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Science Advances

Efficient on-chip platform for coherent light-matter coupling using bound states in the continuum

Pai Zhou, Hui-Zhen Zhang, Tingmei Li, Zhongshan Zhang, Yu‐Hui Chen, Xiangdong Zhang

Published Apr 25, 2025• 15 citations
Publisher

Abstract

Storing and retrieving photonic qubits are key functionalities in future optical quantum networks, and integrating scalable optical-memory units is crucial as these networks expand. However, attempts to combine silicon photonics and erbium ions for telecom memories, without losing the scalable and low-loss properties of silicon chips, face challenges because of limited light-matter interactions and potential extra decoherence. Here, we present an efficient silicon-chip platform using bound states in the continuum to overcome these limitations. In addition to a low propagation loss of 0.5 ± 0.5 decibels per centimeter, our experiments demonstrate an order-of-magnitude enhancement in light absorption compared to previous traditional silicon hybrid designs. Using these properties, we demonstrated photon echoes in our waveguide structures, revealing a coherence time of 2.6 ± 0.6 microseconds at zero magnetic field, closely matching that of bulk crystals. These characteristics make the bound state in the continuum platform a promising candidate for realizing integrated optical memories for quantum network applications.

DOI
10.1126/sciadv.adu0976

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Storing and retrieving photonic qubits are key functionalities in future optical quantum networks, and integrating scalable optical-memory units is crucial as these networks expand. However, attempts to combine silicon photonics and erbium ions for telecom memories, without losing the scalable and low-loss properties of silicon chips, face challenges because of limited light-matter interactions and potential extra decoherence. Here, we present an efficient silicon-chip platform using bound states in the continuum to overcome these limitations. In addition to a low propagation loss of 0.5 ± 0.5 decibels per centimeter, our experiments demonstrate an order-of-magnitude enhancement in light absorption compared to previous traditional silicon hybrid designs. Using these properties, we demonstrated photon echoes in our waveguide structures, revealing a coherence time of 2.6 ± 0.6 microseconds at zero magnetic field, closely matching that of bulk crystals. These characteristics make the bound state in the continuum platform a promising candidate for realizing integrated optical memories for quantum network applications.
The hybrid of silicon and erbium-doped crystals provides a promising photonic platform for the advancement of quantum technologies, combining scalability, telecom compatibility, and complex quantum functionalities. However, the inherent high refractive index of silicon leads to optical field confinement within the silicon, limiting the number of optically active erbium ions and the achievable light absorption, which are essential for various quantum applications. To fully utilize the potential of erbium-doped crystals, we implemented bound states in the continuum within the hybrid silicon structures, which achieve substantial optical energy confinement within the lower-index crystal. Our structure, which is applicable to both transverse magnetic and transverse electric polarization modes, effectively increases the number of optically active erbium ions and demonstrates simultaneous minimal radiative losses and enhanced erbium absorption. Moreover, the core design principle is extensible to other low-refractive-index materials.
A scalable platform for on-chip optical quantum networks will rely on standard top-down nanofabrication techniques and solid-state emitters with long coherence times. We present a new hybrid platform that integrates amorphous silicon photonic waveguides and microresonators fabricated on top of a yttrium orthosilicate substrate doped with erbium ions. The quality factor of one such resonator was measured to exceed 100,000 and the ensemble cooperativity was measured to be 0.54. The resonator-coupled ions exhibited spontaneous emission rate enhancement and increased coupling to the input field, as required for further development of on-chip quantum light-matter interfaces.
Integrated photonic circuits offer great promise for quantum technologies. However, due to the rapid propagation of light, many envisioned applications require efficient on-chip quantum memories with a programmable delay, compact footprint, and high fidelity. Implementing this based on standard semiconductor processing technology is an outstanding challenge. Here, we realize such memories using erbium-doped silicon waveguides, fabricated as part of a multi-wafer project by a nanophotonic foundry. We demonstrate light storage with a 44.2(9) MHz44.2(9)\ \text{MHz}44.2(9) MHz bandwidth and a programmable delay exceeding 1 μs1\ μ\text{s}1 μs in a device with a footprint of only 1.5×10−2 mm21.5\times 10^{-2}\ \text{mm}^21.5×10−2 mm2, outperforming on-chip delay lines by many orders of magnitude. The phase of the read-out light field is preserved with a visibility of 91.3(30) %91.3(30)\ \%91.3(30) %. The efficiency of 1.89(28)×10−81.89(28)\times 10^{-8}1.89(28)×10−8 can be improved in future devices through resonator enhancement and higher dopant concentrations. With this, the demonstrated approach will pave the way towards applications in photonic quantum computing based on scalable silicon processing technology.
Optical quantum memories will enable technologies including long distance quantum communication and modular quantum computing. Rare earth ion doped crystals provide an excellent solid state platform for optical quantum memories. Among rare earths, erbium is particularly appealing due to its long-lived telecom-wavelength resonance, allowing integration with silicon photonics and with existing optical communication technology and infrastructure. We present an on-chip all-optical quantum memory at telecom wavelengths using a nanobeam photonic crystal cavity fabricated directly in erbium-167 doped yttrium orthosilicate. Using an atomic frequency comb protocol, we store coherent pulses for memory times as long as 10 µs, albeit with low efficiency. For shorter memory times, we achieve a memory efficiency of 0.4%, which is limited by the coupling rate between the resonator and the ensemble of ions. By working at dilution refrigerator temperatures, we are able to access a regime where the ions have long optical coherence times and good spectral holeburning properties using only a moderate magnetic field applied with permanent magnets. We characterize the multimode properties and fidelity of the quantum memory in this device, and outline a path toward higher efficiency.
Rare-earth ion defects in solid-state hosts have emerged as a promising candidate for emissive quantum memories owing to their inherent spin–photon interface and long optical and spin coherence times. Erbium (Er3+) in particular has optical transitions in the C-band making it well suited to the use of existing telecom technology infrastructure. In this work, we present a platform to integrate rare-earth ions into standard silicon photonic circuits. Erbium is co-deposited in CMOS-compatible TiO2 host films onto SOI and patterned into high Purcell factor photonic crystal cavities. Purcell factors in excess of 500 are observed and transient spectral hole burning and photoluminescence excitation scans reveal homogeneous linewidths below 15 MHz. Additionally, we show that photonic wirebonding can provide a solution for low-loss and thermally stable fiber-to-chip coupling.
Quantum networks will enable a variety of applications, from secure communication and precision measurements to distributed quantum computing. Storing photonic qubits and controlling their frequency, bandwidth, and retrieval time are important functionalities in future optical quantum networks. Here we demonstrate these functions using an ensemble of erbium ions in yttrium orthosilicate coupled to a silicon photonic resonator and controlled via on-chip electrodes. Light in the telecommunication C-band is stored, manipulated, and retrieved using a dynamic atomic frequency comb protocol controlled by linear DC Stark shifts of the ion ensemble’s transition frequencies. We demonstrate memory time control in a digital fashion in increments of 50 ns, frequency shifting by more than a pulse width ( ± 39 M H z ), and a bandwidth increase by a factor of 3, from 6 to 18 MHz. Using on-chip electrodes, electric fields as high as 3 kV/cm were achieved with a low applied bias of 5 V, making this an appealing platform for rare-earth ions, which experience Stark shifts of the order of 10 kHz/(V/cm).