arXiv subjects

Mikael Afzelius

Publications and source records attributed to Mikael Afzelius.

At least 19 recordsLinked to original sources

arXiv2026

Entanglement distribution and quantum storage of more than 8000 modes over a metropolitan network

Entanglement generation between telecommunication photons and matter is central to fibre-based quantum repeaters. Achieving practical communication rates requires multiplexing, which multimode quantum memories can provide. Rare-earth-ion ensembles offer large temporal multimode storage by exploiting the numerous spectral channels within their absorption spectrum. Here, we report on a quantum repeater node comprised of a $^{171}$Yb$^{3+}$:Y$_2$SiO$_5$ multimode quantum memory, featuring a 250 MHz bandwidth and a $76.6~\mu\mathrm{s}$ lifetime, and a bandwidth-matched entangled photon-pair source. We introduce and validate a quantitative measure of the effective temporal mode capacity using a Schmidt decomposition. With this platform, we demonstrate entanglement between a telecom photon propagating through a 25.3 km fiber spool and a 979 nm photon stored for $125~\mu\mathrm{s}$ across 16340 temporal modes. Finally, we report a field deployment distributing entanglement over 5.66 km through the Geneva metropolitan fibre network while storing 8235 modes for $63~\mu\mathrm{s}$.

quant-ph↗

arXiv2025

Optical investigation of ultra-slow spin relaxation in $^{171}$Yb$^{3+}$:Y$_2$SiO$_5$ single crystals

We present a comprehensive study of spin relaxation dynamics at cryogenic temperatures in a rare-earth-doped crystal used for quantum memory applications: $^{171}$Yb:Y$_2$SiO$_5$. Spin relaxation is indeed a major limiting factor for both the efficiency and storage time of quantum memory protocols based on atomic frequency combs in rare-earth materials. The relaxation dynamics among the four ground-state hyperfine levels were simultaneously investigated by optically perturbing the spin population distribution and monitoring its return to thermal equilibrium through optical absorption spectroscopy. By applying different types of perturbations, we were also able to distinguish between two types of relaxation processes, induced by spin-phonon and spin-spin interactions. Below 1 K, we observed that the re-thermalization of the Yb$^{3+}$ ion population takes several hours, driven solely by direct phonon absorption or emission. However, the effective lifetime of individual spin states is much shorter - on the order of several seconds in low-doped (2 ppm) samples and of milliseconds in 10 ppm samples - due to spin-spin interactions. These findings provide valuable guidelines for optimizing doping levels and operating temperatures in rare-earth-doped crystals for quantum applications. Notably, they suggest that atomic frequency combs with lifetimes of several hours could be realized using $^{171}$Yb:Y$_2$SiO$_5$ crystals with slightly less than 2 ppm doping and operating near 1 K.

physics.atom-ph↗

arXiv2025

Optical pumping simulations and optical Rabi frequency measurements in $^{151}\mathrm{Eu}^{3+}\!:\mathrm{Y}_2\mathrm{SiO}_5$ under magnetic field

Europium-doped crystal $\mathrm{Y}_2\mathrm{SiO}_5$ is an interesting platform for optical quantum memories, due to its long optical and spin coherence times. In this work, we investigate $^{151}\mathrm{Eu}^{3+}\!:\mathrm{Y}_2\mathrm{SiO}_5$ under the application of magnetic field, a complex system with 36 different optical-hyperfine transitions. We present a simple numerical simulator for calculating the effect of optical pumping schemes that allow the isolation of a single frequency class of ions, applicable to any multi-level atom with inhomogeneous broadening. Experimentally, we develop methodologies for estimating the magnetic field vector, demonstrating high precision in predicting spectral features based on the known spin Hamiltonians. We measured the optical Rabi frequency of 21 transitions among the 36 possible optical-hyperfine transitions, allowing for the construction of a 6x6 branching ratio matrix of relative transition strengths. From the Rabi frequency measurements, we derive the optical dipole moment for the $^7\!F_0 \leftrightarrow {}^5\!D_0$ transition, yielding a value of $(6.94 \pm 0.09) \times 10^{-33}~\mathrm{C} \cdot \mathrm{m}$. Our results provide a critical test of the spin Hamiltonian models, demonstrating high precision in predicting energy levels and relative transition strengths, which are key abilities for quantum applications with $^{151}\mathrm{Eu}^{3+}\!:\mathrm{Y}_2\mathrm{SiO}_5$ under magnetic field.

