Quantum particles can move between two locations through a process called tunneling. However, what if a particle suddenly stopped moving because of a magnetic field it never directly interacted with?
Physicists at the University of Oxford have recreated this counterintuitive behavior using a hybrid quantum computer, demonstrating the Aharonov–Bohm effect in a simulated quantum system.
The Aharonov–Bohm effect occurs when a particle traveling around a region containing magnetic flux experiences a change in its quantum phase, even though it never passes through the magnetic field itself.
In the Oxford University experiment, this effect caused the particle’s possible paths to cancel each other out, completely suppressing its tunneling between two locations.
These findings could help scientists simulate fundamental interactions that are increasingly difficult to model using conventional computers.
Building a quantum loop from trapped ions
In everyday physics, a charged particle responds to the electric and magnetic fields it encounters. Quantum mechanics, however, allows something more subtle.
A particle can acquire a measurable phase change when it travels around a region containing magnetic flux, even if it never enters the region where the magnetic field exists. Physicists Yakir Aharonov and David Bohm predicted this effect in 1959, and experiments with real electrons later confirmed it.
The Oxford researchers are now investigating how the same phenomenon behaves in lattice gauge theories, mathematical frameworks used to describe interactions between matter and fields in particle physics. In these theories, matter occupies points on a grid, while gauge fields connect those points.
As these systems grow, calculating their behavior on conventional computers becomes increasingly difficult. Quantum simulation offers an alternative: researchers build a physical system that follows the same underlying rules as the model they want to study, then observe its behavior.
Beginning in 2022, the study authors developed an experiment to simulate a lattice gauge theory, a framework for studying how matter interacts with gauge fields.
Their hybrid system combined two components with different roles. Qubits, or quantum bits, encoded in the internal electronic states of trapped ions represented the gauge fields. Quantum oscillators, corresponding to the ions’ vibrations, represented matter.
The researchers connected two oscillators, representing matter at two locations, using two qubits representing the fields between them. Together, these components formed a loop, an elementary building block of the theory. The team used digital operations to prepare and measure the system, while analog quantum evolution allowed it to simulate the interactions.
The magnetic flux that canceled a particle’s movement
The researchers prepared the two qubits in an entangled state, in which their quantum properties are linked in a way that has no classical equivalent. In the simulation, this state represented magnetic flux passing through the loop.
Encoding the flux this way was initially a practical workaround. The hardware could not provide the interaction needed to introduce the flux as a fixed background. However, the approach also allowed the researchers to investigate something more interesting: a gauge field that could evolve alongside the matter it interacted with.
“For us, the exciting step was to encode the magnetic flux in a gauge field that was itself dynamical. Rather than having matter evolve in a fixed background, the matter and gauge field become part of the same quantum dynamics,” Sebastian Saner, lead researcher and a postdoc researcher at Oxford University, said.
The researchers then observed a matter particle tunneling between the two locations around the loop. Without magnetic flux, the particle could tunnel freely. When the researchers introduced a flux, the two possible paths interfered destructively, canceling the tunneling process and leaving the system in its starting state.
This is when “we observe Aharonov–Bohm interference with dynamical gauge fields encoding the magnetic flux, thereby demonstrating the interplay between charge and flux.”
Extending quantum simulations to fundamental interactions
The experiment offers a way to study how matter and magnetic fields interact in quantum systems. With further development, similar simulations could help researchers investigate more complex interactions that are difficult to model using conventional computers.
In a related study, a team at the University of Maryland, led by Professor Norbert Linke, used a hybrid quantum system to simulate the Yukawa potential, an interaction relevant to nuclear and particle physics.
Together, the studies show how hybrid quantum architectures could help scientists explore the fundamental forces and interactions that shape the physical world.
The study is published in the journal Nature Physics.