Light Storage in Light Cages: A Scalable Platform for Multiplexed Quantum Memories

Quantum memories are essential for photonic quantum technologies, enabling long-distance quantum communication and serving as delay units in quantum computing. Hot atomic vapors using electromagnetically induced transparency provide a simple platform with second-long photon storage capabilities. Light-guiding structures enhance performance, but current hollow-core fiber waveguides face significant limitations in filling time, physical size, fabrication versatility, and large-scale integration potential. In this work, we demonstrate the storage of attenuated coherent light pulses in a cesium (Cs) quantum memory based on a 3D-nanoprinted hollow-core waveguide, known as a light cage (LC), with several hundred nanoseconds of storage times. Leveraging the versatile fabrication process, we successfully integrated multiple LC memories onto a single chip within a Cs vapor cell, achieving consistent performance across all devices. We conducted a detailed investigation into storage efficiency, analyzing memory lifetime and bandwidth. These results represent a significant advancement toward spatially multiplexed quantum memories and have the potential to elevate memory integration to unprecedented levels. We anticipate applications in parallel single-photon synchronization for quantum repeater nodes and photonic quantum computing platforms.

💡 Research Summary

This paper presents a scalable approach to hot‑atomic‑vapor quantum memories by employing three‑dimensional nanoprinted hollow‑core waveguides, termed “light cages” (LCs). Conventional hollow‑core fibers (HCFs) suffer from prohibitively long gas‑filling times (months), limited geometrical flexibility, and poor compatibility with chip‑scale integration, which hinder their use in large‑scale quantum networks. The authors overcome these drawbacks by fabricating LCs directly on silicon substrates using two‑photon polymerization of commercially available photoresins, followed by a thin (100 nm) alumina coating that protects the polymer from reactive cesium (Cs) vapor and fine‑tunes the anti‑resonant guidance to the Cs D1 transition at 894 nm.

The experimental platform consists of a heated (74 °C) Cs vapor cell containing multiple LCs spaced several hundred micrometers apart on a single chip. Two independent diode lasers address the Λ‑system formed by the F = 3 → F′ = 3 (signal) and F = 4 → F′ = 3 (control) transitions of the Cs D1 line. Co‑propagating signal and control beams are coupled into each LC with a measured coupling efficiency of ≈20 %. By varying the control‑beam power from 0 to 40 mW, the authors observe electromagnetically induced transparency (EIT) windows whose transmission approaches unity at 40 mW, with a fitted Rabi frequency Ωc ≈ 232 ± 43 MHz and a transparency bandwidth Δf_EIT ≈ 133 ± 24 MHz.

For storage, weak coherent pulses (average photon number |α|² ≈ 50, temporal width Δt_s ≈ 14 ns) are injected together with a control pulse that first optically pumps the atoms and then maps the optical excitation onto a collective spin wave. After a programmable storage interval t_storage, a third control pulse retrieves the stored light. At a storage time of 52 ns, the internal retrieval efficiency η_int reaches 9.8 %. Notably, the efficiency versus storage time does not follow a simple exponential decay; instead it exhibits damped oscillations attributed to Larmor precession of the spin wave in residual magnetic fields (≈0.2 G from Earth’s field and heating wires). This magnetic‑field‑induced modulation suggests a new avenue for controlling the polarization of stored photons.

The measured 1/e memory lifetime is 83 ± 2 ns, limited primarily by spin‑dephasing from atomic collisions and magnetic‑field inhomogeneities. Increasing the control power compresses the signal pulse inside the LC (compression factor up to 1:143 at 10 mW), but because the LC length is only 5 mm, only a fraction of the compressed pulse is stored, leading to a reduction of η_int with higher control powers for short storage times. Conversely, for longer storage intervals the intrinsic decoherence dominates and the control‑power dependence becomes weaker.

Bandwidth characterization, performed by varying the signal pulse width while keeping the control‑to‑signal width ratio constant (Δt_c = 1.5 Δt_s), yields a –3 dB bandwidth of 35.2 ± 0.6 MHz, defined at the pulse width where η_mem drops by 3 dB. This bandwidth is compatible with many quantum‑communication protocols and demonstrates that the LC memory operates optimally at relatively low control powers, yet can tolerate higher powers without damaging the polymer structure—opening the door to high‑intensity nonlinear experiments within the same waveguide.

A key achievement is the simultaneous integration of multiple LCs with differing geometries (single‑layer and dual‑layer strand designs) on a single chip, each exhibiting consistent EIT spectra, storage lifetimes, and retrieval efficiencies. This spatial multiplexing capability enables parallel synchronization of photons from independent single‑photon sources, a functionality required for quantum repeaters and photonic quantum‑computing architectures that rely on feed‑forward operations and Bell‑state measurements.

Current limitations include modest internal efficiencies (≈10 %) due to the short LC length and limited optical depth of the vapor cell. The authors suggest that extending the LC length, increasing the vapor’s optical depth (e.g., higher temperature, buffer‑gas loading), or employing higher Cs densities could substantially improve performance.

In summary, the work demonstrates that 3D‑nanoprinted light cages provide a versatile, fast‑filling, chip‑compatible platform for hot‑atom quantum memories. Their ability to host multiple, independently addressable memories on a single substrate marks a significant step toward scalable, spatially multiplexed quantum networks. Future research will likely focus on optimizing waveguide geometry for higher optical depth, integrating on‑chip control electronics, and exploiting the observed magnetic‑field‑induced spin dynamics for advanced quantum‑state manipulation.