  Our objectives are to determine the properties of the interstellar medium
(ISM) and of star-formation in typical star-forming galaxies at high redshift.
Following up on our previous multi-wavelength observations with HST, Spitzer,
Herschel, and the Plateau de Bure Interferometer (PdBI), we have studied a
strongly lensed z=2.013 galaxy, the arc behind the galaxy cluster MACS
J0451+0006, with ALMA to measure the [CII] 158 micron emission line, one of the
main coolants of the ISM. [CII] emission from the southern part of this galaxy
is detected at 10 $\sigma$. Taking into account strong gravitational lensing,
which provides a magnification of $\mu=49$, the intrinsic lensing-corrected
[CII]158 micron luminosity is $L(CII)=1.2 \times 10^8 L_\odot$. The observed
ratio of [CII]-to-IR emission, $L(CII)/L(FIR) \approx (1.2-2.4) \times
10^{-3}$, is found to be similar to that in nearby galaxies. The same also
holds for the observed ratio $L(CII)/L(CO)=2.3 \times 10^3$, which is
comparable to that of star-forming galaxies and active galaxy nuclei (AGN) at
low redshift. We utilize strong gravitational lensing to extend diagnostic
studies of the cold ISM to an order of magnitude lower luminosity ($L(IR) \sim
(1.1-1.3) \times 10^{11} L_\odot$) and SFR than previous work at high redshift.
While larger samples are needed, our results provide evidence that the cold ISM
of typical high redshift galaxies has physical characteristics similar to
normal star forming galaxies in the local Universe.
