Abstract
Atomic defects in the solid state are a key component of quantum repeater networks for long-distance quantum communication1. Recently, there has been significant interest in rare earth ions2,3,4, in particular Er3+ for its telecom band optical transition5,6,7 that allows long-distance transmission in optical fibres. However, the development of repeater nodes based on rare earth ions has been hampered by optical spectral diffusion, precluding indistinguishable single-photon generation. Here, we implant Er3+ into CaWO4, a material that combines a non-polar site symmetry, low decoherence from nuclear spins8 and is free of background rare earth ions, to realize significantly reduced optical spectral diffusion. For shallow implanted ions coupled to nanophotonic cavities with large Purcell factor, we observe single-scan optical linewidths of 150 kHz and long-term spectral diffusion of 63 kHz, both close to the Purcell-enhanced radiative linewidth of 21 kHz. This enables the observation of Hong–Ou–Mandel interference9 between successively emitted photons with a visibility of V = 80(4)%, measured after a 36 km delay line. We also observe spin relaxation times T1,s = 3.7 s and T2,s > 200 μs, with the latter limited by paramagnetic impurities in the crystal instead of nuclear spins. This represents a notable step towards the construction of telecom band quantum repeater networks with single Er3+ ions.
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Data availability
Data for all figures are available from the Harvard Dataverse repository (https://doi.org/10.7910/DVN/YLXVLB).
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Acknowledgements
We acknowledge helpful conversations with C. Thiel, P. Goldner and M. Rančić. This work was primarily supported by the US Department of Energy (DOE), Office of Science, National Quantum Information Science Research Centers, Co-design Center for Quantum Advantage (C2QA) under contract number DE-SC0012704. We also acknowledge support from the DOE Early Career award (grant no. DE-SC0020120, for modelling of decoherence mechanisms and spin interactions), as well as AFOSR (grant nos. FA9550-18-1-0334 and YIP FA9550-18-1-0081), the Eric and Wendy Schmidt Transformative Technology Fund, the Princeton Catalysis Initiative and DARPA DRINQS (grant no. D18AC00015) for establishing the materials spectroscopy pipeline and developing integrated nanophotonic devices. We acknowledge the use of Princeton’s Imaging and Analysis Center, which is partially supported by the PCCM, an NSF MRSEC (grant no. DMR-1420541), as well as the Princeton Micro-Nano Fabrication Center.
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S.O., Ł.D., S.P.H. and M.T.U. performed the experiments in the main text and analysed the data. S.P.H., C.M.P. and P.S. developed the materials used in this work, including ion implantation and annealing techniques. C.M.P., M.R. and S.C. performed initial spectroscopy of single erbium ions in CaWO4, together with S.O., Ł.D., S.P.H. and M.T.U. R.J.C. and N.P.d.L. supervised the materials development, and J.D.T. developed the project concept and supervised all aspects of the work. S.O., Ł.D., S.P.H., M.T.U., N.P.d.L. and J.D.T. wrote and edited the manuscript, with input from all authors.
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Extended data figures and tables
Extended Data Fig. 1 Ensemble PLE dependence on annealing.
Blue: the ensemble PLE intensity for a high density sample. Orange: the PLE intensity recorded for the same sample after annealing at a temperature of 300 °C for 1 hour, revealing a 5x increase in fluorescence intensity. This data was obtained using a sample cooled to 4K.
Extended Data Fig. 2 Ensemble spectroscopy of Er3+:CaWO4.
a Site-selective excitation spectrum of Er3+:CaWO4. Green arrows indicate when the excitation laser is resonant with different excited state crystal-field levels, where the corresponding excitation path is labeled using the matching number in (b). Solid orange arrows denote the fluorescence energies due to decay from the 4I13/2Y1 level, whereas the dashed arrows denote decay from the 4I13/2Y2 level. No other Er3+ sites were observed. The prominent line with unity gradient corresponds to laser scatter and has been re-scaled in intensity for clarity. b Assigned transitions. The performed spectroscopy yielded energies of four 4I15/2 and three 4I13/2 levels.
Extended Data Fig. 3 Measuring the cyclicity.
a Decay of the A transition fluorescence count rate from optical pumping after initializing into \(\left|{\uparrow }_{g}\right\rangle \) (blue). The count rate decays as e−n/C revealing a cyclicity of C = 1030(10). The orange trace corresponds to the same readout pulse sequence after initializing into \(\left|{\downarrow }_{g}\right\rangle \). b Intensity autocorrelation g(2)(t1 − t2) of the A transition showing strong suppression of the zero-delay peak with g(2)(0) = 0.018(3).
Extended Data Fig. 4 Optical coherence of Er3+:CaWO4.
a Timing sequence for an optical Hahn echo experiment. The ion is initialized into \(\left|{\uparrow }_{g}\right\rangle \) state using 10 pairs of optical πB and microwave πMWe pulses. b An optical Hahn echo measurement (green) reveals an optical coherence of T2,o = 10.2 μs. Applying XY32 dynamical decoupling sequence (orange) extends the optical coherence to the radiative limit (blue dashed line) T2,o = 18 μs, at a field where the optical T1,o = 9.1(3) μs. c Timing sequence for an optical XYN experiment. d Dephasing rate scaling with the number of refocusing pulses of XY dynamical decoupling sequences (red) compared to the lifetime limit (blue).
Extended Data Fig. 5 Spin lifetime as a function of magnetic field strength.
Solid line is a fit to Eq. (14) as is predicted for the spin-lattice relaxation time.
Extended Data Fig. 6 W bath limited coherence.
a The second order contribution to the CCE simulation for the Hahn experiment for 10 random W-bath configurations, where we assume that the nearest W nuclear spin is located at rW. Fitting each of the curves to a stretched exponential yields T2,s = 22.7(4) ms with n = 2.7(1). We note that this is only the envelope and faster ESEEM features, obtained from the first order CCE simulation, persist as seen in Fig. 4d. This simulation considers W nuclear spins within an 11 nm radius of the Er3+ spin. b CCE simulation of Ramsey experiment for the same W bath configurations. Fitting each of the curves to a Gaussian decay yields \({T}_{2,{\rm{s}}}^{* }=4.0(4)\,\mu s\). Both simulations are performed at our experimental field configuration.
Extended Data Fig. 7 Probability density of paramagnetic impurity concentration.
a Assuming a 3D uniform distribution of impurities, we estimate that the bath concentration is in the range 1.6 × 1016 – 5.7 × 1016 cm−3 with 70% confidence, with the likeliest concentration at 3.7 × 1016 cm−3. b Assuming a 2D distribution on the surface of our crystal, assumed to be located 10 nm away from the Er3+ spin, we estimate an area concentration in the range of 0.5–1.3 nm−2 with 70% confidence, with the likeliest concentration at 0.77 nm−2. Dashed lines indicate the confidence ranges for the impurity concentrations.
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Ourari, S., Dusanowski, Ł., Horvath, S.P. et al. Indistinguishable telecom band photons from a single Er ion in the solid state. Nature 620, 977–981 (2023). https://doi.org/10.1038/s41586-023-06281-4
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DOI: https://doi.org/10.1038/s41586-023-06281-4
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