🔬 optics

A photonic integrated comb engine for ultracold quantum gases

This paper demonstrates a scalable, fully integrated photonic engine that combines self-injection-locked microcombs, atomic referencing, and power amplification to achieve coherent control of ultracold rubidium quantum gases, thereby enabling the deployment of large-scale atomic quantum systems.

Original authors: Wei Sun, Xiaoying Yan, Jinbao Long, Sanli Huang, Zhixin Duan, Zhenyuan Shang, Zeying Zhong, Jiahao Sun, Yue Hu, Shichang Li, Baoqi Shi, Yi-Han Luo, Shuyi Li, Hao Tan, Chen Shen, Zhichuan Niu, Shengjun

Published 2026-09-24

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CC BY 4.0

Original authors: Wei Sun, Xiaoying Yan, Jinbao Long, Sanli Huang, Zhixin Duan, Zhenyuan Shang, Zeying Zhong, Jiahao Sun, Yue Hu, Shichang Li, Baoqi Shi, Yi-Han Luo, Shuyi Li, Hao Tan, Chen Shen, Zhichuan Niu, Shengjun Yang, Junqiu Liu

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

Cold atoms are the workhorses of a new generation of quantum technology. By cooling clouds of gas to temperatures just above absolute zero, scientists can slow these atoms down enough to make them behave like waves rather than particles. In this frozen state, the atoms become incredibly sensitive tools for measuring time, gravity, and magnetic fields with a precision that was once impossible. They also serve as a platform for simulating complex materials and building quantum computers. However, controlling these delicate atoms requires a very specific kind of light. To keep the atoms stable and to manipulate their quantum states, researchers need lasers that are not only powerful but also perfectly synchronized and incredibly steady. For decades, the equipment needed to generate this light has been bulky, fragile, and expensive, consisting of large tables filled with mirrors, separate lasers, and optical cavities. This size and complexity have kept these powerful tools locked inside specialized laboratories, far from the portable sensors or space-based instruments where they could be most useful.

A team of researchers has now built a solution that shrinks this entire optical system down to the size of a postage stamp. They created a "microcomb engine," a single, integrated device that generates, stabilizes, and amplifies the light needed to control ultracold atoms. Instead of using a collection of separate components, they combined a laser diode with a tiny ring-shaped resonator made of silicon nitride. When the laser light is fed into this microscopic ring, it creates a "frequency comb," which is essentially a ruler made of light. This ruler consists of many distinct colors of light, all perfectly spaced and locked together. The researchers used a technique called self-injection locking, where the light bouncing back from the ring stabilizes the laser itself, making it incredibly quiet and steady. To ensure this light remains perfectly tuned to the atoms, they referenced the system to a specific transition in rubidium atoms, a method that keeps the laser from drifting even over long periods.

The team demonstrated that this compact engine could do everything a massive laboratory setup could do. They took the light from their microcomb, selected a specific color, and amplified it to a power of 102 milliwatts, all while preserving the perfect stability of the original signal. Using this amplified light, they successfully trapped a cloud of rubidium atoms and cooled them until they formed a Bose-Einstein condensate, a state of matter where thousands of atoms act as a single quantum entity. They then used the light to build an optical lattice, a grid of light that holds the atoms in place like eggs in a carton, and drove two-photon Raman transitions, a process that changes the internal spin of the atoms. These are the exact operations required for quantum simulation and computing. The device, which fits on a chip measuring just a few square centimeters, replaced the need for multiple independent lasers and reference cavities, proving that a single, integrated source can drive complex quantum experiments.

This achievement marks a significant shift in how quantum systems are built. Previously, scaling up these experiments meant adding more lasers and more reference cavities, which increased the size, cost, and vulnerability of the system. With this microcomb engine, the researchers showed that they could scale the number of control channels without multiplying the hardware. Because the light comes from a single, coherent source, every beam derived from it is automatically synchronized with the others. This means that a single chip can provide the stable, multi-color light needed for cooling, trapping, and manipulating atoms simultaneously. The device was fabricated using standard manufacturing processes, suggesting that these engines could be produced in large quantities at a low cost. By bringing the generation, stabilization, and amplification of light onto a single chip, the researchers have removed a major barrier to deploying quantum sensors and simulators in the field, on satellites, or in mobile laboratories. The work establishes a new architecture where the complexity of controlling quantum matter is handled by a compact, robust, and manufacturable platform.

