The long road to large-scale quantum computers

QuTech, the advanced research center for quantum computers, will host the RF Technology event 2022 this year. One of the speakers is Fabio Sebastiano, assistant professor at TU Delft. He will give a lecture on the application of cmos microwave drivers at cryogenic temperatures. FHI spoke with the scientist.

By: Dimitri Reijerman

At QuTech, a collaboration between TU Delft, TNO and industrial partners, a lot of fundamental research is done on quantum computers and internet. Sebastiano explains his contribution: “My research focuses on cryogenic electronics to enable large-scale quantum computers. The fundamental components of quantum computers are quantum bits or qubits. For a quantum computer to work properly, qubits need to be cooled at cryogenic temperatures. However, qubits alone are not enough to build a quantum computer: they need to be connected to an electrical interface to provide electrical signals to perform operations and to measure the results of the calculations. In today’s state-of-the-art quantum computers, such an electrical interface is used at room temperature and connected to the qubits in a cryogenic chamber with long cables.”

But that’s where the complexity comes in, says Sebastiano: “This approach is feasible for the few qubits available today – less than 100 – but it will be unfeasible for thousands or millions of qubits, because of the unreliability and the volume of so many cables. And we will need millions of qubits to build quantum computers that can solve practical problems. To address this challenge, I want to bring the cryogenic electronic interface as close as possible to the qubits by applying cryogenic electronics.”

The role of cmos microwave drivers

CMOS microwave drivers thus play an important role in controlling qubits. Sebastiano explains how this works: “One of the fundamental requirements in quantum computing is to be able to generate microwave electrical signals that are sent to the qubits. By properly shaping these signals, both in amplitude and duration, a specific operation can be enforced on the qubits. The CMOS microwave drivers are electronic circuits that generate such signals to perform quantum operations on the qubits. In particular, our microwave drivers can generate signals in the frequency range of 2GHz to 20GHz. Already, to control 128 qubits, our proposed integrated circuit uses about 100 million transistors. As mentioned below, the electronics need to operate at cryogenic temperatures to alleviate the wiring requirements. It turns out that standard CMOS devices can operate at cryogenic temperatures, and with specific design techniques we were able to achieve the high performance and low power required for controlling qubits. High performance because qubits are extremely fragile and require very precise electrical signals. Low power because the cooling capacity of cryogenic refrigerators is limited.”

Fundamental research at QuTech faces many challenges. The development of a cryogenic CMOS interface is also a complex task, says the scientist: “Firstly, qubits are fragile and therefore require very precise and low-noise control signals. The electronics must therefore deliver high performance. At the same time, while high performance in a circuit usually translates into higher power consumption, in a cryogenic circuit the maximum power consumption is limited by the cooling capacity of the refrigerator. So we have to build circuits with high efficiency, i.e. achieve high performance with a limited energy budget. In order to optimize the circuit for optimal power consumption, we also have to know exactly what the qubits need. This prevents an unnecessarily complex design. To solve this, we had to study the effect of each characteristic of the electrical signal on the qubit performance and co-simulate and co-design the qubits and the electronics.”

But even the extremely low temperatures put scientists and their designs to the test, says Sebastiano: “CMOS devices are not built to operate at cryogenic temperatures, and there is no standard model to simulate their behavior at cryogenic temperatures, which is very different from room temperature. To address this issue, my group worked on the one hand to characterize CMOS devices at those temperatures and build suitable simulation models, and on the other hand to develop circuit design techniques that take such different behavior into account.”

How much longer do we have to wait?

QuTech, like other researchers, is making steady progress in their research into quantum computers. However, it will still take years before practically usable quantum computers are available, Sebastiano believes: “Functional quantum computers are already a reality. However, the main problems are the very high error rate of state-of-the-art qubits and the low number of qubits available in those machines. I think it will take another ten years before quantum computers can solve complex practical problems, that is, problems that cannot be solved by standard computers and that have practical applications, such as simulating a complex molecule or a new material.”

Meanwhile, the University of New South Wales (UNSW Sydney) is also getting interesting research done on how to embed qubits on chips using artificial atoms. Sebastiano is very interested in this research by his colleagues: “The UNSW researchers have shown a new, more reliable and robust way to build qubits based on spin qubits. This is very exciting, because I am a big fan of these types of qubits as candidates for large-scale quantum computers. Various ways of building qubits are currently being explored, but spin qubits have promising properties: they are extremely small, allowing millions to be built into a small integrated circuit, they can be operated at a relatively higher temperature – above 1K – for which better cooling capabilities are available. This means that they can in principle be integrated on the same chip alongside regular CMOS electronics. So they are the best candidate to be driven by the cryo-CMOS circuits that I am developing, and integrated together with such electronics.”

Would you like to attend Fabio Sebastiano's lecture? Sign up for free for the RF Technology event 2022.