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

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

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.

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