  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.
