⚛️ quantum physics

Demonstration of traveling-wave interactions between spontaneous photon emissions and atoms in a chiral F-P cavity

This study experimentally demonstrates traveling-wave atom-light interactions within a Fabry-Pérot cavity by applying a bias magnetic field to break time-reversal symmetry and preserve light helicity, thereby overcoming the non-uniform coupling limitations of traditional standing-wave cavities.

Original authors: Jiajin Lu, Minjie Wang, Haole Jiao, Xiang Chen, Hongze Zhang, Shujing Li, Hai Wang

Published 2026-09-01

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CC BY 4.0

Original authors: Jiajin Lu, Minjie Wang, Haole Jiao, Xiang Chen, Hongze Zhang, Shujing Li, Hai Wang

Original paper licensed under CC BY 4.0 (http://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 quiet world of quantum physics, scientists are constantly trying to master the delicate dance between light and matter. They want to trap individual atoms and make them talk to photons, the tiny particles that make up light, to build the future of ultra-secure communication and powerful computers. To do this, they often use mirrors to create a cavity, a small box where light bounces back and forth, forcing it to interact with atoms many times. This amplifies the connection, making the atoms and light behave as a single unit. However, a major hurdle has always been the nature of the light inside these boxes. In standard setups, the light waves moving forward and the waves reflecting backward crash into each other, creating a pattern of peaks and valleys known as a standing wave. This uneven pattern means some atoms get a strong signal while others get a weak one, and it causes the stored information to fade away very quickly, often in less than a microsecond. Researchers have long sought a way to make the light travel in only one direction, like a river flowing downstream, so that every atom feels the same steady push and the information lasts longer.

A team of researchers at Shanxi University in China has now demonstrated a way to achieve this one-way flow inside a standard mirror cavity, a setup that is usually much simpler to build than the complex ring-shaped alternatives they have used before. They worked with a cloud of cold rubidium atoms, cooling them down until they moved very slowly, and placed them between two mirrors. To break the natural symmetry that causes light to bounce back and forth, they applied a magnetic field and inserted two special crystal plates, known as quarter-wave plates, on either side of the atoms. These plates act like filters that change the twist of the light. When an atom emits a photon, the light travels forward with a specific twist, passes through a plate, hits a mirror, and returns. But on its way back, the plate changes the light's twist so that it no longer matches the atom's ability to absorb it. The light effectively becomes invisible to the atom on the return trip.

The result is that the light interacts with the atoms only as it moves in one direction, creating a traveling wave rather than a standing wave. The researchers tested this by generating pairs of light particles and atomic excitations, a process where a "write" laser creates a flash of light and a corresponding ripple in the atoms. They measured how well they could retrieve this stored information later. In a normal cavity, the interference between forward and backward light would scramble the storage, but in their new chiral cavity, the light maintained its direction and the atoms remained in sync. They found that the system successfully preserved the unique properties of the light, allowing the atoms to interact with it in a uniform, traveling-wave manner. This achievement proves that it is possible to create these one-way interactions using simple, flat mirrors and clever optical tricks, rather than complex ring structures. It opens a path toward more stable and efficient quantum memory devices, where information can be stored for longer periods without the distortion caused by light bouncing back and forth.

Technical Summary: Demonstration of Traveling-Wave Interactions in a Chiral F–P Cavity

Problem Statement
While Fabry–Pérot (F–P) cavities are standard platforms for enhancing atom–photon interactions in quantum optics, conventional linear F–P cavities support standing-wave modes. In these systems, forward and backward propagating fields superpose, creating spatially non-uniform atom–photon coupling. This standing-wave nature is particularly detrimental to quantum memory applications, such as Duan–Lukin–Cirac–Zoller (DLCZ) spin-wave storage, where the superposition of write-out (Stokes) photons leads to a significantly shortened storage lifetime (limited to ~0.5 μs in previous F–P implementations). While ring cavities can distinguish scattering directions to enable traveling-wave interactions, they suffer from complex configurations and longer cavity lengths that necessitate slower quantum information processing rates. Consequently, there is a need for a simpler cavity architecture that preserves light helicity and enables traveling-wave interactions within an F–P geometry.

Methodology
The authors experimentally realized a chiral F–P cavity using a cold ensemble of $^{87}$ Rb atoms. The experimental setup modifies a standard F–P cavity (comprised of a high-reflectivity flat mirror and an output coupler) by inserting two quarter-wave plates ( $\lambda/4$ ) on either side of the atomic cloud and a beam displacer (BD) to control resonance conditions.

Key operational steps include:

Symmetry Breaking: A bias magnetic field ( $B_0 \approx 10$ G) is applied along the z-axis to define the quantization axis. This lifts the Zeeman degeneracy and breaks time-reversal symmetry (TRS), distinguishing between $\sigma^-$ and $\sigma^+$ atomic transitions.
State Preparation: Atoms are optically pumped into the specific Zeeman state $|F_g=1, m_F=-1\rangle$ .
DLCZ Scheme: A "write" laser pulse (795 nm, $\sim$ 100 ns duration) induces spontaneous Raman scattering. The experiment focuses on the $\sigma^-$ transition, which generates a Stokes photon non-classically correlated with a spin wave.
Chiral Interaction Mechanism:
- Forward Propagation (+z): Stokes photons emitted in the $\sigma^-$ transition are initially left-hand circularly polarized (LHCP). Upon passing through the first quarter-wave plate, they convert to vertical (V) linear polarization. After reflection by the cavity mirror and a second pass through the quarter-wave plate, they revert to LHCP. Crucially, when the reflected field propagates backward through the atoms, its polarization state (LHCP) does not match the atomic transition required for interaction, preventing coupling.
- Backward Propagation (-z): Stokes photons emitted in the backward direction are right-hand circularly polarized (RHCP). The cavity configuration ensures these fields do not resonate with the specific atomic transition in the same manner as the forward field.
Verification: The team measured the time-dependent retrieval efficiencies of spin waves correlated with forward and backward Stokes fields to confirm the traveling-wave nature of the interaction.

Key Contributions and Results

Realization of Chiral F–P Cavity: The study demonstrates an F–P cavity that preserves light helicity within the atom–photon interaction region, effectively converting the system from a standing-wave to a traveling-wave interaction regime without the complexity of a ring cavity.
Nonreciprocal Coupling: The experiment confirms that Stokes photons emitted from the $\sigma^-$ transition interact with atoms in a traveling-wave manner. The forward-propagating field resonates with the cavity and interacts with the atoms, while the backward-propagating field (after reflection and polarization conversion) is effectively decoupled from the specific atomic transition due to polarization mismatch constraints.
Experimental Confirmation: By measuring retrieval efficiencies, the authors verified that the interaction is nonreciprocal. The forward and backward Stokes fields exhibit distinct behaviors, confirming that the superposition of fields characteristic of standard F–P cavities is suppressed.

Significance
The paper claims that this work paves the way for demonstrating traveling-wave atom–photon interactions in F–P cavities. By overcoming the limitations of standing-wave modes (specifically the short storage lifetime of spin-wave quantum memories) while avoiding the structural complexity and processing speed limitations of ring cavities, this chiral F–P configuration offers a suitable platform for studying quantum optics and atomic physics. The results suggest a path toward achieving spatially uniform coupling and longer storage lifetimes in simpler cavity geometries.

Technical Summary: Demonstration of Traveling-Wave Interactions in a Chiral F–P Cavity

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