A Bridge Between Quantum Computers And Quantum Networks: Hungarian Success In The MOCA Project

Two fundamentally different worlds must be connected before quantum computers can be integrated into a global quantum network — and the key to achieving this lies in a handful of laser-cooled atoms. As part of the international MOCA project, Hungarian researchers have built a device demonstrating that the ‘translation’ between microwave and optical signals really works.

One of the greatest technical obstacles to the quantum networks of the future is that the ‘language’ of the most promising quantum computers is fundamentally different from that used in long-distance quantum communication. Quantum processors based on superconducting circuits operate using microwave signals, but these signals cannot be transmitted over long distances through optical fibres without losing their quantum properties. For long-distance transmission, particles of light known as photons are needed. The international MOCA research project — Integrated Microwave-to-Optical Conversion by Atoms on a Superconducting Chip — set out to bridge these two worlds by developing a microwave-to-optical signal converter, with researchers from the HUN-REN Wigner Research Centre for Physics playing a key role.

Atoms as mediators
The key to the solution lies in a special property of cold atoms. Although superconducting circuits can perform the operations in a short duration of time, the quantum information stored in them ‘leaks away’ quickly. Laser-cooled atoms, by contrast, retain their quantum state for much longer and interact naturally with light. The MOCA researchers’ idea was to connect microwave and optical electromagnetic fields using a cloud of atoms. The atoms then act as mediators, transferring the information encoded in the microwave signal to the light.

The project, which has a budget of around €866,000, is funded through the QuantERA programme and involves research groups from five countries: Germany, France, Italy, Greece and Hungary. The consortium is coordinated by the University of Tübingen, while each of the other partners is responsible for developing a specific subtechnology: the French partner LP2N is designing the waveguides, the Italian partner INRIM is responsible for nanofabrication, and the Greek partner FORTH is developing the theoretical framework for the atomic measurements.

Validation at HUN-REN Wigner
While the international partners are focusing primarily on building the final miniaturised, chip-integrated architectures, the Hungarian team, led by Péter Domokos, a member of the Hungarian Academy of Sciences, has taken on a different but essential task: building a larger-scale experimental setup to test and validate the operating principle of the chip-based solutions before they are implemented in miniaturised form.

In the setup operated by the Quantum Optics Research Group of the Quantum Technology Department at HUN-REN Wigner, laser-cooled rubidium atoms are held in an optical dipole trap inside an optical cavity under ultra-high-vacuum conditions. When a microwave signal is resonant with the transition between the atoms’ hyperfine states, it changes their quantum state. This change can be detected through the interaction between the atoms and the laser light circulating in the cavity. The light leaving the cavity therefore carries the information contained in the microwave signal: the conversion has taken place.

‘We control the atom–photon interaction at the quantum-mechanical level. Through the atoms’ quantum states, the optical and microwave excitations become coupled, making it possible to achieve highly efficient signal conversion between fields with very different wavelengths,’ explained Dr András Dombi, Senior Research Fellow at HUN-REN Wigner and co-leader of the MOCA consortium.

Still being fine-tuned, but already a milestone
The complete system will require considerable further fine-tuning, but the results already achieved in the Wigner laboratory mark an important milestone. In the long term, the setup will serve as a reference against which the performance of the miniaturised chips built at the other partner institutes can be measured.

‘Fully characterising and calibrating the system’s operating parameters will still take a long time, but thanks to the diverse range of methods incorporated into the setup, even this initial phase already promises to deliver new results in the field of cold-atom cavity quantum electrodynamics,’ said Péter Domokos.

If the project achieves its goal, it will resolve more than a technical detail: in the future, it could help connect distant superconducting quantum processors through optical links, laying the foundations for a global quantum network.