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We present ALMA CO(2-1) spectroscopy of 6 massive(log$_{10}$M$_{\rm{*}}/\rm{M}_\odot>$11.3) quiescent galaxies at $z\sim1.5$.These data represent the largest sample using CO emission to trace moleculargas in quiescent galaxies above $z>1$, achieving an average 3$\sigma$sensitivity of M$_{\rm{H_{2}}}\sim10^{10}\rm{M}_\odot$. We detect one galaxy at4$\sigma$ significance and place upper limits on the molecular gas reservoirsof the other 5, finding molecular gas mass fractionsM$_{\rm{H_{2}}}$/M$_{\rm{*}}$=f$_{\rm{H_{2}}}<2-6$% (3$\sigma$ upper limits).This is 1-2 orders of magnitude lower than coeval star-forming galaxies atsimilar stellar mass, and comparable to galaxies at $z=0$ with similarly lowsSFR. This indicates that their molecular gas reservoirs were rapidly andefficiently used up or destroyed, and that gas fractions are uniformly low($<$6%) despite the structural diversity of our sample. The implied rapiddepletion time of molecular gas (t$_{\rm{dep}}<0.6$ Gyr) disagrees withextrapolations of empirical scaling relations to low sSFR. We find that our lowgas fractions are instead in agreement with predictions from both the recentSIMBA cosmological simulation, and from analytical "bathtub" models for gasaccretion onto galaxies in massive dark matter halos(log$_{10}M_{\rm{halo}}/\rm{M}_\odot\sim14$ at $z=0$). Such high mass halosreach a critical mass of log$_{10}M_{\rm{halo}}/\rm{M}_\odot>12$ by $z\sim4$that halt the accretion of baryons early in the Universe. Our data isconsistent with a simple picture where galaxies truncate accretion and thenconsume the existing gas at or faster than typical main sequence rates.Alternatively, we cannot rule out that these galaxies reside in lower masshalos, and low gas fractions may instead reflect either stronger feedback, ormore efficient gas consumption.

ALMA measures rapidly depleted molecular gas reservoirs in massive quiescent galaxies at z~1.5