First cryogenic electronic circuits to control diamond spin qubits

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Fujitsu, in collaboration with the Dutch research institute QuTech, has announced the development of the world's first cryogenic electronic circuits for controlling diamond-based quantum bits.

This breakthrough will enable the convenient operation of both the qubits and the control electronics in a compact cryogenic refrigerator, with the new technique addressing the ‘wiring bottleneck’ in cooling qubits while maintaining high quality performance, marking a significant milestone toward the realisation of larger-scale quantum computers.

QuTech presented the results of the joint research project with Fujitsu at the "International Conference on Solid State Device Circuits ISSCC 2024 (IEEE International Solid-State Circuits Conference 2024).

Qubits make use of extremely fragile quantum effects that are disturbed by various influences including small amounts of heat. Heat leaking into quantum computers would immediately destroy the information that a qubit was holding, rendering any quantum computer unreliable and unusable. To assure accurate operation, qubits need to be cooled down to the coldest temperatures possible, close to absolute zero kelvin (-273°C).

Accurate operation of the electronic circuits controlling the qubit represents an ongoing challenge, and conventional methods to keep qubits cold enough require a small cryogenic refrigerator, where qubits are connected with wires to the electronics outside the fridge. However, wires between the cold qubits and the room-temperature electronics significantly impact reliability, manufacturing and the size of quantum computers.

In response to this, Fujitsu in collaboration with researchers and engineers at QuTech - a collaboration between the TU Delft and TNO - have developed a new technique leveraging QuTech’s expertise in cryogenic semiconductor integrated circuit (cryo-CMOS circuit) technology and diamond spin qubit, which is more robust to heat disturbance, to successfully drive a diamond spin qubit using a cryo-CMOS circuit installed in a cryogenic refrigerator.

The new technology enables the installation of a cryo-CMOS circuit at the same temperature as a diamond spin qubit (4 Kelvin), which can simplify wiring and lead to the construction of high-performance, large-scale integrated quantum computers.

The technology cools the whole quantum computer instead of just the qubits. Leveraging cryo-CMOS circuit technology, Fujitsu together with QuTech were able to design a magnetic field application circuit and a microwave driving circuit necessary for driving a diamond spin qubit at 4 Kelvin. By driving this magnetic field application circuit and microwave driving circuit in the same cryogenic refrigerator as the qubit, Fujitsu and QuTech were able to successfully generate a magnetic field and microwaves strong enough to drive the diamond spin qubit.

The new technology simplifies wiring and may one day contribute to the realisation of high-performance, large-scale integrated quantum computers.

Commenting on the announcement Fabio Sebastiano, Lead Investigator, QuTech, said, “In designing electrical systems, there is always a balance between performance and power: the increase of one means a decrease of the other. Our challenge is obtaining high performance, while also not limiting the power consumption.”

Masoud Babaie, Principal Investigator, QuTech added, “This is crucial as too much power could overheat the cryogenic refrigerator used to keep the system at a low temperature. We used specific cryogenic electronic controllers (cryo-CMOS controllers) to alleviate the interconnect bottleneck: now we need fewer wires to enter the cryogenic fridge, which greatly enhances the scalability of the whole quantum computer.”

Dr. Shintaro Sato, Fellow, SVP & Head of Quantum Laboratory at Fujitsu Research, Fujitsu Limited, said "Wiring between control circuits and qubits is a common problem in the process of scaling up quantum computers. Results of our joint research highlight the potential of cryo-CMOS technology for diamond spin qubits to overcome this bottleneck. We anticipate that the new technology will enable us to achieve the high scalability expected in quantum computers using diamond spin qubits."

Realising cryogenic electronic circuits for controlling diamond-based quantum bits, the newly developed technology signifies a significant step toward the realization of large-scale quantum computers.

Moving forward, Fujitsu and QuTech will look to further enhance the newly developed technology, including the expansion from 1-qubit operation to 2-qubit operations, implementation of the qubit read-out functionality, and the scaling up to a larger quantum processors.