Later this summer, physicists at the Argonne and Fermi national laboratories will exchange quantum information across 30 miles of optical fiber running beneath the suburbs of Chicago. One lab will generate a pair of entangled photons—particles that have identical states and are linked in such a way that what happens to one happens to the other—and send them to their colleagues at the other lab, who will extract the quantum information carried by these particles of light. By establishing this two-way link, the labs will become the first nodes in what the researchers hope will one day be a quantum internet linking quantum computers around the nation.
A quantum web is loaded with potential. It would enable ultra-secure data transmission through quantum encryption. Astronomers could study distant galaxies in unprecedented detail by combining the rare intergalactic photons collected by individual optical telescopes to create a distributed superscope. Linking small quantum computers could create a quantum cloud and rapidly scale our computing abilities. The problem is that quantum information hates long-distance travel. Send entangled photons out into the real world through optical fiber and, in less than 50 miles, environmental interference will destroy their quantum state. But if the photons were relayed through a satellite instead, they could be sent to destinations hundreds—and potentially thousands—of miles away. So in 2018, NASA partnered with MIT’s Lincoln Laboratory to develop the technologies needed to make it happen.
The goal of the National Space Quantum Laboratory program, sometimes referred to as Quantum Technology in Space, is to use a laser system on the International Space Station to exchange quantum information between two devices on Earth without a physical link. The refrigerator-sized module would be attached to the outside of the space station and would generate the entangled photons that carry the quantum information to Earth. The demonstration would pave the way for a satellite that could take entangled particles generated in local quantum networks and send them to far-flung locations.
“In the future, we will likely see quantum information from Argonne routed through a sequence of satellites to another location across the country, or the world,” says David Awschalom, a senior scientist and the quantum group leader at Argonne National Laboratory. “Much like with existing telecommunications, developing a global quantum network may involve a combination of space- and ground-based platforms.”
NASA is not the first to take quantum technologies to space. In 2016 China launched a satellite that sent a pair of entangled photons to two cities more than 700 miles apart. It was a critical test for long-distance quantum key distribution, which uses particles to encrypt information in a way that is almost impossible to break. It demonstrated that entangled particles could survive the journey from space to Earth by randomly sending photons to two ground stations and comparing when they arrived. If two photons arrived at the same time, they must have been entangled.
It was a groundbreaking demonstration, but “you can’t use that to generate a quantum network, because the photons are arriving at random times, and it wasn’t sending any quantum information,” says Scott Hamilton, who leads the Optical Communications Technology group at MIT’s Lincoln Lab. In this sense, what NASA is pursuing is totally different. The agency wants to use a technique called entanglement swapping to send quantum information carried by entangled particles from one node on the ground to another. This requires being able to send entangled photons with very precise timing and measure them without destroying the information they carry.
Entanglement is the source of many of the advantages of a quantum network, since it allows for information to be exchanged between two particles no matter how far apart they happen to be—what Einstein famously called “spooky action at a distance.” These particles are typically photons, which can be thought of as the envelopes carrying letters full of quantum information. But this information is notoriously delicate. Too much interference from the outside world will cause the information in the quantum missives to disappear like vanishing ink.
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Typically, entangled photons are generated from a single source. A laser is fired at a special kind of crystal, and two identical photons pop out; one copy stays with the sender, the other goes to the receiver. The problem is that entangled photons can’t be amplified as they travel from sender to receiver, which limits how far they can travel before the information they carry is destroyed. Entanglement swapping is the art of entangling photons generated from two different sources, which allows the photons to be passed from node to node in a network similar to how a repeater relays optical or radio signals in a classical system.
“Entanglement swapping is a necessity to propagate entanglement over large distances,” says Babak Saif, an optical physicist at NASA’s Goddard Flight Center. “It’s the first step toward a quantum internet.”
In NASA’s system, a pair of entangled photons is generated on the International Space Station and another pair of entangled photons is generated at a ground station on Earth. One of the photons from space and one of the photons generated on Earth are sent to a quantum device that performs a bell measurement, which determines the state of each photon. This simultaneous measurement causes the remaining photons from their respective pairs—the one in space and the other on Earth—to become entangled, despite being generated by different sources. The next step is to send the remaining photon in space to a different ground station on Earth and repeat the process. This entangles the photons at each ground station and establishes a connection between the two quantum devices without a physical link.
