  In the last few years, the great utility of PT-symmetric systems in sensing
small perturbations has been recognized. Here, we propose an alternate method
relevant to dissipative systems, especially those coupled to the vacuum of the
electromagnetic fields. In such systems, which typically show anti-PT symmetry
and do not require the incorporation of gain, vacuum induces coherence between
two modes. Owing to this coherence, the linear response acquires a pole on the
real axis. We demonstrate how this coherence can be exploited for the enhanced
sensing of very weak anhamonicities at low pumping rates. Higher drive powers
($\sim 0.1$ W), on the other hand, generate new domains of coherences. Our
results are applicable to a wide class of systems, and we specifically
illustrate the remarkable sensing capabilities in the context of a weakly
anharmonic Yttrium Iron Garnet (YIG) sphere interacting with a cavity via a
tapered fiber waveguide. A small change in the anharmonicity leads to a
substantial change in the induced spin current.
