QphoX and Bluefors Team up to Enable Optical Control of Superconducting Qubits

QphoX and Bluefors today announced the results of their recent collaboration to mature optical control of superconducting qubits. This approach has thus far been limited to academic settings, but the teams at QphoX and Bluefors have now demonstrated out-of-the-box operation of the QphoX Optical Control System with a transmon qubit at the Bluefors Quantum Applications Lab in Helsinki. The work represents an important step towards making such optical interconnects available to quantum scientists and engineers worldwide. The results have been shared on the arXiv preprint server available at https://arxiv.org/abs/2603.18780. 

A significant bottleneck for scaling up quantum processor size is the control and readout of qubits at millikelvin temperature, which necessitates connections to the room-temperature environment. Optical channels can provide a scalable, low heatload solution for realizing massively parallel qubit control. This advantage results from the vast reduction in thermal conductivity and an increase in parallelization gained moving from coaxial to fiber-based signal carriers. At the same time, this qubit control system must be compatible with preserving qubit coherence, and be stable enough to provide repeatable, high-fidelity qubit operations.

Through their collaboration, the teams at QphoX and Bluefors were able to demonstrate the compatibility of the QphoX optical control method with the Bluefors cryogenic platform. The results show the potential of the optical qubit control technique to break through current cryogenic thermal bottlenecks without compromising on performance. 

The work was carried out at the Bluefors Quantum Applications Lab in Helsinki using an LD400 system equipped with a transmon qubit. The teams compared conventional microwave-based qubit control with an optical approach, in which control and readout signals were generated at room temperature using the QphoX Optical Control System, encoded onto laser light, and transmitted into the cryostat via optical fiber. At the still flange of the dilution refrigerator, a cryogenic photodiode converted the optical signals into microwave pulses that drive and read out the qubit. For direct comparison, the same qubit could also be driven through a standard microwave input line. By operating both methods on the same device under identical conditions, the researchers were able to make a detailed comparison of microwave- and optical-based qubit control, demonstrating both the preservation and the long-term stability of qubit coherence. The scalability of the optical control system was further analyzed by in-depth thermal modelling of heatloads for large numbers of control and readout lines, revealing a potential advantage for the optical approach compared to a conventional coaxial infrastructure. 

“Optical qubit control has long held promise, but it’s through this collaboration with Bluefors that we could make the critical steps towards realising these interfaces at a large scale,” says Rob Stockill, CTO of QphoX.

“These results demonstrate that Bluefors platforms handle advanced quantum-optical measurements with ease. Our findings confirm that combining optical links with transmon qubits is a viable, and potentially even scalable approach,” says Russell Lake, Director of Quantum Applications at Bluefors.

This demonstration sets the stage for industry-wide adoption of optical control schemes for superconducting qubits, paving the way for the operation of large quantum processors via optical interconnects.