Quantum Processor Reveals Photon Bound States Remain Strong Even Amid Chaos

The researchers used a quantum processor to make the microwave photons unusually sticky. They coaxed them into clumping together into bound states, then discovered that these photon 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 these platforms are playing in the study of quantum dynamics.

Photons – quantum packets of electromagnetic radiation like light or microwaves – generally do not interact with each other. Two crossed flashlight beams, for example, cross undisturbed. But in a network of superconducting qubits, microwave photons can be made to interact.

In “Robust Bound State Formation of Interacting Photons,” published today in Nature, Google Quantum AI researchers describe how they engineered this unusual situation. They studied a ring of 24 superconducting qubits that could host microwave photons. By applying quantum gates to pairs of neighboring qubits, photons could move by hopping between neighboring sites and interacting with nearby photons.

Interactions between photons affected their so-called “phase”. The phase keeps track of the oscillation of the photon’s wave function. When the photons do not interact, their phase accumulation is rather uninteresting. Like a well-rehearsed choir, they are all in sync with each other. In this case, a photon which was initially next to another photon can move away from its neighbor without becoming desynchronized.

Just as each person in the choir contributes to the song, each possible path the photon can take contributes to the photon’s overall wave function. A group of photons initially clustered at neighboring sites will evolve into a superposition of all the possible paths that each photon could have taken.

When photons interact with their neighbors, this is no longer the case. If a photon jumps away from its neighbor, its rate of phase accumulation changes, becoming out of sync with its neighbors. All the paths in which the photons separate overlap, resulting in destructive interference. It would be as if each member of the choir were singing to their own beat – the song itself was washed out, becoming indistinguishable through the din of individual singers.

Of all the possible configuration paths, the only possible scenario that survives is the configuration in which all photons stay together in a bound state. This is why the interaction can reinforce and lead to the formation of a bound state: by eliminating all other possibilities in which the photons are not bound together.

To show rigorously that the bound states did indeed behave like the particles, with well-defined quantities such as energy and momentum, the researchers developed new techniques to measure how the energy of the particles changed with the momentum. By analyzing how the correlations between the photons varied with time and space, they were able to reconstruct the so-called “energy-momentum dispersion relationship”, confirming the particle nature of the bound states.

The existence of bound states per se was not new: in a regime called the “integrable regime”, where the dynamics are much less complicated, bound states were already predicted and observed ten years ago.

But beyond integrability, chaos reigns. Prior to this experiment, it was reasonably assumed that bound states would collapse amid chaos. To test this, the researchers went beyond integrability by fitting the geometry of the simple ring to a more complex gear-like network of connected qubits. They were surprised to find that the linked states persisted well in the chaotic regime.

The Google Quantum AI team still doesn’t know where these bound states get their unexpected resilience from, but it could have something to do with a phenomenon called “prethermalization,” where mismatched energy scales in the system can prevent a system to reach thermal equilibrium as quickly as it would otherwise.

The researchers hope that studying this system will lead to new insights into quantum many-body dynamics and inspire more fundamental discoveries in physics using quantum processors.