Coherent microwave-to-optical transduction with Yb:YSO spins strongly coupled to a 3D resonator

In the quest to build a quantum internet, scientists face a fundamental disconnect between two powerful technologies. On one side are superconducting qubits, the tiny, ultra-fast processors that currently lead the race for quantum computing power. These machines operate at microwave frequencies and must be kept at temperatures near absolute zero to function. On the other side are optical signals, beams of light that can carry information across vast distances through fiber-optic cables with almost no loss. The challenge is that these two languages of information do not speak to each other. To connect a quantum computer to a global network, researchers need a translator, or transducer, that can convert the microwave signals from the computer into light signals for travel, and back again, without destroying the delicate quantum information they carry.

This translation is difficult because the energy difference between a microwave photon and an optical photon is enormous. Furthermore, the translator must operate in the same freezing environment as the computer and cannot rely on strong magnetic fields, which would disrupt the computer's operation. While some materials have shown promise, they often require magnetic fields that are incompatible with superconducting circuits. A team of researchers at the University of Calgary has now turned their attention to a specific crystal doped with ytterbium ions. This material naturally possesses the right properties to bridge the gap without needing external magnetic fields, offering a potential path to integrate quantum memory and signal conversion into a single, efficient device.

The researchers set out to demonstrate this conversion using a crystal of yttrium orthosilicate, a solid material infused with a small amount of ytterbium. They placed a tiny piece of this crystal, measuring just a few millimeters in size, inside a specialized 3D microwave resonator. This resonator is a hollow metal structure shaped with loops and gaps, designed to trap and amplify microwave signals. The crystal was cooled to a temperature of 30 millikelvin, colder than deep space, to ensure the atoms inside remained stable. The team then fired a continuous beam of laser light into the crystal while simultaneously sending microwave signals through the resonator. The goal was to see if the microwave energy could be absorbed by the atoms and re-emitted as a new color of light, effectively translating the signal from one form to another.

The experiment succeeded in producing this conversion, though the process was currently quite faint. The team measured an internal efficiency of 2 times 10 to the negative 8, meaning that for every hundred million microwave photons sent in, only a tiny fraction emerged as converted optical photons. Despite this low number, the result was significant because it proved the mechanism worked in a continuous stream of data, rather than just in short bursts. The system operated with a bandwidth of 200 kilohertz, a measure of how much information it could handle at once. Crucially, all of this happened without applying any external magnetic field, relying instead on the natural internal structure of the ytterbium atoms to create the necessary energy levels for the conversion.

To understand why the conversion was working, the researchers examined how the atoms in the crystal interacted with the microwave field. They observed a phenomenon known as an avoided crossing, where the energy levels of the atoms and the resonator repel each other when they get close in frequency. This repulsion is a clear signature that the microwave field and the atomic spins are strongly coupled, exchanging energy back and forth faster than they lose it to their surroundings. By measuring the gap between these energy levels, the team calculated a coupling strength of 850 kilohertz. This strong connection confirmed that the crystal was not just passively sitting in the microwave field but was actively participating in the quantum exchange required for transduction.

The team also investigated how many atoms were actually participating in the process. By increasing the power of the microwave signal, they found a point where adding more power stopped increasing the conversion and actually caused the signal to drop. This saturation point indicated that the atoms had been fully excited and could not absorb any more energy. From this behavior, they estimated that only a tiny fraction of the atoms in the crystal, roughly one in a thousand, were being used at any given moment. This suggested that the current setup was not using the full potential of the material, likely because the laser beam was too wide and the microwave field was spread out over a large volume.

Looking ahead, the researchers used their data to calculate what would happen if they optimized the system. They found that simply increasing the concentration of ytterbium in the crystal from 5 parts per million to 50 parts per million could boost the efficiency significantly. They also suggested that focusing the laser beam more tightly and improving the electrical connections of the resonator could lead to further gains. Based on these calculations, they predict that with these adjustments, the internal efficiency could rise to 10 to the negative 4. While this is still a small number, it represents a thousand-fold improvement over their current results and moves the technology closer to a level where it could be useful for real-world quantum networks.

The work demonstrates that ytterbium-doped crystals are a viable candidate for quantum transduction, particularly because they function without the magnetic fields that plague other materials. The researchers have shown that the basic physics works, that the atoms can be strongly coupled to the microwave field, and that the conversion can happen continuously. While the current efficiency is low, the path to improvement is clear. By refining the crystal composition and the optical setup, this platform could eventually serve as a bridge, allowing the powerful quantum computers of the future to speak the language of light and connect to a global quantum internet.