  We report the discovery of large ionized, [O II] emitting circumgalactic
nebulae around the majority of thirty UV luminous quasars at $z=0.4-1.4$
observed with deep, wide-field integral field spectroscopy (IFS) with the
Multi-Unit Spectroscopy Explorer (MUSE) by the Cosmic Ultraviolet Baryon Survey
(CUBS) and MUSE Quasar Blind Emitters Survey (MUSEQuBES). Among the 30 quasars,
seven (23%) exhibit [O II] emitting nebulae with major axis sizes greater than
100 kpc, twenty greater than 50 kpc (67%), and 27 (90%) greater than 20 kpc.
Such large, optically emitting nebulae indicate that cool, dense, and
metal-enriched circumgalactic gas is common in the halos of luminous quasars at
intermediate redshift. Several of the largest nebulae exhibit morphologies that
suggest interaction-related origins. We detect no correlation between the sizes
and cosmological dimming corrected surface brightnesses of the nebulae and
quasar redshift, luminosity, black hole mass, or radio-loudness, but find a
tentative correlation between the nebulae and rest-frame [O II] equivalent
width in the quasar spectra. This potential trend suggests a relationship
between ISM content and gas reservoirs on CGM scales. The [O II]-emitting
nebulae around the $z\approx1$ quasars are smaller and less common than
Ly$\alpha$ nebulae around $z\approx3$ quasars. These smaller sizes can be
explained if the outer regions of the Ly$\alpha$ halos arise from scattering in
more neutral gas, by evolution in the cool CGM content of quasar host halos, by
lower-than-expected metallicities on $\gtrsim50$ kpc scales around $z\approx1$
quasars, or by changes in quasar episodic lifetimes between $z=3$ and $1$.
