  A mechanically compliant element can be set into motion by the interaction
with light. In turn, this light-driven motion can give rise to ponderomotive
correlations in the electromagnetic field. In optomechanical systems, cavities
are often employed to enhance these correlations up to the point where they
generate quantum squeezing of light. In free-space scenarios, where no cavity
is used, observation of squeezing remains possible but challenging due to the
weakness of the interaction, and has not been reported so far. Here, we measure
the ponderomotively squeezed state of light scattered by a nanoparticle
levitated in a free-space optical tweezer. We observe a reduction of the
optical fluctuations by up to $25$~\% below the vacuum level, in a bandwidth of
about $15$~kHz. Our results are well explained by a linearized dipole
interaction between the nanoparticle and the electromagnetic continuum. These
ponderomotive correlations open the door to quantum-enhanced sensing and
metrology with levitated systems, such as force measurements below the standard
quantum limit.
