Hyperfine and Antiferromagnetic Resonances Coupling with Optical Transitions in Rare Earth Crystals for quantum information

Project Information

Harcotrec

Grant agreement ID: 101066781

DOI

10.3030/101066781

Project closed

EC signature date 4 August 2022

Start date 1 September 2022

End date 31 August 2024

Funded under

• Marie Skłodowska-Curie Actions (MSCA)

Total cost

No data

EU contribution

€ 211 754,88

Coordinated by

CENTRE NATIONAL DE LA RECHERCHE SCIENTIFIQUE CNRS
France

Periodic Reporting for period 1 - Harcotrec (Hyperfine and Antiferromagnetic Resonances Coupling with Optical Transitions in Rare Earth Crystals for quantum information)

Reporting period: 2022-09-01 to 2024-08-31

Summary of the context and overall objectives of the project

The potential of quantum computers to efficiently solve complex combinatorial and optimization problems holds promise to revolutionize various fields, from artificial intelligence to biochemical engineering. The Harcotrec project aimed to develop a "quantum transducer," a device designed to translate quantum information from microwave to optical frequencies. This type of device is crucial for enabling communication between quantum processors over long distances. Quantum processors based on superconducting qubits operate at microwave frequencies, which are not easily transmitted over long distances due to high signal loss. Optical photons, on the other hand, can be transmitted over fiber-optic networks with minimal loss, making them ideal carriers of quantum information. The transducer would thus act as a bridge, converting quantum information encoded in microwave photons into optical photons, facilitating the interconnection of quantum processors across large distances and laying the groundwork for a quantum internet.
This network would exponentially enhance computational capacity by enabling distributed quantum computing and secure data transmission through quantum cryptography. However, a major technological hurdle is the low performance of current microwave-to-optical transducers. This project, therefore, aimed to develop a transducer with high efficiency and bandwidth, advancing the strategic objective of interconnected quantum computing systems.
Among the various platforms and approaches for realizing a quantum transducer, electro-optic conversion schemes in crystals with low concentrations of rare-earth elements are among the most promising. Despite this potential, the current conversion efficiency remains below 1%, falling short of practical application requirements. The project aimed to enhance conversion efficiency by focusing on stoichiometric rare-earth crystals—crystals with a significantly high density of rare-earth ions. According to theoretical models, conversion efficiency is expected to scale quadratically with the ion density. Therefore, the use of stoichiometric crystals could potentially increase the conversion efficiency by several orders of magnitude.
The project comprised three main work packages. The first was to identify and grow high-quality stoichiometric rare-earth crystals. The second focused on characterizing the essential properties of these crystals for the conversion scheme, specifically aiming for narrow optical and microwave transition linewidths at cryogenic temperatures. The final work package involved equipping the cryogenic setup with microwave cavities and optical systems, and studying the microwave-to-optical conversion process in terms of efficiency, bandwidth, and noise levels.
Work packages 1 and 2 were especially critical, as high-resolution optical spectroscopy studies on these materials are limited. Thus, successfully growing and characterizing the optimal crystal was essential to the project’s success.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Through extensive literature research, several hydrated rare-earth crystals, particularly those containing praseodymium ions, were chosen for their favorable crystal structure and optical quality. A specialized crystal growth apparatus was developed to facilitate the slow cooling and controlled evaporation processes necessary to grow defect-minimized crystals, which are essential for maintaining narrow linewidths. The grown crystals were initially characterized through low-temperature absorption spectroscopy to assess their optical transition properties. Additionally, the crystal structure and fluorescence were verified using a Laue diffractometer and a Raman spectrometer. Finally, the optical linewidth of the transitions of interest was precisely measured in these crystals using a tunable Dye laser and cooling the samples to ultra-low temperatures, down to 50 mK, using a dilution refrigerator.
Factors such as solution acidity, solvent type, and growth temperatures were meticulously adjusted to reduce the optical linewidth. Through this iterative process, we refined the growth techniques to minimize defects and impurities, achieving high-quality stoichiometric rare-earth crystals. Through these efforts, linewidths as narrow as 5.5 GHz were obtained in the best-performing crystals, marking significant progress toward meeting the targeted spectral properties.
Given that these crystals are prone to dehydration in vacuum environments, a substantial effort was dedicated to preventing this degradation. Specifically, a custom copper cell was designed and constructed to seal the crystals, maintaining them in a controlled atmosphere of helium or air. This cell, with its sealed optical windows, is an important step toward the future integration of the microwave-optical transduction setup. It also demonstrated the feasibility of introducing an optical field in a microwave cavity environment, crucial for the project's next phase. Despite these advances, it was not possible to proceed with Work Package 3 within the project’s timeframe.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The project achieved significant results in the pursuit of developing efficient quantum transducers, building a solid foundation for future advances in microwave-to-optical conversion. We successfully grew high-quality, stoichiometric praseodymium-doped crystals, meeting stringent requirements for resonance frequencies and linewidths essential for transduction applications. The achieved optical linewidths reached approximately 5.5 GHz in optimized crystals, approaching target specifications. In addition, several pathways have been identified to achieve even narrower linewidths. The crystal characterization also highlighted intriguing properties, such as the potential magnetic ordering of praseodymium spins, which could further narrow microwave transition linewidths by decreasing inhomogeneous broadening.
Moreover, the electrically allowed microwave transitions in these praseodymium-doped crystals show considerable promise for applications that leverage strong dipole interactions, making them ideal for strong cavity coupling and sensitive detection schemes.
Although time constraints limited the full realization of the transduction experiments, the groundwork established—including optimized materials, spectroscopy data, and cavity design considerations—provides a solid basis for future quantum transduction applications.

Stoichiometric_praseodymium_crystals