Research: bound states of photons hold strong in chaos

Researchers have used a quantum processor to make microwave photons uncharacteristically sticky; they coaxed them to clump together into bound states, then found that these proton clusters survived in a regime where they were expected to dissolve into their usual, solitary states. The discovery was first made on a quantum processor, marking the growing role that these platforms play in studying quantum dynamics.

Protons — quantum packets of electromagnetic radiation like light or microwaves — typically don’t interact with one another. Two crossed flashlight beams, for example, pass through one another undisturbed. But in an array of superconducting qubits, microwave photons can be made to interact. In a paper published in Nature, researchers at Google Quantum AI describe how they engineered this solution. By applying quantum gates to pairs of neighbouring qubits, photons could travel around by hopping between neighbouring sites and interacting with nearby photons.

The interactions between the photons affected their so-called ‘phase’. The phase keeps track of the oscillation of the photon’s wave function. When the photons are non-interacting, their phase accumulation is rather uninteresting, as they are all in sync with one another. In this case, a photon that was initially next to another photon can hop away from its neighbour without getting out of sync. Every possible path the photon can take contributes to the photon’s overall wave function. A group of photons initially clustered on neighbouring sites will evolve into a superposition of all possible paths each photon might have taken.

When photons interact with their neighbours, this is no longer the case. If one photon hops away from its neighbour, its rate of phase accumulation changes, becoming out of sync with its neighbours. All paths in which the photons split apart overlap, leading to destructive interference. Among all the possible configuration paths, the only possible scenario that survives is the configuration in which all photons remain clustered together in a bound state: by suppressing all other possibilities in which photons are not bound together.

To show that the bound states behaved just as the particles did, with well-defined quantities such as energy and momentum, researchers developed new techniques to measure how the energy of the particles changed with momentum. By analysing how the correlations between the photons varied with time and space, researchers were able to reconstruct the so-called “energy-momentum dispersion relation”, confirming the particle-like nature of the bound states. The existence of the bound states in itself was not new — in a regime called the “integrable regime”, where the dynamic is less complicated, the bound states were already predicted and observed 10 years ago. But beyond integrability, chaos reigns.

Before this experiment, it was assumed that the bound states would fall apart in the midst of chaos. To test this, the researchers pushed beyond integrability by adjusting the simple ring geometry to a more complex, gear-shaped network of connected qubits. They found that bound states persisted well into the chaotic regime. The researchers at Google Quantum AI are still unsure where these bound states derive their unexpected resilience, but it could have something to do with a phenomenon called “prethermalisation”, where incompatible energy scales in the system can prevent a system from reaching thermal equilibrium as quickly as it otherwise would. Researchers hope investigating this system will lead to new insights into many-body quantum dynamics and inspire more fundamental physics discoveries using quantum processors.

Image caption: A ring of superconducting qubits can host “bound states” of microwave photons, where the photons tend to clump on neighbouring qubit sites. Image credit: Google Quantum AI