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 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 that these platforms are playing in studying quantum dynamics.

Photons typically don’t interact with one another. But in an array of super­conducting qubits, microwave photons can be made to interact. Now, researchers at Google Quantum AI 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 travel around by hopping between neighboring sites and interacting with nearby photons.

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

When photons interact with their neighbors, this is no longer the case. If one photon hops away from its neighbor, its rate of phase accu­mulation changes, becoming out of sync with its neighbors. 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. This is why interaction can enhance and lead to the formation of a bound state: by suppressing all other possi­bilities in which photons are not bound together.

To rigorously show that the bound states indeed behaved just as 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 analyzing how the corre­lations between photons varied with time and space, they were able to reconstruct the 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 an integrable regime, where the dynamics is much less complicated, the bound states were already predicted and observed ten years ago. But beyond inte­grability, chaos reigns. Before this experiment, it was reasonably 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 were surprised to find that bound states persisted well into the chaotic regime. 

The team at Google Quantum AI is still unsure where these bound states derive their unexpected resilience, but it could have something to do with prethermali­zation, 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. (Source: Google Quantum AI)

Reference: A. Morvan et al.: Formation of robust bound states of interacting microwave photons, Nature 612, 240 (2022); DOI: 10.1038/s41586-022-05348-y

Link: Google Research, Mountain View, USA