quant-ph↗

arXiv2025

Quantum networks using rare-earth ions

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.

quant-ph↗

arXiv2024

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

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 is 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 memory. The AFC memory is implemented in $^{151}\textrm{Eu}^{3+}:\textrm{Y}_2\textrm{SiO}_5$ with an applied magnetic field of 231 mT, which allows lifting Zeeman transition degeneracy which otherwise cause time-domain interference in the optical-to-spin conversion. By optimizing the conversion using the developed simulation tool, we achieve a total efficiency of up to 96%, including the spin echo sequence and spin dephasing, for a storage time of 500 $μ$s. 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.

quant-ph↗

arXiv2024

Decoherence induced by dipole-dipole couplings between atomic species in rare-earth ion-doped Y$_2$SiO$_5$

Rare-earth ion doped crystals are state-of-the-art platforms for processing quantum information, particularly thanks to their excellent optical and spin coherence properties at cryogenic temperatures. Experimental observations have shown that the application of a static magnetic bias field significantly improves the coherence times in the rare-earth ions ensemble, but only a few studies have focused on its the dependency as a function of both magnetic field direction and amplitude. This is especially true for magnetic field amplitudes under the mT, and for low magnetic dipole moment ions. In this paper, we investigate the relationship between the magnetic field parameters and the decoherence caused by magnetic dipole-dipole coupling with the nearest neighbors nuclear spins in the crystal. The primary non-Kramers rare-earth ions investigated here are europium and praseodymium, but we also extend our study to the ytterbium Kramers ion due to its low magnetic dipole in the mT range. We perform theoretical investigations and simulations of the energy structure and coherence time evolution and identify good correspondences between experimental and simulated spin echo data. This work allows us to pinpoint the most relevant decoherence mechanisms in the considered magnetic field regime, and to predict favorable magnetic configurations.

quant-ph↗

arXiv2023

Optical coherence and spin population dynamics in $^{171}$Yb$^{3+}$:Y$_2$SiO$_5$ single crystals

$^{171}$Yb$^{3+}$-doped Y$_2$SiO$_5$ crystals are a promising platform for optical quantum memories in long-distance quantum communications. The relevance of this material lies in $^{171}$Yb long optical and spin coherence times, along with a large hyperfine splitting, enabling long quantum storage over large bandwidths. Mechanisms affecting the optical decoherence are however not precisely known, especially since low-temperature measurements have so far focused on the 2 to 4 K range. In this work, we performed two- and three-pulse photon echoes and spectral hole burning to determine optical homogeneous linewidths in two 171 Yb:YSO crystals doped at 2 and 10 ppm. Experiments were performed in the 40 mK to 18 K temperature range, leading to linewidths between 320 Hz, among the narrowest reported for rare-earth ions, and several MHz. Our results show that above 6 K the homogeneous linewidth is mainly due to an elastic two-phonon process which results in a slow broadening with temperature, the homogeneous linewidth reaching only 25 kHz at 10 K. At lower temperatures, interactions with $^{89}$Yb nuclear spin-flips, paramagnetic defects or impurities, and also Yb-Yb interactions for the higher concentrated crystal, are likely the main limiting factor to the homogeneous linewidth. In particular, we conclude that the direct effect of spin and optical excited state lifetime is a minor contribution to optical decoherence in the whole temperature range studied. Our results indicate possible paths and regimes for further decreasing the homogeneous linewidths or maintaining narrow lines at higher $^{171}$Yb concentration.

quant-ph↗

arXiv2023

Towards the device-independent certification of a quantum memory

Quantum memories represent one of the main ingredients of future quantum communication networks. Their certification is therefore a key challenge. Here we develop efficient certification methods for quantum memories. Considering a device-independent approach, where no a priori characterisation of sources or measurement devices is required, we develop a robust self-testing method for quantum memories. We then illustrate the practical relevance of our technique in a relaxed scenario by certifying a fidelity of 0.87 in a recent solid-state ensemble quantum memory experiment. More generally, our methods apply for the characterisation of any device implementing a qubit identity quantum channel.

