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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.Progress in quantum computers and their threat to public key cryptography is driving new forms of encryption [1]. One promising alternative is quantum key distribution (QKD) which provides provable security underpinned by quantum physics [2]. In particular, QKD can be achieved using pairs of photons that possess fundamental correlations known as quantum entanglement [3]. Practically, entanglement-based QKD enables a reduction in the number of trusted components [4]. A global quantum network for distributing entangled photon pairs will enable strong encryption keys to be shared between any two points on Earth.Entanglement-based QKD is a mature technology [5][6][7] compared with other entanglement-assisted applications. However, it shares a common range limit with more conventional prepare-send-measure QKD schemes [8]. Metropolitan scale QKD networks are possible using optical fiber or free-space links, but fiber losses and ground-level atmospheric turbulence preclude extending these networks to a global scale. Quantum repeater technology that can overcome these losses is still in the starting stages of being researched [9].Scalable global entanglement distribution can be achieved using a constellation of satellites equipped with spaceto-ground optical links [10]. The technology behind these links are relatively well-documented (e.g. [11][12][13]). Most proposals [14] employ a downlink to minimize transmission loss [15]. A source of photon pairs is placed on board a satellite in Earth orbit that will then either act as a trusted relay between two ground nodes, or beam one photon to one ground station and its pair photon to a different ground station. An additional advantage of space-based entanglement systems is that they allow fundamental tests of the possible overlap between quantum and relativistic regimes for which operation in space is necessary [16].Despite many preliminary studies on space quantum systems [11,12,15,[17][18][19][20][21] there has been limited published work demonstrating relevant technology in space [12,21] due to the prohibitive cost of traditional sp...

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