April 20, 2026 -- Superconducting qubits—bits of quantum information—have been widely considered a promising technology for moving quantum computing forward. But there’s still much work to be done before they can be brought out of a near absolute zero temperature environment. The lab of Professor Hong Tang has recently published two studies that advance the technology.

Infinite qubit storage

To solve practical problems, quantum processors need a lot of qubits - up to thousands to millions. Such a large number of qubits requires significantly complex wiring and a way to store them at a temperature colder than deep space. This is complicated by the physical size the cryogenic devices, known as dilution refrigerators, that maintain qubits at a temperature just above absolute zero. In a study published in Nature Photonics, Tang’s research team has found a way around this obstacle.

A flexible and cost-effective solution is to build a quantum network by connecting qubits inside separate refrigerators. Connecting qubits with standard coaxial cables, however, wouldn’t work if those cables are kept in a room temperature environment. And storing them all in one very cold room would be near impossible. Even under an optimistic assumption of 1,000 qubits per refrigerator, scaling to 1 million qubits would require linking 1,000 refrigerators—an arrangement that is physically impractical within a single room.

“Most of the quantum computing systems are based on superconducting qubits that live in dilution refrigerators, but the number of qubits that you can access or control is limited,” said Tang, the Llewellyn W. Jones Jr. Professor of Electrical & Computer Engineering and Physics. “That limitation is imposed by the cooling power of individual refrigerators. So what we are doing is establishing a link between refrigerators.”

Which is not as straightforward as it sounds. “If you want to communicate between two refrigerators, you can use a cable, but that cable has to be cold.”

That is, if the cable isn’t maintained at a temperature just above absolute zero, the qubits—composed of microwave photons—would lose their quantum state while passing from one refrigerator to the next. But if those refrigerators are at different locations—say, a kilometer apart—a super cold cable isn’t feasible.

So Tang and his team developed a system that converts the microwave photons to optical photons, which are used in fiber optics and other applications, and do not require super cold temperatures.

“Optical photons don't care what the temperature around them is,” Tang said. “They can go across the optical fiber and deliver to the remote site. Then we convert them back to microwave photons.”

Making that conversion is tricky because there’s a big difference in energy levels between microwave photons and optical photons. Tang’s lab built a device that simultaneously confines the optical field and electric field together, causing the light and microwaves to couple strongly.

“The information encoded in microwave circuits can then be brought into optical circuits, and you can then lift the room temperature and propagate the optical circuits in a fiber,” Tang said.

Building qubits, one layer at a time

For another study, Tang and his lab borrowed a technique from the semiconductor industry, atomic layer deposition (ALD), to build qubits for quantum computing.

ALD allows manufacturers to build materials, one atom at a time. It’s most commonly used to make increasingly small computer chips. But Tang’s lab has shown that it can also be used to create qubits. Their results are published in Nature Materials.

Similar to how a 3D printer works, the process creates the qubits precisely and consistently. And because it uses the same equipment that is used in industry, the process can easily be put to commercial use. Specifically, the Tang lab used ALD to create a qubit made of two layers of superconducting niobium nitride separated by one ultrathin layer of aluminum nitride.

“Each layer grows through carefully controlled chemical reactions, allowing the thickness and quality of the structure to be precisely defined,” said Danqing Wang, a Ph.D. student and lead author of the study.

The type of material they’re using is key to the success of their method. Conventional superconducting qubits use aluminum as its main electrodes, which has a critical temperature of 1 Kelvin (that is, the temperature at which the material becomes a superconductor). The nitrides used by Tang’s lab, though, have a critical temperature of about 13 Kelvin— still cold, but much warmer than the aluminum-based materials.

“So we can even go at a much higher temperature,” Tang said. “That will save a lot of cost because the equipment needed to store the qubits is much less expensive. That also means you have better scalability. And this material is compatible with semiconductor foundries, so you can make a large-scale wafer foundry as opposed to doing it in a typical university cleanroom.”

Going forward, Wang said, the lab will focus on refining the material’s quality to improve the qubits’ performance and explore higher-temperature operations.