Technical Summary: A Photonic Integrated Comb Engine for Ultracold Quantum Gases

Problem Statement
The coherent control of ultracold quantum gases is fundamental to quantum sensing, simulation, and computation. However, realizing these applications in field-deployable or spaceborne systems is hindered by the formidable engineering challenges of generating, referencing, and scaling highly stable optical fields. Conventional cold-atom apparatuses rely on macroscopic arrays of discrete lasers, reference cavities, and free-space optics, resulting in prohibitive size, weight, power, and cost (SWaP-C) penalties, alongside vulnerabilities to alignment drift and mechanical vibration. While photonic integrated circuits (PICs) have advanced at telecommunication wavelengths, their extension into the visible and near-infrared spectrum for atomic applications remains functionally fragmented. Existing integrated sources often operate as standalone demonstrations, and atom-facing circuits typically depend on externally generated light. Consequently, a fully integrated laser-comb source capable of coherent manipulation of ultracold quantum gases has not yet been realized.

Methodology
The authors demonstrate a scalable, hybrid-integrated "microcomb engine" operating at 780 nm, designed to bridge frequency synthesis, atomic referencing, and power amplification. The system architecture consists of three primary stages:

Microcomb Generation: The core engine utilizes a hybrid integration of an electrically driven III–V laser diode (either Fabry–Pérot or Distributed Bragg Reflector) edge-coupled to a high-quality factor ( $Q$ ) silicon nitride ( $\text{Si}_3\text{N}_4$ ) microresonator. The diode and resonator are fabricated using mature CMOS and foundry processes on 150-mm wafers. By leveraging self-injection locking (SIL), where Rayleigh backscattering from the resonator locks the diode frequency, the system generates coherent "platicon" (dark-pulse) microcombs. The team characterized microresonators with intrinsic quality factors ( $Q_0$ ) around $9.5 \times 10^6$ and generated combs with mode spacings of 20 GHz and 100 GHz.
Atomic Referencing and Stabilization: To ensure long-term stability, the microcomb is referenced to an $^{85}\text{Rb}$ hyperfine transition ( $F=3 \to F'=4$ ) using modulation-transfer spectroscopy (MTS). This active feedback loop disciplines the pump laser, which, due to the coherence of the microcomb, stabilizes all comb lines simultaneously.
Injection Amplification and Delivery: A specific comb tooth (detuned by approximately 990 GHz from the pump) is isolated via a fiber Bragg grating and used to injection-lock a secondary FP laser diode. This process amplifies the signal from microwatts to over 100 mW while preserving the spectral purity and stability of the original microcomb. The resulting high-power, narrow-linewidth continuous-wave (CW) laser is delivered to a vacuum chamber to manipulate a Bose–Einstein condensate (BEC) of $^{87}\text{Rb}$ .

Key Results

Device Performance: The hybrid-integrated modules achieve a compact footprint of 29 mm². The microcomb generation demonstrates intrinsic coherence, verified by the absence of low-frequency amplitude noise and the presence of pristine microwave beatnotes.
Stability Metrics: The MTS locking mechanism suppresses long-term frequency drift of the pump laser from over 200 MHz (free-running) to the 100-kHz level over a 2,000-second measurement window. The injection-amplified output inherits this stability, exhibiting a fractional frequency instability of $1.6 \times 10^{-11}$ at 33 seconds and a $\beta$ -separation linewidth of 23.6 kHz.
Amplification: The system successfully amplifies a selected comb line to 102 mW with a side-mode suppression ratio (SMSR) of 41.7 dB and a gain of 40.6 dB.
Quantum Control: The synthesized optical fields were used to:
- Construct an optical lattice, calibrating a lattice depth of $V_L = 4.2 E_r$ (recoil energy) via Kapitza–Dirac diffraction.
- Drive coherent two-photon Raman transitions between spin states ( $|1, -1\rangle$ and $|1, 0\rangle$ ) with a coupling energy of $\Omega = 0.7 E_r$ .
- Adiabatically prepare stationary quantum states, including spin-orbit-coupled and Raman-lattice configurations, without significant atomic heating or decoherence.
Comparative Advantage: The authors note that disabling the MTS lock results in frequency noise orders of magnitude higher, which prevents the successful preparation of Raman-lattice ground states, highlighting the critical necessity of atomic referencing for phase-sensitive operations.

Significance and Claims
The paper claims to establish a transformative, fully integrated frequency-synthesis architecture that is essential for realizing deployable, large-scale atomic quantum systems. By seamlessly integrating device fabrication with frequency synthesis, power amplification, and coherent atomic operations, the work overcomes the SWaP-C limitations of traditional setups.

The authors emphasize that this architecture allows multiple optical-control channels to scale without a proportional multiplication of independent frequency references. The microcomb acts as a common coherent frequency grid, capable of seeding lasers for cooling, lattice formation, and Raman coupling simultaneously. The work suggests that this platform is a versatile engine for next-generation quantum simulation and sensing, potentially enabling the widespread deployment of integrated photonics in transportable atomic sensors, simulators, and clocks, including spaceborne applications. The estimated raw cost per engine is below $20, further supporting the potential for scalable manufacturing.

Technical Summary: A Photonic Integrated Comb Engine for Ultracold Quantum Gases

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