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.
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.
Abstract Long-duration quantum memories for photonic qubits are essential components for achieving long-distance quantum networks and repeaters. The mapping of optical states onto coherent spin-waves in rare earth ensembles is a particularly promising approach to quantum storage. However, it remains challenging to achieve long-duration storage at the quantum level due to read-out noise caused by the required spin-wave manipulation. In this work, we apply dynamical decoupling techniques and a small magnetic field to achieve the storage of six temporal modes for 20, 50, and 100 ms in a 151Eu3+:Y2SiO5 crystal, based on an atomic frequency comb memory, where each temporal mode contains around one photon on average. The quantum coherence of the memory is verified by storing two time-bin qubits for 20 ms, with an average memory output fidelity of F = (85 ± 2)% for an average number of photons per qubit of μin = 0.92 ± 0.04. The qubit analysis is done at the read-out of the memory, using a type of composite adiabatic read-out pulse we developed.
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.
Abstract Ensemble-based quantum memories are key to developing multiplexed quantum repeaters, able to overcome the intrinsic rate limitation imposed by finite communication times over long distances. Rare-earth ion doped crystals are main candidates for highly multimode quantum memories, where time, frequency and spatial multiplexing can be exploited to store multiple modes. In this context the atomic frequency comb (AFC) quantum memory provides large temporal multimode capacity, which can readily be combined with multiplexing in frequency and space. In this article, we derive theoretical formulas for quantifying the temporal multimode capacity of AFC-based memories, for both optical memories with fixed storage time and spin-wave memories with longer storage times and on-demand read out. The temporal multimode capacity is expressed in key memory parameters, such as AFC bandwidth, fixed-delay storage time, memory efficiency, and control field Rabi frequency. Current experiments in europium- and praseodymium-doped Y2SiO5 are analyzed within this theoretical framework, which is also tested with newly acquired data, as prospects for higher temporal capacity in these materials are considered. In addition we consider the possibility of spectral and spatial multiplexing to further increase the mode capacity, with examples given for praseodymium doped Y2SiO5.
153 Eu 3 + :Y 2 SiO 5 is a very attractive candidate for a long-lived, multimode quantum memory due to the long spin coherence time (∼15 ms), the relatively large hyperfine splitting (100 MHz) and the narrow optical homogeneous linewidth (∼100 Hz). Here we show an atomic frequency comb memory with spin-wave storage in a promising material 153 Eu 3 + :Y 2 SiO 5 , reaching storage times slightly beyond 10 μs. We analyse the efficiency of the storage process and discuss ways of improving it. We also measure the inhomogeneous spin linewidth of 153 Eu 3 + :Y 2 SiO 5 , which we find to be 69 ± 3 kHz. These results represent a further step towards realizing a long-lived, multimode solid-state quantum memory.
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.
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.