  Satellites carrying sources of entangled photons could establish a global
quantum network, enabling private encryption keys between any two points on
Earth. Despite numerous proposals, demonstration of space-based quantum systems
has been limited due to the cost of traditional satellites. We are using very
small spacecraft to accelerate progress. We report the in-orbit operation of a
photon pair source aboard a 1.65 kg nanosatellite and demonstrate pair
generation and polarization correlation under space conditions. The in-orbit
photon correlations exhibit a contrast of 97+/-2%, matching ground-based tests.
This pathfinding mission overcomes the challenge of demonstrating in-orbit
performance for the components of future entangled photon experiments. Ongoing
operation establishes the in-orbit lifetime of these critical components. More
generally, this demonstrates the ability for nanosatellites to enable faster
progress in space-based research.

Quantum Physics;Instrumentation and Detectors;Space Physics