  Despite decades of study, we still do not fully understand why some massive
galaxies abruptly switch off their star formation in the early Universe, and
what causes their rapid transition to the red sequence. Post-starburst galaxies
provide a rare opportunity to study this transition phase, but few have
currently been spectroscopically identified at high redshift ($z>1$). In this
paper we present the spectroscopic verification of a new photometric technique
to identify post-starbursts in high-redshift surveys. The method classifies the
broad-band optical-near--infrared spectral energy distributions (SEDs) of
galaxies using three spectral shape parameters (super-colours), derived from a
principal component analysis of model SEDs. When applied to the multiwavelength
photometric data in the UKIDSS Ultra Deep Survey (UDS), this technique
identified over 900 candidate post-starbursts at redshifts $0.5<z<2.0$. In this
study we present deep optical spectroscopy for a subset of these galaxies, in
order to confirm their post-starburst nature. Where a spectroscopic assessment
was possible, we find the majority (19/24 galaxies; ~80 per cent) exhibit the
strong Balmer absorption (H $\delta$ equivalent width $W_{\lambda}$ >5 Ang.)
and Balmer break, characteristic of post-starburst galaxies. We conclude that
photometric methods can be used to select large samples of recently-quenched
galaxies in the distant Universe.
