Efficient transcontinental entanglement distribution is necessary to build a global quantum network. Without quantum repeaters, distribution through optical fibers is assailed by loss and scattering, limiting the network’s reach to around 100 kilometers. This distance can be greatly extended however, by transmitting photons through free space, where the transmission falls only as the reciprocal square of the propagation distance. Our experiment aims to prove the viability of one proposed satellite intermediary scheme: a down-link architecture using entanglement swapping. In this scheme, a satellite generates a pair of entangled photons that are spectrally unentangled – and therefore able to interfere with other photons. The satellite transmits the telecom pump and the entangled telecom photons down to a station on Earth’s surface; by doing so, both channels experience the same temporal drift due to Doppler shift and dispersion. The transmitted pump can then be collected, reamplified, and used to pump a second terrestrial entanglement source. Synchronizing and interfering the satellite and terrestrial entangled, telecom photons will then swap entanglement to the unused photons. We are implementing this by pumping non-degenerate entanglement sources, which produce daughter photons at 773 nm and 1588 nm, with a 520 nm pump, generated from third harmonic generation.
Efficient transcontinental entanglement distribution is necessary to build a global quantum network. Without quantum repeaters, distribution through optical fibers is assailed by loss and scattering, limiting the network’s reach to around 100 kilometers. This distance can be greatly extended however, by transmitting photons through free space, where the transmission falls only as the reciprocal square of the propagation distance. Our experiment aims to prove the viability of one proposed satellite intermediary scheme: a down-link architecture using entanglement swapping. In this scheme, a satellite generates a pair of entangled photons that are spectrally unentangled – and therefore able to interfere with other photons. The satellite transmits the telecom pump and the entangled telecom photons down to a station on Earth’s surface; by doing so, both channels experience the same temporal drift due to Doppler shift and dispersion. The transmitted pump can then be collected, reamplified, and used to pump a second terrestrial entanglement source. Synchronizing and interfering the satellite and terrestrial entangled, telecom photons will then swap entanglement to the unused photons. We are implementing this by pumping non-degenerate entanglement sources, which produce daughter photons at 773 nm and 1588 nm, with a 520 nm pump, generated from third harmonic generation.
Long-distance, entanglement distribution over thousands of kilometers is necessary to build a global-scale quantum network. Directly transferring entangled, telecom photons through fiber is relatively straight forward in local networks where the separation between nodes is below 100 km, corresponding to a 1% transmission. However, the exponentially greater loss at larger distances (e.g. only 10−6 transmission at 300 km) leads to unacceptably low transmission rates, which drop below the noise rates of typical photon detectors. Alternatively, the Earth’s rarefied atmosphere at high altitudes reduces the absorption and scattering of infrared photons. Therefore, free-space links between satellite and ground stations have been shown to enable entangled photon transmission over 2000 km. While this demonstration showed the viability of satellite-enhanced entanglement distribution, the source used in this demonstration produced distinguishable photons, which cannot be used for many fundamental protocols like teleportation. We are building a testbed to demonstrate the feasibility of distributing entangled, indistinguishable photons over a space-to-ground link using entanglement swapping.
The realization of the quantum internet as an interconnected network of quantum hardware requires the distribution of entanglement over long distances. Due to the exponential photon loss in optical fibers, the necessary communication rates for end-user applications cannot be achieved if direct links over several hundred kilometers are required. Two possible solutions to increase the link distance are the implementation of quantum repeaters and the use of free-space optical satellite links. A natural extension would be to combine both approaches, by using satellites as quantum repeater nodes and take advantage of the low losses in free-space. However, due to the higher technological requirements for space-based technologies and the already low communications losses in free-space, it is not yet clear whether satellite- based quantum repeaters can lead to an additional improvement in the communication rate. Here we show for which operating scenarios a satellite-based quantum repeater outperforms a single satellite for entanglement distribution. To this end, we developed a satellite simulation software that includes a model of the optical link model and of entanglement swapping in order to analyze entanglement distribution rates of the satellite-based quantum repeater and single satellite. The simulation captures important effects such as the single-mode fiber coupling efficiency, adaptive optics corrections, decoherence and decay of information in the quantum memory as well as its retrieval efficiency. Based on these effects, we show advantages and challenges of satellite constellations for quantum communications in low, medium and geosynchronous orbits.
