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arXiv:2608.08221

Millisecond optical coherence and strong collective coupling in an integrated telecom rare-earth photonic platform

Kah Jen Wo, Pavel A. Dmitriev, Karthik Dasigi, Fumiya Hanamura, Steven Touzard

quant-phphysics.opticsSubmitted Aug 8, 2026• Announced Aug 11, 2026• 0 citations
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Abstract

Long-range quantum network nodes require the combination of strong light-matter coupling, long coherence times and in situ spectral control at telecom wavelengths. The coherence of erbium in integrated devices is held back by its hosts, which do not simultaneously provide the weakly magnetic nuclear-spin environment and the well-defined substitutional sites found in coherence-optimised bulk crystals. Here we bring such an optimised crystal onto a photonic chip, by bonding an Er${^{3+}}$:CaWO${_{4}}$ host without an adhesive interlayer to a high-${Q}$ electro-optically tuneable thin-film lithium niobate microring resonator. At an effective temperature of ${75}$ mK and a field of only ${0.2}$ T, the bonded ensemble retains an effective homogeneous linewidth of ${289\pm34}$ Hz (${T_\text{M}=1.10\pm0.13}$ ms), with spectral diffusion proceeding at ${86\pm18}$ Hz and saturating at ${1.5\pm0.2}$ kHz. Electro-optically tuning the resonator through the erbium optical transition resolves an avoided crossing with a collective cooperativity of ${C=6.7\pm0.4}$. Exploiting superhyperfine coupling to the host's ${^{183}}$W nuclear spins, we store and retrieve optical phase information over ${5}$ s with a visibility of ${0.935\pm0.015}$. Strong collective coupling, millisecond coherence and in situ spectral tuning in a single device thus establish heterogeneous integration leveraging coherence-optimised hosts as a route to scalable telecom quantum networks.

