Light meets superconducting circuits

Leading technology companies such as Google, Microsoft, and IBM, have been investing heavily in quantum computing systems based on microwave superconducting circuit platforms and have been looking to scale them in order to develop commercial computing platforms.

A successful quantum computer requires a significant numbers of qubits, the building blocks of quantum computers, so that it can store and manipulate quantum information.

However, quantum signals can be contaminated by thermal noise generated by the movement of electrons and, in order to prevent this, superconducting quantum systems need to operate at ultra-low temperatures – less than 20 milli-Kelvin – which is achieved using cryogenic helium-dilution refrigerators.

The output microwave signals from such systems are amplified by low-noise high-electron mobility transistors (HEMTs) at low temperatures. Signals are then routed outside the refrigerator by microwave coaxial cables, which are the easiest solutions to control and read superconducting devices but are poor heat isolators, and take up a lot of space. This becomes a critical problem when companies are looking to scale up qubits in the thousands in order to develop commercial platforms.

In response, researchers at EPFL’s School of Basic Sciences have developed a novel approach that uses light to read out superconducting circuits, overcoming the scaling challenges of quantum systems.

The scientists replaced HEMT amplifiers and coaxial cables with a lithium niobate electro-optical phase modulator and optical fibres respectively. Microwave signals from superconducting circuits modulate a laser carrier and encode information on the output light at cryogenic temperatures. Optical fibres are about 100 times better heat isolators than coaxial cables and are 100 times more compact. This enables the engineering of large-scale quantum systems without requiring enormous cryogenic cooling power. In addition, the direct conversion of microwave signals to the optical domain facilitates long-range transfer and networking between quantum systems.

“We’ve demonstrated a proof-of-principle experiment using a novel optical readout protocol to optically measure a superconducting device at cryogenic temperatures,” said Amir Youssefi, a PhD student working on the project. “It opens up a new avenue to scale future quantum systems.”

To verify this approach, the team performed conventional coherent and incoherent spectroscopic measurements on a superconducting electromechanical circuit, which showed perfect agreement between optical and traditional HEMT measurements.

Although this project used a commercial electro-optical phase modulator, the researchers are currently developing advanced electro-optical devices based on integrated lithium niobate technology to significantly enhance their method’s conversion efficiency and lower noise.

The results of this work were first published in Nature Electronics.