quant-ph↗

arXiv2022

Proposal for spin squeezing in rare-earth ion-doped crystals with a four-color scheme

Achieving spin squeezing within solid-state devices is a long standing research goal, due to the promise of their particularities, for instance their long coherence times, the possibility of low-temperature experiments or integration of entanglement-assisted sensors on-chip. In this work, we investigate an interferometer-free four-color scheme to achieve spin squeezing of rare-earth ion-doped crystals. The proposal relies on an analytic derivation that starts from a Tavis-Cummings model for light-matter interaction, providing microscopic insights onto spin-squeezing generation. We evidence spin squeezing signature in the light intensity variance. We consider the two particular cases of europium- and praseodymium-doped yttrium orthosilicates, workhorses of quantum technology developments. We show that up to 8 dB of spin squeezing can be obtained with readily accessible experimental resources, including noise due to photon scattering. Our results for rare-earth ion-doped crystals add to promising properties of these platforms for manipulating many-body entangled states and for high-precision measurements.

quant-ph↗

arXiv2022

Coherent optical-microwave interface for manipulation of low-field electronic clock transitions in $^{171}$Yb$^{3+}$:Y$_2$SiO$_5$

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, but it is challenging to simultaneously and efficiently drive both optical and microwave transitions over a long crystal. In this work, we use a loop-gap microwave resonator to coherently drive optical and microwave clock transitions in $^{171}$Yb$^{3+}$:Y$_2$SiO$_5$, at close to zero external magnetic field. The low magnetic field regime is particularly interesting for interfacing these spin transitions with superconducting circuits. We achieve a Rabi frequency of 0.56 MHz at 2.497 GHz, over a 1-cm long crystal. Furthermore, we provide new insights into the spin dephasing mechanism at very low fields, showing that superhyperfine-induced collapse of the Hahn echo signal plays an important role at low fields. Our calculations and measurements reveal that the effective magnetic moment can be manipulated in $^{171}$Yb$^{3+}$:Y$_2$SiO$_5$, allowing to suppress the superhyperfine interaction at the clock transition. At a doping concentration of 2 ppm and a temperature of $3.4$ K, we achieve the longest spin coherence time of $10.0 \pm 0.4 ~\text{ms}$ reported in $^{171}$Yb$^{3+}$:Y$_2$SiO$_5$.

quant-ph↗

arXiv2022

Remote distribution of non-classical correlations over 1250 modes between a telecom photon and a $^{171}$Yb$^{3+}$:Y$_2$SiO$_{5}$ crystal

Quantum repeaters based on heralded entanglement require quantum nodes that are able to generate multimode quantum correlations between memories and telecommunication photons. The communication rate scales linearly with the number of modes, yet highly multimode quantum storage remains challenging. In this work, we demonstrate an atomic frequency comb quantum memory with a time-domain mode capacity of 1250 modes and a bandwidth of 100 MHz, to our knowledge the largest number of modes stored in the quantum regime. The memory is based on a $Y_{2}SiO_{5}$ crystal doped with $^{171}Yb^{3+}$ ions, with a memory wavelength of 979 nm. The memory is interfaced with a source of non-degenerate photon pairs at 979 and 1550 nm, bandwidth-matched to the quantum memory. We obtain strong non-classical second-order cross correlations over all modes, for storage times of up to $25$ $μs$. The telecommunication photons propagated through 5 km of fiber before the release of the memory photons, a key capability for quantum repeaters based on heralded entanglement and feed-forward operations. Building on this experiment should allow distribution of entanglement between remote quantum nodes, with enhanced rates owing to the high multimode capacity.

quant-ph↗

arXiv2022

A readout-integrated time-bin qutrit analyzer for echo-based quantum memories

We present a method to project time-bin qutrits stored in an echo-based quantum memory using several successive partial readouts of the memory. We demonstrate how this scheme can be used to implement projections onto a full set of mutually unbiased bases and, therefore, enables the characterization of arbitrary quantum states. Further, we study the integration of this protocol for the case of atomic frequency comb spin-wave storage by simulating the full storage process and performing a storage experiment with bright time-bin pulses in $^{151}$Eu$^{3+}$:Y$_2$SiO$_5$. In this context, a compound pulse for implementing partial readouts in quick succession is introduced and characterized.

