Satellites have played a pivotal role in establishing global Internet coverage. They can also be used to extend the reach of existing entanglement-based aquantum networks to connect distant ground stations on Earth, surpassing the limitations of terrestrial fiber networks without using quantum repeaters. As quantum communications technology matures, a quantum satellite infrastructure will be needed to enable a global Quantum Internet.

Using Satellites for Entanglement Distribution

The Quantum Internet will use a combination of terrestrial networks on the ground and segments based in space. Above is an example of how satellites could be employed to distribute entangled pairs of photons to very distant ground stations on Earth. The objective here is to connect two ground stations on Earth with entanglement. These stations are located very far apart - too distant for a purely terrestrial link. To achieve this, two satellites, each with an entangled photon pair source, emit a pair of photons. Each satellite sends one photon to a ground station and another photon to the middle node (the terrestrial repeater node). This middle node captures the incoming photons, one photon from each pair emitted by the two satellites, and then performs entanglement swapping. This process effectively stitches together the entanglement from the first ground stations to the middle node with the entanglement from the last ground station to the middle node, creating long-range entanglement between the two ground stations. This is one method for delivering entanglement to very distant ground stations using satellite downlinks. 

There are different architectures to consider as well. The example above used two satellites with downlinks to distribute entanglement. Uplink architectures are an option as well, in which the ground stations emit entangled photon pairs up to the satellite which hosts a quantum repeater. There are trade-offs for each of these architectures, so choosing the right one will depend on the particular objective and implementation constraints. 

There has also been demonstrated potential for using high altitude UAVs as a way to more effectively connect satellites with these terrestrial quantum networks.^[UAV]

Satellites for scaling entanglement-based quantum networks

 In which instances should a space-based link be used to scale a quantum network, and in which instances should a terrestrial repeater chain using fiber be used? The figure below begins to address this exact question.

There is no one-size-fits-all architecture for scaling entanglement-based quantum networks. The application informs the architecture needed to scale. In other words, the link and hardware configuration that is best suited to the application dictates the requirements for expanding the network. The application may require a certain coverage: full global connectivity, regional connectivity, and metropolitan region connectivity would each have different needs to meet for coverage. For example: to achieve Quantum Secure Communication from London to New York, a satellite link is likely more viable than a chain of terrestrial repeaters. Distance between nodes, coverage, and key rates will all factor into the considerations for this example. For quantum key distribution, the throughput, network uptime, and the cost of implementation will all factor heavily into determining the appropriate architecture. 

There are many other applications that quantum satellites will enable. Constellations of Low Earth Orbit satellites could scale a secure communications application to global quantum connectivity. Pushing this same technology to orbits further from Earth, it’s possible to enable applications at the intersection of quantum physics and relativity, developing a theory for quantum gravity, and other critical questions about the nature of our universe.^[UNIVERSE]

Quantum satellites are a promising platform for expanding the impact of quantum technology enabled by quantum networks. Building for scalability and looking to the vision of a global Quantum Internet requires building for diverse applications to run on the network, as well as preparing for advancements in technology: from improvements to quantum hardware and protocols to the engineering of quantum repeaters and robust quantum satellites. Taking a hardware-agnostic, multipurpose-application approach to quantum networks will help to create the most flexibility for scaling, whether that scale up is from local area network to regional network, or regional network to global Quantum Internet.

Satellites Free-space quantum communication projects at a glance

There are many quantum satellite efforts across the globe.  

Above is a timeline of different global efforts around free-space quantum communication until 2020. The first demonstration of free-space quantum entanglement distribution occurred in 2005. This was a terrestrial demonstration of entanglement distribution over 13 km. In blue are demonstrated milestones: quantum teleportation has been demonstrated, quantum key distribution has been demonstrated. In red are proposed missions going forward. Most of these projects involve a number of collaborators, both with public participation from government agencies, private companies, satellite companies, startups and bigger corporations, as well as universities and academics. Some notable projects include:

• QEYSSat (Quantum EncrYption and Science Satellite) driven by the Canadian Space Agency, with planned launch in 2025. The goal for this project is to launch three Low Earth Orbit satellites to study, demonstrate, and validate space-based quantum secure communications. 
• Eagle-1, driven by the European Space Agency, launches in 2024. This will build the first European end-to-end space-based QKD system, a step toward future European quantum communications.
• QKDSat (Quantum Key Distribution Satellite) launched in 2023 by ARQIT and collaborators in the UK. This project aims to demonstrate how a space-based quantum infrastructure can be used to exchange sensitive information between several parties.
• Shenzhou 16 is the follow-up of the Micius project launched by China in 2023. This project will go on to launch a Geostationary satellite in 2026. 
• SpooQySats projects launched in 2018 and 2020 to demonstrate entanglement generation on a satellite. This project is led by the National University of Singapore Center for Quantum Technologies, in collaboration with a company called SpeQtral. 
• There's a collaboration between Oak Ridge National Labs and University of Illinois, Urbana Champaign to study how satellites could enable more efficient and secure quantum networks through experiments, emulation, and simulation. This study concluded in 2023.
• The Deep Space Quantum Link project, led by NASA, aims to establish long-baseline quantum links between the Lunar Gateway moon-orbiting space station and nodes on, or near, the Earth. 
• QUDICE is a project led by ThalesAlenia Space, but is a large collaboration of partners in the public sector, private sector, and in academia. This project has several goals:
  • Launch two sources for QKD: one for discrete variable encoding, and one for continuous variable encoding in order to explore the trade off between these different mechanisms for encoding quantum information. Study the effects of quantum random number generators. Develop satellite pointing, acquisition, and tracking systems. Use a 5G communication system to perform the necessary post processing for QKD. 

This is not an exhaustive list, but gives a sense of the breadth and depth of the projects being pursued to implement free-space quantum communication. Each of these projects has a different angle of approach to space-based quantum communications. While advancements are needed to build the ideal quantum-enabled satellite, this technology is already being deployed for Quantum Secure Communications applications.

The convergence of entanglement-based technology and satellite infrastructure represents a groundbreaking leap in optical communication systems, offering unprecedented security with global coverage. This technology will scale small local area networks and metropolitan-scale terrestrial networks to achieve a global Quantum Internet. Entanglement-based satellites are the next frontier of free-space communications technology, and free-space entanglement-based communication is the new space race. Now is a great time to shape your organization’s Quantum Networking strategy.

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