  Quantum communication implementations require efficient and reliable quantum
channels. Optical fibers have proven to be an ideal candidate for distributing
quantum states. Thus, today's efforts address overcoming issues towards high
data transmission and long-distance implementations. Here, we experimentally
demonstrate the secret key rate enhancement via space-division multiplexing
using a multicore fiber. Our multiplexing technique exploits the momentum
correlation of photon pairs generated by spontaneous parametric
down-conversion. We distributed polarization-entangled photon pairs into
opposite cores within a 19-core multicore fiber. We estimated the secret key
rates in a configuration with 6 and 12 cores from the entanglement visibility
after transmission through 411 m long multicore fiber.
