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arXiv:2410.14551 · Quantum Science and Technology

Efficient and reversible optical-to-spin conversion for solid-state quantum memories

Jingjing Chen, Mikael Afzelius

Published Apr 1, 2025• 7 citations
arXivHTMLPDF

Abstract

Abstract Long-duration and efficient quantum memories for photons are key components of quantum repeater and network applications. To achieve long-duration storage in atomic systems, a short-lived optical coherence can be mapped into a long-lived spin coherence, which forms the basis for many quantum memory schemes. In this work, we present modeling and measurements of the back-and-forth, i.e. reversible, optical-to-spin conversion for an atomic frequency comb (AFC) memory. The AFC memory is implemented in 151Eu3+:Y2SiO5 with an applied magnetic field of 231 mT, to suppress time-domain interference effects in the conversion efficiency. By optimizing the conversion using the developed simulation tool, experimentally we achieve a total efficiency of up to 96%, including the spin echo sequence and spin dephasing, for a storage time of 500 µs. Our methods and results pave the way for long-duration storage of single photon states in 151Eu3+:Y2SiO5 with high signal-to-noise, at the millisecond timescale.

DOI
10.1088/2058-9565/adc7d3

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Abstract Long-duration and efficient quantum memories for photons are key components of quantum repeater and network applications. To achieve long-duration storage in atomic systems, a short-lived optical coherence can be mapped into a long-lived spin coherence, which forms the basis for many quantum memory schemes. In this work, we present modeling and measurements of the back-and-forth, i.e. reversible, optical-to-spin conversion for an atomic frequency comb (AFC) memory. The AFC memory is implemented in 151Eu3+:Y2SiO5 with an applied magnetic field of 231 mT, to suppress time-domain interference effects in the conversion efficiency. By optimizing the conversion using the developed simulation tool, experimentally we achieve a total efficiency of up to 96%, including the spin echo sequence and spin dephasing, for a storage time of 500 µs. Our methods and results pave the way for long-duration storage of single photon states in 151Eu3+:Y2SiO5 with high signal-to-noise, at the millisecond timescale.
Quantum memories that store and retrieve photonic quantum states are key components for future quantum networks. This thesis investigates efficient optical-to-spin population transfer in Eu³⁺:Y₂SiO₅, a leading platform for atomic frequency comb (AFC) quantum memories due to its long coherence times and excellent spectral properties. Through combined experimental and theoretical work, the optical-to-spin transfer efficiency is increased to 96%, demonstrating near-unity conversion between optical and spin states—an essential requirement for high-performance spin-wave storage. A validated spin Hamiltonian is established by reconstructing the branching-ratio matrix under a 231 mT magnetic field aligned near the D₂ axis, ensuring isolated transitions and accurate modeling of energy levels and transition strengths. Numerical simulations based on optical Bloch equations reproduce the measured Rabi dynamics and identify optimal control-pulse parameters, while a second model captures optical-pumping dynamics and spectral preparation. Together, these tools provide a quantitative framework for optimizing Eu³⁺:Y₂SiO₅ quantum memories and advancing scalable solid-state quantum storage.
Abstract Quantum memories with long storage times are key elements in long-distance quantum networks. The atomic frequency comb (AFC) memory in particular has shown great promise to fulfill this role, having demonstrated multimode capacity and spin–photon quantum correlations. However, the memory storage times have so-far been limited to about 1 ms, realized in a Eu3+ doped Y2SiO5 crystal at zero applied magnetic field. Motivated by studies showing increased spin coherence times under applied magnetic field, we developed an AFC spin-wave memory utilizing a weak 15 mT magnetic field in a specific direction that allows efficient optical and spin manipulation for AFC memory operations. With this field configuration the AFC spin-wave storage time increased to 40 ms using a simple spin-echo sequence. Furthermore, by applying dynamical decoupling techniques the spin-wave coherence time reaches 530 ms, a 300-fold increase with respect to previous AFC spin-wave storage experiments. This result paves the way towards long duration storage of quantum information in solid-state ensemble memories.
A long-lived quantum memory is a firm requirement for implementing a quantum repeater scheme. Recent progress in solid-state rare-earth-ion-doped systems justifies their status as very strong candidates for such systems. Nonetheless an optical memory based on spin-wave storage at the single-photon level has not been shown in such a system to date, which is crucial for achieving the long storage times required for quantum repeaters. In this paper we show that it is possible to execute a complete atomic frequency comb (AFC) scheme, including spin-wave storage, with weak coherent pulses of $\overline{n}=2.5\ifmmode\pm\else\textpm\fi{}0.6$ photons per pulse. We discuss in detail the experimental steps required to obtain this result and demonstrate the coherence of a stored time-bin pulse. We show a noise level of $(7.1\ifmmode\pm\else\textpm\fi{}2.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ photons per mode during storage, and this relatively low noise level paves the way for future quantum optics experiments using spin waves in rare-earth-doped crystals.
We report on the experimental demonstration of an optical spin-wave memory, based on the atomic frequency comb (AFC) scheme, where the storage efficiency is strongly enhanced by an optical cavity. The cavity is of low finesse, but operated in an impedance matching regime to achieve high absorption in our intrinsically low-absorbing Eu 3+ :Y 2 SiO 5 crystal. For storage of optical pulses as an optical excitation (AFC echoes), we reach efficiencies of 53% and 28% for 2 μ s and 10 μ s delays, respectively. For a complete AFC spin-wave memory we reach an efficiency of 12%, including spin-wave dephasing, which is a 12-fold increase with respect to previous results in this material. This result is an important step towards the goal of making efficient and long-lived quantum memories based on spin waves, in the context of quantum repeaters and quantum networks.
A major challenge in quantum technologies is to build an efficient and long-lived quantum memory, particularly using solid-state devices. I will here report on an experiment where we combine the AFC optical memory with spin-echo techniques to extend the memory time from a few microseconds to about 1 ms, using an Europium-doped crystal. In general the spin-echo technique allows one to control the inhomogeneous spin dephasing which often sets the storage-time limit both in solid-state systems and laser-cooled gases. However, theoretically it is has been argued that spin-echo techniques would be extremely difficult to apply without creating noise in the case of a single quanta stored in a large spin ensemble. We here show how this noise can be limited and demonstrate high signal-to-noise ratio in the output mode when storing pulses at the single-photon level. Furthermore we stored polarization qubits encoded onto weak coherent , with fidelities surpassing a classical storage scheme.
Optical quantum memories are essential components for realizing the full potential of quantum networks. Among these, rare-earth-doped crystal memories stand out due to their large multimode storage capabilities. To maximize the multimode capacity in the time domain, it is key to simultaneously achieve large memory bandwidth and long optical storage time. Here, we demonstrate an atomic frequency comb optical memory in 171Yb3+:Y2SiO5 , with a memory bandwidth of 250 MHz and a storage time of up to 125 µs. The efficiency reaches 20% at short storage times, and 5% at 125 µs. These results were enabled by an optimized optical pumping scheme, guided by numerical modelling. Our approach is specifically designed for future spin-wave storage experiments, with the theoretical bandwidth limit set at 288 MHz by the hyperfine structure of 171Yb3+:Y2SiO5 . Additionally, we introduce an efficient method for synthesizing the optical pumping waveforms required for generating combs with tens of thousands of teeth, as well as a simple yet frequency-agile laser setup for optical pumping across a 10 GHz bandwidth.