Quantum Repeaters for a Global Quantum Internet

The physics that makes quantum computers tick is very sensitive to disturbances. Currently, this means that these computers struggle to share their data with one another over long distances in the way that classical computers readily can, where an intermediate calculation might be handed off from one server to another on the other side of the country. But in a boost for quantum distributed computing, scientists have developed a “quantum repeater” that can help connect such computers over the kind of fiber-optic cables used by telecom companies today. This would allow quantum computers to be separated by dozens and theoretically hundreds of kilometers without the need for satellite links, a new study finds.

Quantum physics makes possible a strange phenomenon known as entanglement. Essentially, two or more particles such as photons that get linked or “entangled” can theoretically influence each other no matter how far apart they are. Entanglement is essential to the workings of quantum computers, which can in theory solve problems no conventional computer ever could.

Quantum networks could connect quantum computers, and also help enable quantum communication of messages protected by theoretically hackproof quantum encryption. In addition, they could help extraordinarily accurate quantum sensors link together in arrays for even greater precision in a multitude of applications, such as helping detect hidden underground resources and structures for mining and the military.

“As a scientist, I’m personally more interested in the sensing applications and what insight they could provide about the world around us,” says study senior author Ben Lanyon, a quantum physicist at the University of Innsbruck, in Austria.

Optical fiber supports much greater bandwidth; experiences less latency because it can directly connect two points instead of requiring signals to bounce off satellites; and is not vulnerable to noise from sunlight and weather, explains Ben Lanyon of the University of Innsbruck.

The amount of funding on quantum networking projects is slowly growing. For instance, the Quantum Internet Alliance started a seven-year program in 2022 to build an innovative quantum Internet ecosystem in Europe, and its first phase was given a budget of €24 million (about US $26 million). In addition, in 2021, the U.S. Department of Energy announced it was devoting $6 million to developing new devices to send and receive quantum network traffic and a further $25 million to developing regional-scale quantum network test-beds. Quantum Internet startups are receiving funding as well—for example, Qunnect, a spinoff from Stony Brook University, in New York, raised $8.5 million in series A funding in 2022.

Previous research has shown that satellites can help transmit quantum signals between ground stations located more than 1,000 kilometers apart. However, scientists would also like to set up quantum networks based on fiber optics for many of the same reasons the vast majority of modern Internet traffic runs via fiber, not satellites. Optical fiber supports much greater bandwidth; experiences less latency because it can directly connect two points instead of requiring signals to bounce off satellites; and is not vulnerable to noise from sunlight and weather, Lanyon explains.

Still, over long distances, chances grow that photons will get lost over fiber optics. To overcome this problem, scientists have sought to create quantum repeaters, devices that can serve as intermediate relay nodes between transmitters and receivers to help quantum signals go the distance. The first blueprints for a quantum repeater were developed 25 years ago.

Previously Lanyon and his colleagues used optical fiber to help keep two ions entangled over a distance of 230 meters. Now they have built a quantum repeater that helped quantum signals cross 50 km. Moreover, these findings suggest that chains of these devices could help quantum signals travel more than 10 times that distance, the kind of lengths needed for practical quantum networks in the real world.

Ideally, the scientists note quantum repeaters should possess three key capabilities. First, they should use standard telecom-wavelength photons, which suffer fewer losses traveling down optical fibers when compared with other wavelengths. Second, they should possess quantum memories, devices that can help the repeaters store and then relay entanglement data. Third, repeaters should prove capable of swapping this data between nodes in a network in a predictable way and not one too subject to the vagaries of chance.

The researchers have now for the first time developed a quantum repeater with all these capabilities united in one system. “Pieces of a fully fledged quantum repeater node have been shown separately before, but they had not all been combined together,” Lanyon says.

One of the new repeaters possesses a pair of calcium ions captured in an ion trap used as two quantum memories. When illuminated by violet laser pulses, they each emit a single photon that stays entangled with its ion. Another device then converts each of these photons to 1,550-nanometer telecom-wavelength light. One photon is then sent down a 25-km-long fiber-optic spool to one node, while the other photon is guided through another spool to a different node. The repeater then swaps the entanglement data of the ions, entangling the photons and their nodes across a combined distance of 50 km.

The scientists found the repeater could help transmit entangled photons at the rate of 9.2 per second. In contrast, in experiments where they directly transmitted entangled photons from one point to another across 50 km without a repeater, they achieved a rate of about 6.7 per second. Although the repeater might supply only a minor advantage at 50 km, the researchers calculated that without a repeater, transmission rates drop significantly at distances more than 100 km.

In addition, Lanyon and his colleagues calculated that, with minor tweaks, using 17 copies of this repeater in a chain could transmit entangled photons over distances of 800 km, albeit with a tenfold drop in transmission rate. “The system would not have to be improved all that much in order to allow for atoms to be entangled in different countries,” Lanyon says.

Lanyon notes that although trapped ions offer the most precise control over the quantum states of light and matter today, ion traps are currently “rather large and cumbersome.” Others have investigated quantum repeaters based on solid-state systems such as nitrogen-vacancy centers—microscopic artificial diamonds with defects in which a carbon atom is replaced with a nitrogen atom and the adjacent carbon atom is missing, he says.

However, while solid-state quantum repeaters may prove more streamlined and scalable, right now “the level of quantum control achieved by solid state systems is not at the level of ions,” Lanyon says. “The best thing to do is to keep developing and exploring a range of different systems for future quantum technologies and perhaps even to combine the best parts of them together.”

In the future, in addition to setting up chains of repeaters, the scientists want to experiment with sending large numbers of photons in parallel. “This ‘multimode’ quantum networking is where speedups can come in the future,” Lanyon says.

The scientists detailed their findings 22 May in the journal Physical Review Letters.