  We have used scanning SQUID magnetometry to image vortices in ultra-thin
[Ba$_{0.9}$Nd$_{0.1}$CuO$_{2+x}$]$_m$/[CaCuO$_2$]$_n$ (CBCO) high temperature
superconductor samples, with as few as three superconducting CuO$_2$ planes.
The Pearl lengths ($\Lambda = 2\lambda_{L}^2/d$, $\lambda_{L}$ the London
penetration depth, $d$ the superconducting film thickness) in these samples, as
determined by fits to the vortex images, agree with those by local
susceptibility measurements, and can be as long as 1mm. The in-plane
penetration depths $\lambda_{ab}$ inferred from the Pearl lengths are longer
than many bulk cuprates with comparable critical temperatures. We speculate on
the causes of the long penetration depths, and on the possibility of exploiting
the unique properties of these superconductors for basic experiments.
