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arXiv:2107.04498 · Physical review. B./Physical review. B

Extending the spin coherence lifetimes of Er3+167:Y2SiO5 at subkelvin temperatures

Jian-Yin Huang, Pei-Yun Li, Zong‐Quan Zhou, Chuan‐Feng Li, Guang‐Can Guo

Published Jun 23, 2022• 7 citations
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Abstract

${\mathrm{Er}}^{3+}\text{:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ is a material of particular interest due to its suitability for telecom-band quantum memories and quantum transducers interfacing optical communication with quantum computers working in the microwave regime. Extending the coherence lifetimes of the electron spins and the nuclear spins is essential for implementing efficient quantum information processing based on such hybrid electron-nuclear spin systems. The electron spin coherence time of ${\mathrm{Er}}^{3+}\text{:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ is so far limited to several microseconds, and there are significant challenges in optimizing coherence lifetimes simultaneously for both the electron and nuclear spins. Here we perform a pulsed-electron-nuclear-double-resonance investigation for an ${\mathrm{Er}}^{3+}$-doped material at subkelvin temperatures. At the lowest working temperature, the electron spin coherence time reaches 290 $\ifmmode\pm\else\textpm\fi{}$ 17 $\mathrm{\ensuremath{\mu}}\mathrm{s}$, which has been enhanced by 40 times compared with the previous results. In the subkelvin regime, a rapid increase in the nuclear spin coherence time is observed, and the longest coherence time of 738 $\ifmmode\pm\else\textpm\fi{}$ 6 $\mathrm{\ensuremath{\mu}}\mathrm{s}$ is obtained. These extended coherence lifetimes could be valuable resources for further applications of ${\mathrm{Er}}^{3+}\text{:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}$ in fiber-based quantum networks.

