Qubits are the basic units of quantum information. Superconducting qubits could help build large-scale quantum computers, but they rely on electrical signals and are hard to scale.
Physicists at the Institute of Science and Technology Austria (ISTA) achieved a fully optical readout of superconducting qubits. By using fiber optics, they reduced the amount of cryogenic hardware needed to measure the qubits.
Co-first author Georg Arnold, a former PhD student in the Fink group at ISTA, said, “This new approach might allow us to increase the number of qubits so they become useful for computation. It also lays the foundation for building a network of superconducting quantum computers connected via optical fibers at room temperature.”
Using optics in quantum hardware is challenging. Superconducting quantum computers rely on unique properties of materials at temperatures near absolute zero. Tiny electrical circuits are cooled to extremely low temperatures, losing all electrical resistance and maintaining a continuous current indefinitely.
Arnold said, “Thus, superconducting qubits are electrical by definition. To make them, we must reach temperatures of only a few thousandths of a degree above absolute zero. That’s even colder than space.”
A New Kind of Quantum Computer
Electrical signals have low bandwidth, transmit little information, and are prone to noise and information loss. They also require colossal cryogenic and expensive components for measuring qubits, which generate a lot of heat.
Optical signals, like those at telecom wavelengths, travel through thin fibers with minimal losses, lower heat dissipation, and higher bandwidth. They are ideal for superconducting quantum hardware but must be translated for qubits.
The team needed to find a way to translate optical signals to qubits and back for a fully optical readout in superconducting quantum hardware.
“Ideally, one would try to eliminate all electrical signals, as the required wiring transports much heat into the cooling chambers where the qubits are. But this is impossible,” says co-first author Thomas Werner, a PhD student in the Fink group at ISTA.
The researchers used an electro-optic transducer to convert optical signals to microwave frequencies, which qubits can understand. The qubits reflect a microwave signal, which the transducer converts back to optics. Werner emphasized the complexity of this task.
“We showed that we can send infrared light close to the qubits without making them lose their superconductivity.”
The team was able to connect the qubits directly to the outside world by using the electro-optic transducer as a switch.
“Our technology can decrease the heat load of measuring superconductive qubits considerably. This will allow us to break the qubit barrier and scale up the number of qubits that can be used in quantum computing,” says Arnold.
Achieving a fully optical readout of superconducting qubits allowed researchers to eliminate many cumbersome electrical components. Conventional electrical readout systems are error-prone and require expensive cooling to cryogenic temperatures.
Using an electro-optic transducer to disconnect qubits from the electrical infrastructure, the team replaced the setup with optics, making the system more robust, efficient, and cost-effective.
This technology could increase the number of usable superconducting qubits and allow scientists to connect multiple quantum computers using light. Quantum computers need “dilution refrigerators” to cool the entire setup, including connections between modules.
“But these dilution refrigerators also have practical limitations and can’t be made infinitely large,” says Arnold.
Space and cooling limitations restrict the number of usable qubits. However, researchers believe connecting two qubits in separate dilution refrigerators using an optical fiber is now possible.
“The infrastructure is available, and we can now build the first simple quantum computing networks,” says Arnold.
While the ISTA physicists have made significant progress in developing superconducting quantum hardware, more work is needed. Their prototype has limited performance, especially in terms of optical power. Nevertheless, it proves that a fully optical readout of superconducting qubits is possible, and further advancements will depend on the industry.
Journal Reference:
- Georg Arnold et al., All-optical superconducting qubit readout, Nature Physics (2025). DOI: 10.1038/s41567-024-02741-4
Source: Tech Explorist
