Fujitsu and QuTech partner on diamond qubit tech

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

QuTech will present 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), one of the largest conferences on semiconductor technology to be held in San Francisco, US, from 18 to 22 February 2024.

Background

Qubits make use of extremely fragile quantum effects that are perturbed by various influences including smallest amount 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^oC).

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.

To address this, Fujitsu in collaboration with researchers and engineers at QuTech – a collaboration between the TU Delft and TNO – 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.

Newly developed technology to cool electronics

Fujitsu in collaboration with QuTech developed a new technology that cools the whole quantum computer instead of just the qubits. Leveraging cryo-CMOS circuit technology, Fujitsu together with QuTech designed 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 successfully generated 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.

Fabio Sebastiano, Lead Investigator, QuTech, explains: “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, adds: “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.”

Future plans

Realising cryogenic electronic circuits for controlling diamond-based quantum bits, the newly developed technology signifies a significant step toward the realisation of large-scale quantum computers. Moving forward, Fujitsu and QuTech will 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.