I am Kenichi Kawaguchi, Project Director of the Modular Quantum Computing Project. In this article, I will explain in detail the world's first multi-qubit operation of an NV module using Cryo-CMOS, achieved through a collaborative research effort between Fujitsu, Delft University of Technology, and QuTech (1). This achievement was presented at ISSCC in February 2026, the highest-level international conference in the circuit field, and a press release has been issued on QuTech's official website (2).
1. Overview of the Latest Achievement
First, let's provide an overview of our latest achievement. Cryo-CMOS refers to CMOS integrated circuits designed and manufactured to operate in cryogenic environments (below a few Kelvin (K) in absolute temperature). In this instance, we successfully drove both types of qubits that constitute a quantum module in the diamond spin approach: an electron spin and nuclear spins, using Cryo-CMOS.
2. Why is Cryo-CMOS Important?
Currently, various hardware approaches for quantum computers are under research and development. Many types of qubits require cryogenic environments, cooled by a cryostat, to operate. The control systems for driving these qubits are typically placed at room temperature, and the qubit chip and control circuits are connected by cables installed within the cryostat (Figure 1 Left). In such a configuration, as the number of qubits increases, so does the number of cables connecting room temperature to the cryogenic environment. This leads to challenges such as difficulty fitting them into the limited space of the cryostat, and heat leakage through the cables, which can affect the stable operation of the qubits. If qubit control circuits can be made with Cryo-CMOS, the number of analog signal lines for controlling individual qubits can be reduced. Only a limited number of digital signal lines for driving the Cryo-CMOS would be needed to connect the cryogenic environment and room temperature, significantly enhancing scalability (Figure 1 Right).
3. Previous Cryo-CMOS for Diamond Spins and the Newly Developed Technology
Two years ago, in our collaborative research, we successfully demonstrated the first driving of an NV center electron spin qubit using Cryo-CMOS (3). However, we had not yet achieved the driving of nuclear spin qubits, which are another type of qubit constituting the diamond spin quantum module. A major breakthrough this time is the realization of a Cryo-CMOS that can drive both electron spin qubits and nuclear spin qubits by simply placing a single coil near the qubits. A characteristic of nuclear spin qubits is that multiple of them can exist within a single quantum module, allowing for the use of up to about 10 qubits. Each nuclear spin qubit can be manipulated by applying an RF wave at a resonant frequency determined by its positional relationship with the NV center. We have now developed a Cryo-CMOS circuit capable of driving 10 nuclear spin qubits, which is the maximum number used in a single quantum module. For the electron spin driving circuit, we have adopted a more flexible method for controlling multiple quantum modules (NV centers). In the previous circuit, the frequency of the microwave generated by the electron spin driving circuit was fixed (one type), and adjustments to the qubit's resonant frequency were made using a magnetic field circuit. This approach necessitated an independent magnetic field control circuit for each module. This time, we have newly enabled variable microwave frequencies, allowing for efficient driving of electron spin qubits without the need for fine-tuning the DC magnetic field. Specifically, by adopting a method that mixes an intermediate frequency generated by a digital aggregation scheme with a local oscillator, it is now possible to efficiently drive the electron spin qubits of each module even if they are not perfectly aligned, simply by changing the intermediate frequency for each module.
4. NV Module Driving with the Developed Cryo-CMOS Circuits
We mounted the newly developed Cryo-CMOS chip and a diamond chip containing NV centers on a single PCB. The Cryo-CMOS was connected to metal wiring placed near the NV centers to enable NV module driving, and experiments were conducted at approximately 6 K (Figure 2). Laser lights were used for spin initialization and readout. We successfully performed quantum state manipulation by driving an electron spin qubit with a resonant frequency of approximately 2.5 GHz at a Rabi frequency of approximately 2 MHz, and two nuclear spin qubits with resonant frequencies near 2 MHz at Rabi frequencies of approximately 1-2 kHz, respectively. Furthermore, we were able to perform dynamical decoupling with Cryo-CMOS, which is commonly used in room-temperature control systems to extend the coherence time of electron spins by isolating them from environmental spins. We confirmed that the electron spin coherence time could be extended from 0.8 ms to 50 ms. Additionally, by applying Gate Set Tomography, we achieved quantum gate fidelities of 99.3% for electron spins and 99.8% for nuclear spins.
5. Outlook
A persistent challenge for Cryo-CMOS lies in power consumption. The power consumption of the Cryo-CMOS circuit developed this time is approximately 45 mW for the electron spin driving circuit and 17 mW for the nuclear spin driving circuit. Even though diamond spin qubits benefit from operating at relatively high temperatures that do not require dilution refrigeration, allowing the use of sample stages with higher cooling power, it is always desirable to minimize the increase in power consumption as the number of qubits grows. Furthermore, in the collaborative research project between Fujitsu and Delft University of Technology, we are developing multi-module quantum chips using optical interconnects. The next crucial step will be to further reduce Cryo-CMOS power consumption and demonstrate multi-module operation using the newly developed quantum chips. Fujitsu will continue to challenge itself in the technological development of scalable quantum computers, together with Delft University of Technology and QuTech.
(2) Scalable diamond Quantum Computing with cryogenic chip integration - QuTech
