Quantum computers are able to solve certain problems far faster than any conventional computer, a feat known as quantum advantage. But simply owning a quantum computer doesn’t guarantee that advantage. What really matters is how the quantum bits, or qubits, inside the machine are prepared.
Each specific arrangement of qubits is called a quantum state, and only some of these states have the special properties needed to outperform conventional computers. One such property is nonstabilizerness, or what physicists call “magic.” The more magic a quantum state has, the more potential it carries for enabling powerful, high-speed computations.
In a new study published in Physical Review Letters, NYU Shanghai Associate Professor of Physics Tim Byrnes and Visiting Associate Professor of Practice in Computer Science Chandrashekar Radhakrishnan, with their team, have developed a faster, more versatile way to measure this magic, even in imperfect, real-world quantum states.
“There are already ways to measure magic,” said Radhakrishnan. “But many only work for perfectly clean states, and others become too slow as systems grow. Our method works for both pure and imperfect mixed states, and it’s much more efficient.”
The team’s method introduces a mathematical tool called a magic monotone, which can detect whether a state has magic and quantify how much it has. Their approach is based on a clever use of geometry: picture all “non-magical” states forming a shape known as the stabilizer polytope. The distance between a state and this geometric shape reveals how much magic it contains.
“The tricky part,” Radhakrishnan explained, “is that you can’t just draw the shape once you get to higher dimensions. We had to find the right mathematical formulation and create algorithms to compute it efficiently.”
Tests confirmed the new approach works — and does so much faster than previous methods.
“Knowing how to measure a quantum resource is the first step to using it,” said Professor Byrnes. “With this tool, we can now start exploring applications where magic plays a direct role in powering quantum technologies.”
The team hopes the magic monotone will help researchers identify the quantum states best suited for tasks like quantum computing, cryptography, and simulation — pushing the limits of what these machines can do.