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—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 "Formation of robust bound states of
interacting photons," published today in Nature, 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
so-called "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. Like a well-rehearsed choir, they're all
in sync with one another. In this case, a photon that was initially next to
another photon can hop away from its neighbor without getting out of sync.

Just as every person in the choir contributes to the
song, 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.
It would be like each choir member singing at their own pace—the song itself
gets washed out, becoming 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 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 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 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 a phenomenon called "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.

Reference: Nature

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