A translator who can hear every rustle in the hall cannot translate a single word. Gravitational waves passing through ordinary space are much the same: they stay invisible to light, because conservation of energy and momentum forbids a photon from turning into a graviton. But put the light in a box with conducting walls, and the rules change. The walls break the homogeneity of space, and a coupling is born that never existed in the vacuum.
In such a cavity, a gravitational wave can rock a pair of photons the way a powerful pump does. In the classical picture, with the gravitational field treated as fixed, the photon number grows exponentially, like a snowball rolling down a hill. This growth is described by the squeezing amplitude r, which is proportional to the spacetime strain and to the interaction time. For realistic parameters, a frequency of 3.9 GHz, a strain of 10⁻²¹, and a photon lifetime of 2 seconds, r comes out to about 10⁻¹², which gives on average 3.8×10⁻²⁴ photons. That is less than a single photon over the entire lifetime of the Universe.
But when the gravitational field is quantized, the picture changes. Gravitons are the quanta of the gravitational field, just as photons are the quanta of light. In the quantum regime the pump becomes depleted: gravitons hand their energy to photons and weaken in the process. The growth of the photons saturates, and the dynamics turn oscillatory, like a pendulum swinging first one way and then the other. The characteristic time of this energy exchange is inversely proportional to the square root of the number of gravitons. If the cavity holds 10²⁴ gravitons, the interaction time shrinks by a factor of 10¹² compared with a single graviton. This is reminiscent of Dicke superradiance, in which many particles emit in concert and faster than each of them would alone.
Another important effect of the quantum regime is the loss of purity in the photonic subsystem. The photons become entangled with the gravitons, and their state ceases to be independent. This can be measured with quantum tomography: if the purity drops, then gravity really is quantum and not classical. In this way the resonator becomes a window into the quantum world of gravity.
At a typical gravitational wave strain of 10⁻²¹ and a frequency of 3.9 GHz, a cavity holds about 10²⁴ gravitons, yet generating a single photon would take 10¹² seconds, which exceeds the age of the Universe.
The idea of a cavity that converts gravitational waves into light echoes the concept of "gravitational radio" from science fiction, for instance in Carl Sagan's novel Contact, where an alien signal uses gravitational waves for communication.