International Collaboration in Chemistry: Suppressing Decoherence in Crystals Through Controlled Chemical Disorder – DISCRYS

A series of Er:Y2SiO5 crystals, a system known for its good coherence properties, will first be grown by high temperature techniques (Czochralski pulling) in which specific co-doping will induce a controlled disorder. We will then study static disorder through inhomogeneous linewidth measurements, and dynamical disorder by photon echo techniques. Dynamical measurements will be performed on different timescales and as a function of temperature and magnetic field to reveal the different contributions to dephasing, in particular those linked to chemical bond instabilities. Moreover, electron spin resonance will directly determine Er3+ electron spin inhomogeneous linewidths.
From these measurements, a quantitative model of the effects of weak disorder on coherence lifetimes will be designed. It will include analysis of static disorder through the inhomogeneous widths and shapes of optical and spin transitions, as well as detailed analysis of coherence lifetime measurements. Our goal is to be able to predict spectroscopic properties obtained from material parameters, like co-dopant nature and concentration, in order to direct the growth effort. Finally optimized materials will be used to build a quantum memory operating at 1.5 µm with three orders of magnitude increase in the storage time–processing bandwidth product over existing systems.

During this period, several Er3+ doped Y2SiO5 (YSO) single crystals were grown by Czochralski pulling with different Er3+ concentrations and/or codopants. Er,Sc:YSO was identified as the most promising system and we carried out a detailed study by optical and EPR spectroscopies. The optical inhomogeneous linewidth shows a 40-fold increase in 0.003 Er, 0.1 Sc:YSO compared to samples doped with Er3+ only, reaching about 20 GHz at zero magnetic field. In the same way, a strong increase in the EPR linewidth is observed, as shown in Fig. 1. To our knowledge, this is the first time that direct evidence of EPR line broadening is reported in a crystal with induced disorder. Coherence lifetimes (T2) were recorded for magnetic fields between 0 and 3 T. At the highest field, T2 is about 200 µs, a value comparable to what is found in non co-doped crystals. These data are well accounted for by the spectral diffusion model developed at MSU, which shows that Er spin flip-flops do not dominate decoherence processes. Moreover no signs of disorder induced decoherence was found.
In summary, measured optical linewidths and coherence lifetimes suggest that this crystal is a promising candidate for broadband and long storage time quantum memories at 1.5 µm (manuscript in preparation).
MSU has also carried out extensive measurements of spectroscopy and optical decoherence for different erbium-doped materials, conclusively verifying and extending their proposed model for suppressing decoherence and spin dynamics through engineered chemical disorder. This work allowed quantitatively explaining unexpected behavior observed in quantum memory demonstrations with erbium-doped glass fibers (E. Saglamyurek et al., Phys. Rev. B, 2015).

For the next period, we plan to further develop Er3+ doped crystals, especially to increase optical depth and investigate quantum storage in optimized samples.

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N. Kunkel, J. Bartholomew, L. Binet, A. Ikesue, and P. Goldner, «High-Resolution Optical Line Width Measurements as a Material Characterization Tool,« J. Phys. Chem. C 120, 13725–13731 (2016).
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The goal of DISCRYS is to use controlled chemical disorder in crystals to suppress decoherence, i.e. processes that corrupt quantum states. This approach will be developed with high quality rare earth doped crystals where disorder will be induced by co-doping with specific impurities. Our main challenges are growing crystals with very low level of optical decoherence; quantitatively understanding the relation between material composition, disorder and decoherence; and ultimately demonstrating high performance storage of telecom wavelength photons in optimized samples.
Quantum information science (QIS) uses specific properties of quantum systems to process, store and transmit data in ways that are impossible to achieve with classical systems. As it fundamentally requires superposition states that remain uncorrupted during the storage and processing of information, only systems with low decoherence are useful in QIS. In this field, quantum light-matter interfaces, called quantum memories are urgently needed for applications in quantum computing, metrology and single photon sources. In addition, quantum memories are essential to extend quantum cryptography, an already commercial technology for extremely secure communications, over long distances.
Rare earth doped crystals have been recently identified as very promising systems in QIS but their performance is still limited by decoherence that affects their optical transitions. At low temperature, this decoherence is due to fluctuating nuclear and electronic spins in the rare earth environment. To address this issue, complex experimental setups and protocols have been put forward. In contrast, DISCRYS proposes to suppress decoherence by a solid-state chemistry approach. By inducing controlled disorder in high quality crystals, resonance between neighboring spin transition can be disrupted, which inhibits relaxation by simultaneous spin flips and finally eliminates decoherence.
DISCRYS will combine single crystal growth, advanced optical spectroscopy and electron paramagnetic resonance, as well as modeling to reach a quantitative understanding of the relations between disorder and decoherence. We will further demonstrate the effectiveness of our material chemistry approach by building a quantum memory operating at the 1.5 µm telecom wavelength, ideally suited for fiber based long distance quantum cryptography. We expect optimized crystals to allow an improvement of three orders of magnitude in memory performance over existing systems.
The results of DISCRYS will be directly useful to groups working on rare earth doped materials for applications in QIS, and more generally to the broad scientific community dealing with quantum coherence in semi-conductors, other impurities in solids or molecules for applications in QIS, sensing, biology etc.. DISCRYS also addresses a key material issue in the development of quantum memories for long-range quantum cryptography. Our project will enable quantum memories compatible with existing fiber telecom networks and therefore has a large potential impact on quantum cryptography technology.

DISCRYS gathers world-leading teams in optical material growth and design, spectroscopy, modeling, and optical quantum information processing. Our international collaboration will combine expertise from solid-state chemistry to quantum physics, and will be essential to achieve major advances in materials for quantum information science.