Smart programming for the quantum computer that does not exist yet

Designing innovative algorithms, thinking outside the box, and brainstorming over coffee with his colleagues—this is what physicist Stefano Polla enjoys most. His success shines through in his nomination for the C.J. Kok Jury Award 2024 for Ph.D. thesis of the Year.

"It might seem strange, but we do not know yet what quantum computers will be useful for," says Stefano Polla. "We only know they can solve very different problems than classical computers. That is enough to convince us that we will find important applications across various fields."

From classical simulation to quantum simulation

Simulation helps researchers to find solutions to problems more quickly. In fields like aircraft design, for example, the performance of new airplanes is first tested on computers. Only the most promising designs are then tested in wind tunnels, saving significant time and money.

In the future, quantum computers might enable similar simulations for quantum systems—such as complex molecules and materials. We design software for these quantum computers and have started doing this while full-scale quantum computers are not available yet. This means we cannot test the software we design. Instead, the researchers from the Applied Quantum Algorithms Group characterize it using tools from theoretical physics, mathematics, and computer science.

Isn't it premature to start designing software for a computer that hasn't been built yet? "No, not at all," Polla responds. By preparing smart algorithms for future quantum systems, we ensure that investments in building quantum computers will pay off in the future.

Under-explored directions

"Probably it has something to do with my character, but I tend to approach research questions creatively, from less-explored angles," Polla admits. In total, he started tens of research projects during his Ph.D. "As a result, many of the paths I explored didn't perform as well as I'd hoped." However, Polla sees this as a valuable outcome of scientific research: Understanding what "does not work as expected" is extremely valuable. Although it is sometimes frustrating.

His approach also led to significant successes. Five of his new design principles passed rigorous testing and characterization and are therefore included in Polla's thesis. Polla even proposes a new application area for quantum algorithms in chemistry.

A special role for one qubit

While collecting his research results for his thesis, Polla realized that a common thread connected the design principles he proposed. "I suggested assigning a specific role to one of the qubits in the computer. One qubit, the smallest unit of quantum information, can have the power to drive complex algorithms if used carefully."

In his algorithms, this single qubit is tasked with specific functions, such as assisting in cooling the system to almost absolute zero, measuring and extracting information from the quantum system, or correcting errors as they occur.

Of course, using only a single qubit rather than many for these operations affects the system's overall capacity. "But in one of my papers, I prove that if you do it smartly, you don't lose much capacity," Polla notes.

Pandemic challenges

"The biggest challenge during my research was working through COVID," says Polla. "We often think of theoretical research as a solitary activity, but that's far from the truth. The development of ideas is usually a collaborative process. Science advances most quickly in meetings, conferences, and even chats over coffee."

During the pandemic, all meetings shifted online, and much of the spontaneity was lost. This made both research and the learning process, especially in the early years of a Ph.D., far more difficult. "Fortunately, I had strong support from my supervisors, my research group, and my friends throughout," Polla says.

Second nomination

Polla's nomination for best Ph.D. thesis of the year 2024 at the Leiden Institute of Physics is the second step in his already successful scientific career. In 2019, he received the Lorentzprijs for his master's thesis and graduated summa cum laude. After completing his bachelor's degree in Italy, he came to Leiden to deepen his knowledge of experimental physics.

He switched to theoretical research, and stayed ever since, drawn by the vibrant atmosphere and his colleagues. "There are so many wonderful people here. It's a young, diverse group, and everyone is very social," Polla says with a smile.

"Now that I've finished my Ph.D., I realize just how much I learned. Alongside this, Polla gained many technical skills. "I've become better at math and coding. And I've also learned to write more effectively—something most people don't think much about, but I believe it's important."

The differences between a quantum computer and a classical computer

A quantum computer uses quantum bits, or qubits. Unlike classical bits, qubits can be 0, 1, or any complex superposition of the two. Quantum computers use this property as well as entanglement to process information in powerful new ways. This enables them to solve problems that are inaccessible to classical computers in fields like cryptography, optimization, and simulation.

Leiden researchers greatly contribute to research on quantum technologies, developing new quantum theories, performing quantum experiments, and developing software for the quantum computer of the future.

Once those quantum computers are finally built, the hope is to use them to design new materials and drugs, or to help solve open questions in physics, chemistry, and molecular biology.

Provided by Leiden University