Observation of Nonclassical Photon Statistics in Single-Bubble Sonoluminescence

A cavitation bubble inside a liquid, under a specific set of conditions, can get trapped in an antinode of the ultrasonically driven standing wave and periodically emits visible photons (1,2). This conversion of sound to light phenomenon, known as sonoluminescence, can be seen with unaided eyes and occurs in multi- or single-bubble regimes. The sonoluminescence radiation spectrum analysis attributes a temperature of about 6,000 Kelvin to the bubble (3-5) -- close to the sun's surface temperature. The bubble dynamics, thermodynamics, and other physicochemical properties are well-explored and studied over the past decades (6-8). Notwithstanding that several theories such as the thermal (black-body radiation) model (9-11), shock wave theory (12), phase transition radiation (13,14), and vacuum quantum electrodynamics (15,16) -- were proposed, the underlying physics of sonoluminescence has hitherto remained a puzzle. Here, we experimentally investigated the photon number statistics of the emitted photons from single-bubble sonoluminescence (SBSL), employing two distinctly different techniques, i.e., measuring multiphoton correlations and photon number (using a photon-number resolving detector). Our findings show that the emitted photons from SBSL possess sub-Poissonian statistics, indicating the nonclassical nature of the SBSL. Our observation of sub-Poissonian statistics of emitted photons may help to explain the physics of SBSL. In addition, considering the importance of nonclassical light sources in quantum technologies, this discovery would be an exciting milestone in paving the route towards a bright quantum photonics source.