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...

Silicon avalanche photodiodes (APDs) are sensitive to operating temperature fluctuations and are also susceptible to radiation flux expected in satellite-based quantum experiments. We introduce a low power voltage adjusting mechanism to overcome the effects of in-orbit temperature fluctuations. We also present data on the performance of Si APDs after irradiation (γ-ray and proton beam). Combined with an analysis of expected orbital irradiation, we propose that a Si APD in a 400 km equatorial orbit may operate beyond the lifetime of the satellite.

We report the successful test flight of a device for generating and monitoring correlated photon pairs under near-space conditions up to 35.5 km altitude. Data from ground based qualification tests and the high altitude experiment demonstrate that the device continues to operate even under harsh environmental conditions. The design of the rugged, compact and power-efficient photon pair system is presented. This design enables autonomous photon pair systems to be deployed on low-resource platforms such as nanosatellites hosting remote nodes of a quantum key distribution network. These results pave the way for tests of entangled photon technology in low earth orbit.