Mikael Afzelius

We report a detailed investigation of spin relaxation dynamics in the rare-earth-doped crystal ¹⁷¹ Yb:Y ₂ SiO ₅ , a promising platform for quantum memory based on atomic frequency combs. The population dynamics among the four ground-state hyperfine levels were studied in the 50 mK–6 K temperature range by optically modifying the spin populations and monitoring their return to thermal equilibrium via absorption spectroscopy. Below 1 K, population rethermalization occurs on timescales of several hours and is governed by direct spin-phonon relaxation. In contrast, the lifetime of individual spin states is strongly reduced by spin-spin interactions, ranging from seconds in low-doped (2 ppm) samples to milliseconds at higher doping (10 ppm). These results provide clear guidelines for optimizing doping concentration and operating temperature, indicating that atomic frequency comb lifetimes on the order of one hour are achievable in ¹⁷¹ Yb:Y ₂ SiO ₅ crystals with Yb concentrations not much lower than 2 ppm and at temperatures at or below 1 K.

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 ¹⁷¹ Yb ³⁺ :Y ₂ SiO ₅ , with a memory bandwidth of 250 MHz and a storage time of up to 125 µ s. The efficiency reaches 20% at short storage times, and 5% at 125 µ s. 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 ¹⁷¹ Yb ³⁺ :Y ₂ SiO ₅ . 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.

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 Yb:YSO, with a memory bandwidth of 250 MHz and a storage time of up to 125 $μ$s. The efficiency reaches 20 % at short storage times, and 5 % at 125 $μ$s. 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 Yb:YSO. 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.

We review concepts and recent work related to creating light–matter interfaces for future quantum networks based on rare-earth ion-doped crystals. More precisely, we explore their unique suitability for creating photon sources, optical quantum memories for light, and qubits that allow quantum information processing. In addition, we review the state-of-the-art of elementary quantum repeater links, and provide suggestions for future research.

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.

The coherent interaction of solid-state spins with both optical and microwave fields provides a platform for a range of quantum technologies, such as quantum sensing, microwave-to-optical quantum transduction and optical quantum memories. Rare-earth ions with electronic spins are interesting in this context. In this work, we use a loop-gap microwave resonator to coherently drive optical and microwave clock transitions simultaneously in a ¹⁷¹Yb³⁺:Y₂SiO₅ crystal, achieving a Rabi frequency of 0.56 MHz at 2.497 GHz over a 1-cm long crystal. Furthermore, we provide insights into the spin dephasing at very low fields, showing that superhyperfine-induced collapse of the Hahn echo plays an important role. Our calculations and measurements reveal that the effective magnetic moment can be manipulated in ¹⁷¹Yb³⁺:Y₂SiO₅, which suppresses the superhyperfine interaction at the clock transition. At a doping concentration of 2 ppm and 3.4 K, we achieve spin coherence time of 10.0 ± 0.4 ms.