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Long-range quantum network nodes require the combination of strong light-matter coupling, long coherence times and in situ spectral control at telecom wavelengths. The coherence of erbium in integrated devices is held back by its hosts, which do not simultaneously provide the weakly magnetic nuclear-spin environment and the well-defined substitutional sites found in coherence-optimised bulk crystals. Here we bring such an optimised crystal onto a photonic chip, by bonding an Er3+{^{3+}}3+:CaWO4{_{4}}4​ host without an adhesive interlayer to a high-Q{Q}Q electro-optically tuneable thin-film lithium niobate microring resonator. At an effective temperature of 75{75}75 mK and a field of only 0.2{0.2}0.2 T, the bonded ensemble retains an effective homogeneous linewidth of 289±34{289\pm34}289±34 Hz (TM=1.10±0.13{T_\text{M}=1.10\pm0.13}TM​=1.10±0.13 ms), with spectral diffusion proceeding at 86±18{86\pm18}86±18 Hz and saturating at 1.5±0.2{1.5\pm0.2}1.5±0.2 kHz. Electro-optically tuning the resonator through the erbium optical transition resolves an avoided crossing with a collective cooperativity of C=6.7±0.4{C=6.7\pm0.4}C=6.7±0.4. Exploiting superhyperfine coupling to the host's 183{^{183}}183W nuclear spins, we store and retrieve optical phase information over 5{5}5 s with a visibility of 0.935±0.015{0.935\pm0.015}0.935±0.015. Strong collective coupling, millisecond coherence and in situ spectral tuning in a single device thus establish heterogeneous integration leveraging coherence-optimised hosts as a route to scalable telecom quantum networks.
Spectrally multiplexed telecom quantum networks require quantum memories combining efficient storage with programmable frequency addressing. An integrated implementation should therefore unite a native telecom transition, efficient storage, and fast on-chip spectral control. Here we demonstrate a cavity-enhanced memory in an isotopically purified 167Er3+^{167}\mathrm{Er}^{3+}167Er3+-doped thin-film lithium niobate microring. Long-lived hyperfine shelving states enable persistent, high-contrast atomic frequency comb preparation with a single-component lifetime of 277.6(52.6)277.6(52.6)277.6(52.6)~s, while cavity impedance matching yields 23.3(5)%23.3(5)\%23.3(5)% on-chip efficiency for 100-ns storage. The intrinsic electro-optic response enables frequency-selective storage and routing at rates up to 20~MHz. We further store and retrieve time-energy-entangled telecom photons, violating an entanglement-witness bound by more than 11 standard deviations. Our results establish erbium-doped thin-film lithium niobate as a programmable light--matter interface for spectrally multiplexed quantum networks.
Optically active solid-state spin qubits thrive as an appealing technology for quantum interconnects and quantum networks, thanks to their atomic size, scalable synthesis, long-lived coherence, and ability to coherently interface with flying qubits. Trivalent erbium dopants, in particular, emerge as an attractive candidate due to their emission in the telecom C band and shielded 4f intra-shell spin and optical transitions. Nevertheless, prevailing top-down architectures for rare-earth qubits and devices have not yet achieved simultaneous long optical and spin coherence, which is necessary for efficient long-distance quantum networks. Here, we demonstrate dual Er3+ telecom spin-photon interfaces in two distinct lattice symmetry sites within an epitaxial thin-film platform. By leveraging high matrix crystallinity, controlled proximity of dopants to surfaces, and exploiting host lattice symmetry, we simultaneously achieve kilohertz-level optical linewidth in a strongly symmetry-protected site, and erbium qubit spin coherence times exceeding 10 milliseconds. Additionally, we realize single-shot readout and microwave coherent control of erbium qubits in a fiber-integrated package, enabling rapid deployment and scalability. These advancements highlight the significant potential of high-quality rare-earth qubits and quantum memories assembled using a bottom-up method, paving the way for scalable development of quantum light-matter interfaces tailored for telecommunication quantum networks. Erbium quantum emitters operating in the telecom C band are promising for spin-photon interfaces but achieving both optical and spin coherence has been challenging. Gupta et al. report two types of erbium dopants in epitaxial Y2O3 thin films occupying distinct lattice sites and exhibiting long spin and optical coherence times
Rare earth ions are known as promising candidates for building quantum light-matter interface. However, tunable photonic cavity access to rare earth ions in their desired host crystal remains challenging. Here, we demonstrate the integration of erbium doped yttrium orthosilicate (Er3+:Y2SiO5) with thin-film lithium niobate photonic circuit by plasma-activated direct flip chip bonding. Resonant coupling to erbium ions is realized by on-chip electro-optically tuned high Q lithium niobate micro-ring resonators. Fluorescence and absorption of erbium ions at 1536.48 nm are measured in the waveguides, while the collective ion-cavity cooperativity with micro-ring resonators is assessed to be 0.36. This work presents a versatile scheme for future rare earth ion integrated quantum devices.
Integrated photonics capable of incorporating rare-earth ions with high optical coherence is desirable for realizing efficient quantum transducers, compact quantum memories, and hybrid quantum systems. Here we describe a photonic platform based on the SmartCut erbium-doped lithium niobate thin film, and explore its stable optical transitions at telecom wavelength in a dilution refrigerator. Optical coherence time of up to $180\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{s}$, rivaling the value of bulk crystals, is achieved in optical ridge waveguides and ring resonators. With this integrated platform, we demonstrate tunable light-ion interaction and flexible control of optical thickness by exploiting long waveguides, whose lengths are in principle variable. This unique ability to obtain high optical density using low-concentration ions further leads to the observation of multiecho pulse trains in centimeter-long waveguides. Our results establish a promising photonic platform for quantum information processing with rare-earth ions.
Integrated photonics capable of incorporating rare earth ions with high optical coherence is desirable for realizing efficient quantum transducers, compact quantum memories, and hybrid quantum systems. Here we describe a photonic platform based on the SmartCut erbium-doped lithium niobate thin film, and explore its stable optical transitions at telecom wavelength in a dilution refrigerator. Optical coherence time of up to 180\,μμμs, rivaling the value of bulk crystals, is achieved in optical ridge waveguides and ring resonators. With this integrated platform, we demonstrate tunable light-ion interaction and flexible control of optical thickness by exploiting long waveguides, whose lengths are in principle variable. This unique ability to obtain high optical density using a low concentration ions further leads to the observation of multi-echo pulse trains in centimeter-long waveguides. Our results establish a promising photonic platform for quantum information processing with rare earth ions.
We report dual er qubits in two distinct symmetry sites of an epitaxially grown single crystal Y2O3 thin film. Coupling of Er spins to a superconducting microwave resonator and a fiber micro-cavity enables simultaneous optical-spin coherence spectroscopy at millikelvin temperature. At optimal field orientations, over millisecond spin coherence times are observed for Er qubits in both C2 and C3i symmetry sites. Single Er ions in both C2 and C3i sites are addressed optically, respectively at the field configuration where their millisecond spin coherence times are measured. By correlating the spin spectral diffusion and optical dephasing rate of C3i Er qubits, we report a magnetic noise limited spin-optical dephasing rate of only 2.6 kHz, demonstrating significant prospect of Er in epitaxial Y2O3 thin film as a highly coherent spin-photon interface for quantum network.