the researchers proposed a method to use this technology to create entanglement between two distant quantum computers. Entanglement is a phenomenon where two particles become linked so that the state of one instantly influences the other, regardless of distance. Their protocol involves sending light from two separate chips into a central beam splitter. Because the magnetic field ensures that the light travels in only one direction with high efficiency, the system can detect a single photon and, based on that detection, confirm that the two distant electron spins have become entangled. The paper suggests that this method is much more robust against signal loss than previous approaches, making it a practical candidate for building a real-world quantum internet.

The work highlights a shift in how scientists approach nanophotonic devices. Rather than trying to build a perfect, static structure that is difficult to manufacture, they are using magnetic fields to dynamically reconfigure the device's properties. This approach means that standard, imperfect chips can be upgraded to perform at the highest levels of precision simply by adjusting an external knob. The findings provide a versatile toolkit for enhancing how light and matter interact on a chip, offering a clear and reconfigurable route to building the complex quantum networks needed for the future.

Technical Summary: A Tunable Chiral Light–Matter Interface with On-Chip Spin Control

Problem Statement
Chiral light–matter interactions are a critical resource for realizing quantum networks, non-reciprocal quantum optics, and directional photon routing. While strong spin–momentum locking in nanophotonic structures can enable directional emission, current approaches face significant limitations. Strategies relying on symmetry-engineered structures, such as glide-plane waveguides (GPWs), require precise fabrication to position emitters at specific "chiral points" and often suffer from reduced directionality due to back-reflections and imperfect local circular polarization. Furthermore, many demonstrations focus on neutral excitons, which lack an addressable spin qubit necessary for many quantum networking protocols. There is a need for a reconfigurable interface that achieves near-unity directionality and high optical cyclicity in standard photonic-crystal waveguides (PCWs) without requiring complex geometric engineering or perfect emitter placement, while simultaneously enabling coherent spin control.

Methodology
The authors demonstrate a tunable chiral interface by coupling a negatively charged exciton ( $X^-$ ) in a quantum dot (QD) to a standard W1 line-defect photonic-crystal waveguide. The $X^-$ forms a four-level system with two spin ground states and two trion excited states. The core methodology involves controlling the orientation and strength of an external magnetic field to tune the polarization of the optical transition dipoles relative to the fixed, elliptically polarized guided mode of the waveguide.

The study explores three magnetic field geometries:

Faraday ( $B_z$ ): Aligned with the growth axis, yielding circularly polarized transitions.
Voigt ( $B_x$ ): In-plane, yielding linearly polarized transitions.
Oblique: A combination of $B_x$ and $B_z$ , allowing continuous tuning of dipole polarization from linear to elliptical.

The researchers utilize p-shell excitation to populate the system and employ a scanning Fabry–Pérot cavity for spectral analysis. They measure emission directionality, chiral branching ratios (the directional analogue of optical cyclicity), and perform coherent spin control via waveguide-mediated optical Raman processes. A theoretical model based on the heavy-hole model and Stokes parameters is used to fit the experimental data and reconstruct the local guided-mode polarization.

Key Contributions and Results

Near-Unity Directionality in Standard PCWs: Despite the waveguide exhibiting imperfect circular polarization (Stokes parameter $S_3 \approx 0.72$ ), the authors achieved near-unity directional emission ( $0.99^{+0.01}_{-0.02}$ ) for a selected transition. This was accomplished by optimizing an oblique magnetic field to align an elliptically polarized transition dipole orthogonally to the waveguide mode, effectively canceling emission in the undesired direction.
Tunable Chiral Branching Ratios: The study demonstrated the ability to tune the chiral branching ratios for two $\Lambda$ -systems within the same platform. By varying the magnetic field angle, they achieved a branching ratio of $134^{+\infty}_{-77}$ for one system, representing a significant enhancement over previous reports in similar platforms. This tunability allows for the suppression of spin-flip transitions relative to spin-conserving ones in a specific propagation direction.
Reconstruction of Local Polarization: By analyzing the magnetic-field-dependent spectroscopy, the authors reconstructed the complete polarization environment (Stokes parameters) of the guided mode at the emitter's position, confirming the theoretical model of dipole-waveguide coupling.
Coherent On-Chip Spin Control: Using the oblique magnetic field configuration, the team demonstrated coherent control of the electron spin via an optical Raman process mediated by the waveguide. They observed Rabi oscillations with effective Rabi frequencies up to $\sim 160$ MHz and $\pi$ -pulse fidelities exceeding 90%, utilizing the waveguide's in-plane driving to potentially reduce required optical power and suppress incoherent spin flips.
Remote Entanglement Protocol: The paper proposes a protocol for generating remote spin–spin entanglement using two such interfaces. By leveraging the simultaneous high directionality and high branching ratio, the protocol uses single-photon detection after interference on a beam splitter to herald entanglement. The authors note that this single-photon approach is more robust to photon loss (scaling linearly with efficiency) compared to two-photon heralding schemes.

Significance
The paper establishes that magnetic-field control offers a reconfigurable and generic route to enhancing chiral spin–photon coupling in standard nanophotonic interfaces. Unlike previous approaches that rely on static, geometry-limited designs, this method allows multiple functionalities—directional emission, tunable optical cyclicity, and coherent spin control—to be accessed on a single platform without modifying the device geometry. The authors claim this work provides a versatile toolkit for enhancing chiral coupling in optical nanostructures, paving the way for scalable, chirality-enabled quantum networks based on on-chip spin–photon nodes. The ability to achieve high directionality and branching ratios in a standard W1 waveguide relaxes the stringent fabrication requirements typically associated with chiral quantum optics.

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