Using ultrathin materials to reduce the size of superconducting qubits could pave the way for personal-sized quantum devices
Like transistors in a classical computer, superconducting qubits are the building blocks of a quantum computer. While engineers have been able to scale transistors down to nanometer scales, superconducting qubits are still measured in millimeters. This is one of the reasons why a practical quantum computing device could not be miniaturized to the size of a smartphone, for example.
MIT researchers have now used ultrathin materials to build superconducting qubits that are at least one-hundredth the size of conventional designs and suffer from less interference between neighboring qubits. This breakthrough could improve the performance of quantum computers and enable the development of smaller quantum devices.
The researchers demonstrated that hexagonal boron nitride, a material made up of only a few monolayers of atoms, can be stacked together to form the insulation in the capacitors of a superconducting qubit. This flawless material allows for much smaller capacitors than those typically used in a qubit, reducing its footprint without significantly sacrificing performance.
Additionally, the researchers show that the structure of these smaller capacitors should significantly reduce crosstalk, which occurs when one qubit unintentionally affects surrounding qubits.
“Right now, we can have maybe 50 or 100 qubits in a device, but for practical use in the future, we will need thousands or millions of qubits in a device. It will therefore be very important to miniaturize the size of each individual. qubit and at the same time avoid unwanted crosstalk between these hundreds of thousands of qubits. It’s one of the very few materials we’ve found that can be used in this type of construction,” says co-lead author Joel Wang, a researcher with the Engineering Quantum Systems group at MIT Research Laboratory for Electronics.
Wang’s co-lead author is Megan Yamoah ’20, a former student in the Engineering Quantum Systems group who is currently studying at the University of Oxford on a Rhodes Scholarship. Pablo Jarillo-Herrero, Cecil and Ida Green Professor of Physics, is a corresponding author, and lead author is William D. Oliver, Professor of Electrical Engineering, Computer Science, and Physics, MIT Lincoln Laboratory Fellow, Director of the Center for Quantum Engineering, and Associate Director of the Electronics Research Laboratory. The research is published today in Natural materials.
Superconducting qubits, a special type of quantum computing platform that uses superconducting circuits, contain inductors and capacitors. Just like in a radio or other electronic device, these capacitors store energy from the electric field. A capacitor is often constructed as a sandwich, with metal plates either side of an insulating or dielectric material.
But unlike a radio, superconducting quantum computers operate at super cold temperatures – less than 0.02 degrees above absolute zero (-273.15 degrees Celsius) – and have very high frequency electric fields, similar to cell phones today. Most insulating materials that operate in this regime have defects. Although this does not harm most classical applications, when quantum coherent information passes through the dielectric layer, it can be lost or randomly absorbed.
“Most dielectrics commonly used for integrated circuits, such as silicon oxides or silicon nitrides, have many defects, resulting in quality factors of around 500 to 1,000. That’s just too much. expensive for quantum computing applications,” says Oliver.
To circumvent this problem, conventional qubit capacitors are more like open-faced sandwiches, with no top plate and a vacuum placed above the bottom plate to act as an insulating layer.
“The price to pay is that the plates are much larger because you dilute the electric field and use a much larger layer for the vacuum,” says Wang. “The size of each individual qubit will be much larger than if you could fit it all in a small device. And the other problem is, when you have two qubits next to each other, and each qubit has its own electric field open to free space, there can be unwanted conversations between them, which can make it difficult to control a single qubit. is just two electrical plates with a very clean insulator sandwiched in between.”
So that’s what these researchers did.
They thought hexagonal boron nitride, which belongs to a family known as van der Waals materials (also called 2D materials), would be a good candidate for building a capacitor. This unique material can be thinned into a layer of atoms that is crystalline in structure and contains no defects. Researchers can then stack these thin layers in the desired configurations.
To test hexagonal boron nitride, they conducted experiments to characterize the cleanliness of the material when interacting with a high-frequency electric field at ultra-cold temperatures, and found that very little energy is lost when it passes through the material.
“Much of the previous work characterizing hBN (hexagonal boron nitride) has been done at or near zero frequency using DC transport measurements. However, qubits operate in the gigahertz regime. It’s great to see that hBN capacitors have quality factors over 100,000 at these frequencies, among the highest Qs I’ve seen for lithographically defined integrated parallel plate capacitors,” says Oliver.
They used hexagonal boron nitride to build a parallel plate capacitor for a qubit. To make the capacitor, they sandwiched hexagonal boron nitride between very thin layers of another van der Waals material, niobium diselenide.
The complex manufacturing process involved preparing atom-thick layers of material under a microscope, then using a sticky polymer to grip each layer and stack it on top of each other. They placed the sticky polymer, along with the stack of 2D materials, on the qubit circuit, then melted the polymer and took it away.
Then they connected the capacitor to the existing structure and cooled the qubit to 20 millikelvins (-273.13 C).
“One of the biggest challenges in the manufacturing process is working with niobium diselenide, which will oxidize within seconds if exposed to air. To avoid this, the entire assembly of this structure must be done in what we call the glove box, which is a big box filled with argon, which is an inert gas that has a very low level of oxygen in it. We have to do everything inside of that box” , says Wang.
The resulting qubit is about 100 times smaller than what they made with traditional techniques on the same chip. The coherence time, or lifetime, of the qubit is only a few microseconds shorter with their new design. And capacitors built with hexagonal boron nitride contain more than 90% of the electric field between the top and bottom plates, suggesting they will significantly suppress crosstalk between neighboring qubits, Wang says. This work is complementary to recent research by a team from Columbia University and Raytheon.
In the future, the researchers want to use this method to build many qubits on a chip to verify that their technique reduces crosstalk. They also want to improve the qubit’s performance by refining the manufacturing process, or even building the entire qubit from 2D materials.
“Now we’ve paved the way to show that you can safely use as much hexagonal boron nitride as you want without worrying too much about defects. This opens up a lot of opportunities where you can create all kinds of different heterostructures and combine them with a microwave, and there’s a lot more room for you to explore. In a way, we’re giving people the green light: you can use this material however you want without worrying too much about the loss associated with the dielectric,” says Wang.
This research was funded, in part, by the United States Army Research Office, the National Science Foundation, and the Assistant Secretary of Defense for Research and Engineering through MIT’s Lincoln Laboratory.