  The localisation of fluorophores is an important aspect of the determination
of the biological function of cellular systems. Quantum correlation microscopy
(QCM) is a promising technique for providing diffraction unlimited emitter
localisation that can be used with either confocal or widefield modalities.
However, so far, QCM has not been applied to three dimensional localisation
problems. Here we show that quantum correlation microscopy provides
diffraction-unlimited three-dimensional localisation for two emitters within a
single diffraction-limited spot. By introducing a two-stage maximum likelihood
estimator, our modelling shows that localisation precision scales as
$1/\sqrt{t}$ where $t$ is the total detection time. Diffraction unlimited
localisation is achieved using both intensity and photon correlation from
Hanbury Brown and Twiss measurements at as few as four measurement locations.