DOI
10.1103/physrevb.105.245134

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Er3+:Y2SiO5{\mathrm{Er}}^{3+}\text{:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Er3+:Y2​SiO5​ is a material of particular interest due to its suitability for telecom-band quantum memories and quantum transducers interfacing optical communication with quantum computers working in the microwave regime. Extending the coherence lifetimes of the electron spins and the nuclear spins is essential for implementing efficient quantum information processing based on such hybrid electron-nuclear spin systems. The electron spin coherence time of Er3+:Y2SiO5{\mathrm{Er}}^{3+}\text{:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Er3+:Y2​SiO5​ is so far limited to several microseconds, and there are significant challenges in optimizing coherence lifetimes simultaneously for both the electron and nuclear spins. Here we perform a pulsed-electron-nuclear-double-resonance investigation for an Er3+{\mathrm{Er}}^{3+}Er3+-doped material at subkelvin temperatures. At the lowest working temperature, the electron spin coherence time reaches 290 \ifmmode±\else\textpm\fi\ifmmode\pm\else\textpm\fi{}\ifmmode±\else\textpm\fi 17 \ensuremathμs\mathrm{\ensuremath{\mu}}\mathrm{s}\ensuremathμs, which has been enhanced by 40 times compared with the previous results. In the subkelvin regime, a rapid increase in the nuclear spin coherence time is observed, and the longest coherence time of 738 \ifmmode±\else\textpm\fi\ifmmode\pm\else\textpm\fi{}\ifmmode±\else\textpm\fi 6 \ensuremathμs\mathrm{\ensuremath{\mu}}\mathrm{s}\ensuremathμs is obtained. These extended coherence lifetimes could be valuable resources for further applications of Er3+:Y2SiO5{\mathrm{Er}}^{3+}\text{:}{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Er3+:Y2​SiO5​ in fiber-based quantum networks.
Erbium-doped solids are prime candidates for quantum memories in optical quantum networks given their telecom-compatible photon emission. An electron spin of erbium with millisecond coherence time is desirable for generating remote entanglement between adjacent quantum network nodes. Here we report GHz-range electron-spin transitions of 167Er3+{}^{167}{\mathrm{Er}}^{3+}167Er3+ in a yttrium oxide (Y2O3{\mathrm{Y}}_{2}{\mathrm{O}}_{3}Y2​O3​) matrix with coherence times that are consistently longer than a millisecond. By polarizing paramagnetic impurity spins we achieve a spin T2{T}_{2}T2​ up to 1.46 ms, and up to 7.1 ms after dynamical decoupling. These coherence lifetimes are among the longest found for erbium in crystalline matrices despite the presence of host nuclear spins. We further enhance the coherence time beyond conventional dynamical decoupling, using customized sequences to simultaneously mitigate spectral diffusion and dipolar interactions. Our study not only establishes 167Er3+{}^{167}{\mathrm{Er}}^{3+}167Er3+: Y2O3{\mathrm{Y}}_{2}{\mathrm{O}}_{3}Y2​O3​ as a significant quantum memory platform but also provides a guideline for engineering long-lived erbium spins in a variety of host materials for quantum technologies.
Interfacing between various elements of a computer--from memory to processors to long range communication--will be as critical for quantum computers as it is for classical computers today. Paramagnetic rare-earth doped crystals, such as Nd(3+):Y2SiO5(YSO), are excellent candidates for such a quantum interface: they are known to exhibit long optical coherence lifetimes (for communication via optical photons), possess a nuclear spin (memory), and have in addition an electron spin that can offer hybrid coupling with superconducting qubits (processing). Here we study two of these three elements, demonstrating coherent storage and retrieval between electron and (145)Nd nuclear spin states in Nd(3+):YSO. We find nuclear spin coherence times can reach 9 ms at ∼5 K, about 2 orders of magnitude longer than the electron spin coherence, while quantum state and process tomography of the storage or retrieval operation between the electron and nuclear spin reveal an average state fidelity of 0.86. The times and fidelities are expected to further improve at lower temperatures and with more homogeneous radio-frequency excitation.
Hybrid quantum systems combining circuit QED with spin-doped solids are an attractive platform for distributed quantum information processing. There, the magnetic ions serve as coherent memory elements and reversible conversion elements of microwave to optical qubits. Among many possible spin-doped solids, erbium ions offer the unique opportunity for a coherent conversion of microwave photons into the telecom C band at $1.54\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{m}$ employed for long distance communication. In our work, we perform a time-resolved electron spin resonance study of an Er3+:Y2SiO5{\mathrm{Er}}^{3+}:{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Er3+:Y2​SiO5​ spin ensemble at millidegrees Kelvin temperatures and demonstrate multimode storage and retrieval of up to 16 coherent microwave pulses. The memory efficiency is measured to be ${10}^{\ensuremath{-}4}$ at a coherence time of ${T}_{2}=5.6\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{s}$. We observe a saturation of the spin coherence time below 50 mK due to full polarization of the surrounding electronic spin bath.
Eu3+{\mathrm{Eu}}^{3+}Eu3+:Y2SiO5{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Y2​SiO5​ crystals are promising candidates for quantum memories because their nuclear spin coherence lifetime can be extended to 47 seconds at a particular magnetic field. Although an extended coherence lifetime of 6 hours can be achieved with a large number of dynamic decoupling sequences, these pulses will also introduce unwanted noise. To realize quantum storage with a lifetime of hours, it is necessary to quantitatively characterize the spin decoherence process of Eu3+{\mathrm{Eu}}^{3+}Eu3+ ions to guide the experimental efforts. In practice, a number of limiting factors could impose a limit on the achievable spin coherence lifetime, which includes the spin inhomogeneous broadening of the Eu3+{\mathrm{Eu}}^{3+}Eu3+ ensemble and the homogeneity and stability of the external magnetic field. Here, we provide a Monte Carlo simulation model of the nuclear spin decoherence process of Eu3+{\mathrm{Eu}}^{3+}Eu3+ ions in Y2SiO5{\mathrm{Y}}_{2}{\mathrm{SiO}}_{5}Y2​SiO5​ crystals, and these limiting factors are considered to provide an accurate prediction of the spin coherence lifetimes. This method is universal and can be applied to other rare-earth-ion-doped crystals.
Sub-second spin and lifetime-limited optical coherences in 171^{171}171Yb3+^{3+}3+:CaWO4_44​
Optically addressable solid-state spins have been extensively studied for quantum technologies, offering unique advantages for quantum computing, communication, and sensing. Advancing these applications is generally limited by finding materials that simultaneously provide lifetime-limited optical and long spin coherences. Here, we introduce 171^{171}171Yb3+^{3+}3+ ions doped into a CaWO4_44​ crystal. We perform high-resolution spectroscopy of the excited state, and demonstrate all-optical coherent control of the electron-nuclear spin ensemble. We find narrow inhomogeneous broadening of the optical transitions of 185 MHz and radiative-lifetime-limited coherence time up to 0.75~ms. Next to this, we measure a spin-transition ensemble line width of 5 kHz and electron-nuclear spin coherence time reaching 0.15~seconds at zero magnetic field between 50~mK and 1~K temperatures. These results demonstrate the potential of 171^{171}171Yb3+^{3+}3+:CaWO4_44​ as a low-noise platform for building quantum technologies with ensemble-based memories, microwave-to-optical transducers, and optically addressable single-ion spin qubits.
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