One step closer to quantum computers that work properly

Quantum computers are computers that are much faster at performing some important types of computational tasks than many of today's machines. Sounds perfect, right?

But for now, the building blocks that perform the calculations in the quantum computer, so-called quantum bits or "qubits", are too unstable to make quantum processors that are large enough to be really useful.

"Quantum computers are completely dependent on qubits remaining stable in order to perform the special calculations they are designed for," says Jacob Benestad, who recently earned his doctorate on qubit physics at the Norwegian University of Science and Technology's (NTNU) Department of Physics.

Quantum bits? Qubits? The terminology is a little intimidating, but not as difficult as you might think.

Not just 0 and 1, but everything in between

Ordinary computers process data in the form of what are called "bits". The impressive flow of information of images, videos and text through our computers is based on a simple process: power is either "off" or "on" in the transistors that form the brains of these machines. The off and on are translated into 0 or 1. There is nothing in between.

But quantum bits, which quantum computers use to process information, can also have intermediate forms. It's not just on or off, the information can be anything in between.At the same time, each individual qubit can "know" the state of all the other qubits. At least as long as everything works. Together, this enables quantum computers to be incredibly fast at performing certain types of calculations. 

Quantum computers have specific applications

Quantum computers will only be better than regular computers for some specific types of calculations, Jeroen Danon, a professor at NTNU’s Department of Physics and Benestad's supervisor, said.

The main difference is that a regular computer stores and processes one number at a time, while a quantum computer can work with all possible numbers simultaneously through something called "superposition".

The challenge is that when we read the answer, we only get one random result out of all the possible ones. If we want to have all the results, we have to repeat the process many times. In this case, a regular computer is more efficient. Therefore, quantum computers are useful primarily in situations where an enormous number of possibilities must be checked, but where one correct answer is needed in the end. In addition, these computers allow for many applications in optimization and simulation, Danon said.

Sensitive qubits

But these qubits have a problem.

"Quantum bits are extremely sensitive to their surroundings. Even small disturbances can cause them to lose their unique properties," says Danon, who is among the researchers working to make these qubits more stable, thus solving this problem.

Constantly adjusting qubits

"We have developed a method that monitors the qubits in real time and adjusts their frequency continuously to adjust for the disturbances from the environment," Benestad says.

Benestad worked on this method with his colleagues at NTNU, Leiden University in the Netherlands, the Niels Bohr Institute at the University of Copenhagen and the Massachusetts Institute of Technology.

With the help of what is called an FPGA controller, the system detects when the qubit begins to become unstable. Then the controller corrects the frequency of the qubit immediately, so that it remains stable.

You can compare it a bit to a guitar string. A guitar string can create very nice tones as long as it is tuned correctly, but at the same time it very quickly goes out of tune if disturbed.

The group of researchers was able to find a way to tune the guitar string in real time, while it is in use. Thus, the quantum bit keeps its tone longer and plays cleaner.

"This provides a longer lifetime, better precision and more robust quantum operations. It is an important step towards reliable quantum computers," says Danon.This finding is important because building quantum computers that will actually work requires many stable qubits, the researchers say.

Reference: F. Berritta, J. Benestad, L. Pahl, M. Mathews, J.A. Krzywda, R. Assouly, Y. Sung, D.K. Kim, B.M. Niedzielski, K. Serniak, M.E. Schwartz, J.L. Yoder, A. Chatterjee, J.A. Grover, J. Danon, W.D. Oliver, and F. Kuemmeth.  Efficient Qubit Calibration by Binary-Search Hamiltonian Tracking. PRX Quantum 6, 030335, Aug 2025.   Doi: 10.1103/77qg-p