quant-ph↗

arXiv2022

Storage of photonic time-bin qubits for up to 20 ms in a rare-earth doped crystal

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 $^{151}$Eu$^{3+}$:Y$_2$SiO$_5$ 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 $F=(85\pm 2)\%$ for an average number of photons per qubit of $μ_\text{in}$ = 0.92$\pm$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.

quant-ph↗

arXiv2022

Multimode capacity of atomic-frequency comb quantum memories

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 Y$_2$SiO$_5$ are analyzed within this theoretical framework, and 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 both rare earh ions.

quant-ph↗

arXiv2021

Optical and spin manipulation of non-Kramers rare-earth ions under weak magnetic field for quantum memory applications

Rare-earth ion doped crystals have proven to be solid platforms for implementing quantum memories. Their potential use for integrated photonics with large multiplexing capability and unprecedented coherence times is at the core of their attractiveness. The best performances of these ions are however usually obtained when subject to a dc magnetic field, but consequences of such fields on the quantum memory protocols have only received little attention. In this article, we focus on the effect of a dc bias magnetic field on the population manipulation of non-Kramers ions with nuclear quadrupole states, both in the spin and optical domains, by developing a simple theoretical model. We apply this model to explain experimental observations in a ${}^{151}$Eu:Y$_2$SiO$_5$ crystal, and highlight specific consequences on the AFC spin-wave protocol. The developed analysis should allow to predict optimal magnetic field configurations for various protocols.

quant-ph↗

arXiv2020

Optical storage for 0.53 seconds in a solid-state atomic frequency comb memory using dynamical decoupling

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 one millisecond, realized in a Eu${}^{3+}$ doped Y${}_2$SiO${}_5$ crystal at zero applied magnetic field. Motivated by studies showing increased spin coherence times under applied magnetic field, we developed a 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.

quant-ph↗

arXiv2020

Optical spin-wave storage in a solid-state hybridized electron-nuclear spin ensemble

Solid-state impurity spins with optical control are currently investigated for quantum networks and repeaters. Among these, rare-earth-ion doped crystals are promising as quantum memories for light, with potentially long storage time, high multimode capacity, and high bandwidth. However, with spins there is often a tradeoff between bandwidth, which favors electronic spin, and memory time, which favors nuclear spins. Here, we present optical storage experiments using highly hybridized electron-nuclear hyperfine states in $^{171}$Yb$^{3+}$:Y$_2$SiO$_5$, where the hybridization can potentially offer both long storage time and high bandwidth. We reach a storage time of 1.2 ms and an optical storage bandwidth of 10 MHz that is currently only limited by the Rabi frequency of the optical control pulses. The memory efficiency in this proof-of-principle demonstration was about 3%. The experiment constitutes the first optical storage using spin states in any rare-earth ion with electronic spin. These results pave the way for rare-earth based quantum memories with high bandwidth, long storage time and high multimode capacity, a key resource for quantum repeaters.

quant-ph↗

arXiv2019

Coherence Time Extension by Large Scale Optical Spin Polarization in a Rare-Earth Doped Crystal

Optically addressable spins are actively investigated in quantum communication, processing and sensing. Optical and spin coherence lifetimes, which determine quantum operation fidelity and storage time, are often limited by spin-spin interactions, which can be decreased by polarizing spins in their lower energy state using large magnetic fields and/or mK range temperatures. Here, we show that optical pumping of a small fraction of ions with a fixed frequency laser, coupled with spin-spin interactions and spin diffusion, leads to substantial spin polarization in a paramagnetic rare earth doped crystal, $^{171}$Yb$^{3+}$:YSO. Indeed, up to more than 90 % spin polarizations have been achieved at 2 K and zero magnetic field. Using this spin polarization mechanism, we furthermore demonstrate an increase in optical coherence lifetime from 0.3 ms to 0.8 ms, due to a strong decrease in spin-spin interactions. This effect opens the way to new schemes for obtaining long optical and spin coherence lifetimes in various solid-state systems such as ensembles of rare earth ions or color centers in diamond, which is of interest for a broad range of quantum technologies.

cond-mat.mtrl-sci↗