What is a 'quantum break'? In the July issue of Scientific American, in a brief article by Charlie Wood on detecting gravitons by looking for long-wavelength radio waves that some of the graviton have turned into, it says,

...other massless particles can abruptly change state in large numbers (a phenomenon known as a quantum break).

Perhaps needless to say, Wikipedia only talks about that stupid video game 'Quantum Break' and its even stupider tie-in tv show...
What is a "quantum break?"
 A: Asking in google "quantum break"  arxiv gives a number of physics preprints,
This seems relevant to gravity:

Quantum break in high intensity gravitational wave interactions


The lowest order amplitudes for [graviton+graviton→photon+photon] lead to cross-sections of order G2, where G is the gravitational constant. These are too small to be of any interest. However in dense clouds of pure gravitons there are collective effects utilizing these same amplitudes that under the right circumstances can lead to copious production of photons.

In the pdf it states:

The comparative suddenness of the transition earns it the designation of "quantum break"

So it is a collective phenomenon which under the right circumstances leads to a sudden copious production of photons., as far as I can see. Quantum because it needs quantum mechanics to get the effect.
In an axion paper:

Once we include self-interaction of the axion, the quantum state evolves so that the expectation value over it starts to deviate from the classical solution. The time-scale of this process defines the quantum break-time.

So it seems to be used to define when quantum effects take over.
A: Quantum breaking is usually used for localized condensed matter systems containing atoms in BEC state.
https://www.news.ucsb.edu/2020/019817/hunt-gravitons
When we usually describe this state, we describe a homogeneous field having constant charge and energy.
It is very important to understand that classically this configuration is stable. At the quantum level, this is not true. The reason for this is that the state is a coherent state, and not a pure eigenstate of the Hamiltonian. Evolution will change this state. The departure from this stable classical state is what is called Quantum Breaking.
https://arxiv.org/abs/2003.10283
