Google Quantum AI Reveals the United States of Photons Stays Strong Even Amid Chaos
Using a quantum processor, the researchers made unusually sticky microwave photons. After persuading them to clump together in the United States, they found that these photon clumps survived in a regime in which they were expected to dissolve into their usual solitary states. Since the finding was made for the first time on a quantum processor, it marks the increasing 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. For example, two crossed flashlight beams pass through each other without being disturbed. However, microwave photons can be made to interact in an array of superconducting qubits.
Google Quantum AI researchers describe how they engineered this unusual situation in “Robust US Formation of Interacting Photons,” published December 7 in the journal. Nature. They investigated a ring of 24 superconducting qubits that could host microwave photons. By applying quantum gates to pairs of neighboring qubits, photons could travel by hopping between neighboring 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 do not interact, their phase accumulation is uninteresting. Like a well-rehearsed chorus, everyone is in sync with each other. In this case, a photon that was initially next to another photon can move away from its neighbor without losing synchronization. Just as each person in the chorus contributes to the song, all possible paths the photon can take contribute to the photon’s overall wave function. A group of photons initially clustered at neighboring sites will become a superposition of all the possible paths each photon could have taken.
When photons interact with their neighbors, this is no longer the case. If a photon moves away from its neighbor, its phase accumulation rate changes and it loses synchronization with its neighbors. All the paths in which the photons separate overlap, leading to destructive interference. It would be as if each member of the choir sang to their own beat: the song itself fades out and becomes impossible to discern through the din of the individual singers. Among all the possible configuration paths, the only possible scenario that survives is the configuration in which all photons remain bundled in a bound state. This is the reason why the interaction can be enhanced and lead to the formation of a bound state: by suppressing all other possibilities where the photons are not bound together.
To rigorously demonstrate that bound states behaved like particles, with well-defined quantities such as energy and momentum, the researchers developed new techniques to measure how the particles’ energy changed with momentum. By analyzing how the correlations between photons varied with time and space, they were able to reconstruct the so-called “energy-momentum dispersion relation”, confirming the particulate nature of the bound states.
The existence of bound states itself 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 linked states would fall apart in chaos. To test this, the researchers went beyond integrability by fitting the simple ring geometry to a more complex gear-shaped network of connected qubits. They were surprised to find that the linked states persisted well into the chaotic regime.
The Google Quantum AI team isn’t yet sure where these bound states derive their unexpected resistance, but it could have something to do with a phenomenon called “prethermalization,” where incompatible energy scales in the system can prevent a system from reaching the thermal equilibrium so fast. like you would otherwise.
The researchers anticipate that the study of this system will provide new insights into the quantum dynamics of many bodies and will inspire more fundamental physical discoveries using quantum processors.
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