  Compact fiber-to-chip couplers play an important role in optical
interconnections, especially in data centers. However, the development of
couplers has been mostly limited to standard single-mode fibers, with few
devices compatible with multicore and multimode fibers. Through the use of
state-of-the-art optimization algorithms, we designed two ultra-compact
couplers for use in a seven-core fiber with 32 $\mu$m core spacing. We
demonstrate for the first time a compact dual-polarization coupler capable of
addressing all cores simultaneously, with measured coupling efficiency of
$-$4.3 dB and with a 3-dB bandwidth of 48 nm. We also demonstrate a
single-polarization coupler with efficiency of $-$2.9 dB and an operating
bandwidth of 77 nm. The dual-polarization coupler has footprint of 20 $\mu$m
$\times$ 10 $\mu$m per core, which makes it the smallest fiber-to-chip coupler
experimentally demonstrated on a standard silicon-on-insulator platform.
