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
Optically active solid-state spin qubits thrive as an appealing technology for quantum interconnect and quantum networking, owing to their atomic size, scalable creation, long-lived coherence, and ability to coherently interface with flying qubits. Trivalent erbium dopants in particular emerge as a compelling candidate with their telecom C band emission and shielded 4f intra-shell spin-optical transitions. However, prevailing top-down architecture for rare-earth qubits and devices has not allowed simultaneous long optical and spin coherence necessary for long-distance quantum networks. Here we demonstrate dual erbium telecom spin-photon interfaces in an epitaxial thin-film platform via wafer-scale bottom-up synthesis. Harnessing precise controls over the matrix purity, dopant placement, and symmetry unique to this platform, we simultaneously achieve millisecond erbium spin coherence time and $<$3 kilohertz optical dephasing rate in an inversion-symmetry protected site and realize both optical and microwave control in a fiber-integrated package for rapid scaling up. These results demonstrate a significant prospect for high-quality rare-earth qubits and quantum memories assembled using a bottom-up method and pave the way for the large-scale development of quantum light-matter interfaces for telecommunication quantum networks.
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±34 Hz (
TM=1.10±0.13 ms), with spectral diffusion proceeding at
86±18 Hz and saturating at
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. Exploiting superhyperfine coupling to the host's
183W nuclear spins, we store and retrieve optical phase information over
5 s with a visibility of
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.
Rare earth ion (REI)-doped oxide thin films are attractive for the application of quantum interconnects due to their stable optical levels and scalability
1−3. Among them, Er
3+ doped CaWO
4 is promising because it possesses narrow optical linewidth transitions and a long spin coherence time
4−6. The electron spin coherence is limited at high temperatures by paramagnetic impurities and by the presence of the 14.3%
183W nuclear spin. To further increase the spin coherence time at millikelvin temperatures, where the paramagnetic impurities are frozen out, our approach is to synthesize chemically and isotopically purified thin films as a host material. We first grow non-isotopically enriched Er
3+ doped CaWO
4 thin films, which exhibit a 214(13) MHz photoluminescence (PL) inhomogeneous linewidth, indicating the thin film has high crystalline quality. We then grow isotopically enriched CaWO
4 thin films using an isotopically purified
186WO
3 source. Time of flight secondary ion mass spectrometry (ToF-SIMS) was used to measure the relative concentration of W isotopes.
183W, the only W isotope that has a net nuclear spin and is the major cause of spin decoherence, was at a relative abundance of 1.2%, a factor of 10 lower than natural abundance. We also observed PL emission from single ions after integrating nano-photonic devices with the thin film. These results establish isotopically engineered CaWO
4 thin films as a promising platform for future studies of nuclear-spin-limited coherence and for scalable rare-earth-ion-based quantum nanophotonic devices.