Gravitational-wave observatories are like a keen ear listening for distant rumbles of cosmic catastrophes. To catch the faint whisper of merging black holes, detectors of gravitational waves use specially prepared light — known as squeezed vacuum. This light suppresses natural quantum noise, letting instruments peer beyond the standard quantum limit.

However, this approach has an Achilles' heel: phase instability (simply put, jitter in the light wave) reduces its effectiveness. Squeezed vacuum works perfectly only when light losses are low and the phase is stable. But in reality, there are always small losses (say, 5% of photons lost) and phase noise, for instance, 200 milliradians. Under these conditions, the authors set out to find more robust quantum states.

Using numerical optimization, the scientists combed the landscape of losses and phase noise to find states that outperform any Gaussian (ordinary, 'bell-shaped' in the mathematical sense) states. They found three classes of such stealth states: Fock-like (with a definite photon number), cubic-phase-like, and states with discrete rotational symmetry. With an average photon number in the input beam of no more than five, these nonstandard states deliver gains up to 2.2 decibels — roughly a 65% improvement in sensitivity.

Interestingly, the advantage of non-Gaussian states persists even when using the simplest measurement method — homodyne detection (essentially measuring one quadrature of the field). This means the new approach does not require ultra-complex equipment. Future detectors could pick up even fainter signals from the depths of the universe, whether gravitational waves or something else.