Oxford University physicists use hybrid quantum computer to reproduce Aharonov–Bohm effect, confirming particles stop moving without contacting magnetic field

Quantum particles can move between two locations through a process called tunneling. But what happens if a particle suddenly stops moving because of a magnetic field it never directly interacted with? Physicists at the University of Oxford have used a hybrid quantum computer to reproduce this counterintuitive behavior, demonstrating the Aharonov–Bohm effect emerging in a simulated quantum system. The Aharonov–Bohm effect occurs when a particle encircles a region containing magnetic flux, experiencing a change in its quantum phase even though it never passes through the magnetic field itself.

In the Oxford experiment, this effect caused the particle's possible paths to cancel each other out, completely suppressing its tunneling between the two locations. The findings could help scientists simulate fundamental interactions that are increasingly difficult to model with classical computers.

Oxford University's experimental method

Building a quantum loop from trapped ions. In everyday physics, charged particles respond to the electric and magnetic fields they encounter. However, quantum mechanics allows more subtle phenomena. A particle encircling a region containing magnetic flux can still acquire a measurable phase change even if it never enters the region where the magnetic field resides. Physicists Yakir Aharonov and David Bohm predicted this effect in 1959, and it was subsequently confirmed in experiments with real electrons. Oxford researchers are now studying the same phenomenon's behavior in lattice gauge theory, a mathematical framework used to describe interactions between matter and fields in particle physics.

In these theories, matter occupies points on a grid, while gauge fields connect these points. As these systems grow, calculating their behavior using classical computers becomes increasingly difficult. Quantum simulation offers an alternative: researchers build a physical system that follows the underlying rules of the model they want to study, then observe its behavior.

Starting in 2022, the study authors conducted an experiment to simulate lattice gauge theory, a framework for studying how matter interacts with gauge fields. Their hybrid system combined components playing two distinct roles. Qubits encoded in the internal electronic states of trapped ions represented the gauge fields. Quantum oscillators corresponding to ion vibrations represented matter. The researchers connected two oscillators with two qubits representing the field between them, representing matter at two locations. Together these components formed a loop, a fundamental building block of the theory.

The team used digital operations to prepare and measure the system, while analog quantum evolution enabled it to simulate interactions.

Dynamic encoding of magnetic flux

Magnetic flux that cancels particle motion. The researchers prepared the two qubits in an entangled state, their quantum properties linked in a way with no classical equivalent. In the simulation, this state represented magnetic flux passing through the loop. Initially encoding flux this way was a practical workaround. The hardware could not provide the interactions needed to introduce flux as a fixed background. However, this approach also allowed researchers to explore something more interesting: a gauge field that can evolve alongside the matter it interacts with. "For us, the exciting step is encoding the flux in a gauge field that is itself dynamic,"

"Matter no longer evolves in a fixed background; instead, matter and gauge field become part of the same quantum dynamics," said Sebastian Saner, lead author and postdoctoral researcher at the University of Oxford.

The researchers then observed a matter particle tunneling between the loop's two locations. Without magnetic flux, the particle could tunnel freely. When the researchers introduced magnetic flux, the two possible paths underwent destructive interference, canceling the tunneling process and returning the system to its initial state. "At this point, we observe Aharonov–Bohm interference, using a dynamic gauge field encoding the flux, thereby demonstrating the interaction between charge and flux."

Future quantum simulation applications

Extending quantum simulation to fundamental interactions. This experiment provides a way to study how matter and magnetic fields interact in a quantum system. With further development, similar simulations could help researchers probe more complex interactions that are difficult to model with classical computers. In a related study, a team led by Professor Norbert Linke at the University of Maryland used a hybrid quantum system to simulate the Yukawa potential, relevant to nuclear and particle physics. Together, these studies show how hybrid quantum architectures can help scientists explore the fundamental forces and interactions shaping the physical world.

The study has been published in the journal Nature Physics.