A large scale quantum network requires the distribution of high-fidelity end-to-end entanglement. To overcome the range limitations inherent to terrestrial fiber, a leading architecture has emerged: satellite-based sources transmitting entanglement to quantum repeaters on the ground. By bridging the gap between abstract analytical frameworks and computationally heavy numerical simulations, this paper provides the first quantitative answer to the question of such a network's achievable performance with current and near-term space technology, while accounting for entanglement swapping and purification. This is achieved by integrating a detailed physical model of a satellite-to-ground link into an analytical entanglement resource estimation framework for quantum repeaters, enabling an optimization of the end-to-end entanglement rate. Our analysis, performed across leading quantum hardware platforms, shows that Low Earth Orbit satellite constellations combined with quantum repeaters employing Neutral Atom or Nitrogen and Silicon Vacancy qubits, could enable a global quantum network, distributing entanglement over distances up to 20,000 km, sufficient for connecting any two points on Earth. This work highlights the major bottlenecks in space and quantum hardware technologies, which need to be addressed, thereby guiding informed investments necessary for enabling a large scale quantum network.
Finding a realistic and efficient scheme that allows for the distribution of entanglement at global scales constitutes a challenging task with a satisfactory solution yet to be found. Due to the exponential losses experienced in optical fibers and the absence of a direct line-of-sight between opposite sides of Earth, employing a quantum repeater chain will likely be inevitable to achieve this task. Long-distance terrestrial fiber connections would require a huge number of intermediate repeater nodes and is further hindered by inaccessible terrain, so that quantum repeaters realized via free-space optical links between satellites constitute a promising alternative. These, however, introduce their own set of challenges, such as the dynamic behaviour of the satellites in orbit and a high baseline of losses due to the satellite-to-ground connection. In this work, we discuss the general scaling properties of free-space optical quantum repeaters and combine them with the geometric constraints imposed by satellite connections to propose and assess a two-node two-satellite quantum repeater architecture that allows entanglement distribution at truly global scales whilst also being able to adaptively be used as a one-node one-satellite repeater to efficiently cover shorter distances. We provide an analysis of the connection time and the effective transmission and show that two MEO satellites are sufficient to distribute entanglement between ground stations with arbitrary distances inside the orbital plane. We deem our architecture as one of the most promising in terms of performance, connection distance and flexibility whilst using only two satellites, alleviating the extreme technical challenge of realizing a quantum repeater constellation.
Quantum communication rates in terrestrial quantum networks are fundamentally limited by fiber loss, even in the presence of quantum repeaters. Satellites offer a solution for long-distance communication, with the most commonly explored scenario involving prepare-and-measure protocols connecting from orbit to a trusted-node ground station via free-space down-links. In contrast, up-link scenarios allow for entanglement to be distributed between a satellite and remote end-users in terrestrial networks, eliminating any trust requirement on the ground station. Here, we demonstrate an ultra-bright source of far-non-degenerate entangled photons and perform quantum key distribution in emulated high-loss satellite scenarios. With a loss profile corresponding to that of one of the pioneering Micius up-link experiments, and a terrestrial end-user separated by 10 km of telecom fiber, we achieve secure key bit accumulation of 5.2 kbit in a single emulated overpass in the asymptotic limit. Our results confirm the viability of upcoming low-Earth orbit receiver satellite missions.
Abstract Long-distance entanglement is a very precious resource, but its distribution is very difficult due to the exponential losses of light in optical fibres. A possible solution consists in the use of quantum repeaters, based on entanglement swapping (ES) or quantum error correction. Alternatively, satellite-based free-space optical links can be exploited, achieving better loss-distance scaling. We propose to combine these two ingredients, quantum repeaters and satellite-based links, into a scheme that allows to achieve entanglement distribution over global distances with a small number of intermediate untrusted nodes. The entanglement sources, placed on satellites, send quantum states encoded in photons towards orbiting quantum repeater stations, where ES is performed. The performance of this repeater chain is assessed in terms of the secret key rate achievable by the BBM92 cryptographic protocol. We perform a comparison with other repeater chain architectures and show that our scheme, even though more technically demanding, is superior in many situations of interest. Finally, we analyse strengths and weaknesses of the proposed scheme and discuss exemplary orbital configurations. The integration of satellite-based links with ground repeater networks can be envisaged to represent the backbone of the future quantum internet.