It all sounds good in theory, but Saif says just getting the timing right is a major challenge. Entanglement swapping requires both photons—the one from space and the one from Earth—to arrive in the measurement system on Earth at the exact same time. Moreover, the photons need to be able to hit a small receiver with perfect accuracy. Achieving this level of precision from a spacecraft 250 miles away moving 17,000 miles per hour is every bit as hard as it sounds. To make it happen, NASA needs a damn good space laser.
NASA’s last major experiment in space laser communications was in 2013, when the agency sent data to and from a satellite orbiting the moon. The experiment was a huge success and allowed researchers to send data from the lunar satellite to Earth at over 600 megabits per second—that’s faster than the internet connections in most homes. But the lunar laser link wasn’t long for this world. Shortly after the experiment, NASA plowed the satellite into the moon so researchers could study the dust it kicked up on impact.
“Unfortunately, they crashed a perfectly good laser communication system on purpose,” says David Israel, the Exploration and Space Communications Projects Division architect at NASA’s Goddard Flight Center. But he says the experiment laid the groundwork for the Laser Communications Relay Demonstration (LCRD) satellite, which is scheduled to launch early next year. This new satellite will spend its first few years in orbit relaying laser communications from a ground station in California to one in Hawaii so Israel and his colleagues can study how the weather affects laser communications.
The long-term vision is to transition the satellite from an experiment to a data relay for future missions. Israel says its first operational user will be the ILLUMA-T experiment, an acronym so tortuous that I am not even going to spell it out here. ILLUMA-T is a laser communication station that is scheduled to be installed on the International Space Station in 2022 and will relay data through the LCRD satellite to the ground to experiment with laser cross-links in space. “The goal is to connect it to the onboard systems so that LCRD and ILLUMA-T are not so much experiments anymore, but another path to get data to and from the space station,” says Israel.
Together, ILLUMA-T and the LCRD satellite will lay the foundation for an optical communications network in space, which will enable the next generation of lunar explorers to send back high-definition video from the surface of the moon. But they will also be used as test beds to qualify the laser technologies needed for NASA’s quantum communication ambitions. “Since we were already building an optical thing for the space station, the idea was, why not go the extra mile and make it quantum enhanced?” says Nasser Barghouty, who leads the Quantum Sciences and Technology Group at NASA.
Hamilton and his colleagues at MIT Lincoln Lab are already building a tabletop prototype of the quantum systems that could be connected to ILLUMA-T. He says it will be used to demonstrate entanglement swapping on Earth and that a space-ready version could be ready within five years. But whether or not the system will ever be installed on the space station is an open question.
Earlier this year, Hamilton, Barghouty, and other quantum physicists gathered for a workshop at the University of California, Berkeley, to discuss the future of quantum communications at NASA. One of the main topics of discussion was whether to start with a quantum communication demo on the space station or proceed directly to a quantum communication satellite. While the space station is a useful test platform for advanced technologies, its low orbit means it can only see a relatively small portion of the Earth’s surface at a time. To establish a quantum link between locations that are thousands of miles apart requires a satellite orbiting higher than the ISS.
NASA’s plan to build a quantum satellite link is referred to as “Marconi 2.0,” a nod to the Italian inventor Guglielmo Marconi, who was the first to achieve a long-distance radio transmission. Barghouty says the main idea behind Marconi 2.0 is to establish a space-based quantum link between Europe and North America by the mid- to late-2020s. But the details are still being discussed. “Marconi 2.0 is not a specific mission, but a vaguely defined class of missions,” says Barghouty. “There are a lot of variations on the concept.”
Hamilton says he expects NASA will have a finalized road map for its quantum communication program in the next year or two. In the meantime, he and his colleagues are focused on building the technologies that will make the first long-distance quantum network possible. Although the exact form this network will take is still being discussed, one thing is for certain—the road to a quantum internet passes through space.