Quantum repeaters for a global quantum internet

The physics that makes quantum computers work is very sensitive to disturbances. Currently this means that these computers struggle to share their data with each other over long distances as classical computers easily do, where an intermediate computation could be transferred from one server to another across the country. But in a push for distributed quantum computing, scientists have developed a quantum repeater that can help connect such computers via the kind of fiber-optic cables used by telecommunications 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 connected or entangled can theoretically affect each other, no matter how far apart they are. Entanglement is essential to the functioning of quantum computers, which in theory can solve problems that no conventional computer could ever solve.

Quantum networks could connect quantum computers and also help enable quantum communication of messages protected by theoretically hackproof quantum cryptography. Also, they could help extraordinarily accurately quantum sensors connect to each other in arrays for even greater accuracy in a multitude of applications, such as helping to detect hidden underground assets and structures for mining and the military.

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

Optical fiber supports much greater bandwidth; experiences lower latency because it can connect two points directly instead of requiring signals to bounce off satellites; and is not vulnerable to noise from sunlight and weather

The amount of funding for quantum networking projects is slowly increasing. For example, the Quantum Internet Alliance launched a seven-year program in 2022 to build an innovative quantum internet ecosystem in Europe, and its first phase has received a budget of 24 million euros (about $26 million). Furthermore, in 2021, the United States Department of Energy announced it will devote $6 million to the development of new devices to send and receive quantum network traffic and an additional $25 million to develop regional-scale quantum networking testbeds. Quantum Internet startups are also receiving funding, such as Qunnect, a spin-off from State University of New York at Stony Brooklifted up $8.5 million in Series A funding in 2022.

Previous research has shown this satellites it can help transmit quantum signals between earth stations located more than 1,000 kilometers apart. However, scientists would also like to create fiber-optic-based quantum networks for many of the same reasons that the vast majority of modern internet traffic goes through fiber, not satellites. Optical fiber supports much greater bandwidth; experiences lower latency because it can connect two points directly instead of requiring signals to bounce off satellites; and is not vulnerable to noise from sunlight and the weather, explains Lanyon.

However, over long distances, the chances of photons being lost on the optical fiber increase. To overcome this problem, scientists have tried to create quantum repeaters, devices that can act as intermediate transmission nodes between transmitters and receivers to help quantum signals travel the distance. THE early designs for a quantum repeater they were developed 25 years ago.

Previously Lanyon and his colleagues used optical fiber to help keep the two entangled ions over a distance of 230 meters. Now they built a quantum repeater that helped quantum signals travel 50 kilometers. Furthermore, these results suggest that the 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 that quantum boosters should possess three key capabilities. First, they would have to use standard telecommunications wavelength photons, which suffer less loss traveling along optical fibers than other wavelengths. Second, they should own quantum memories, devices that can help cell towers store and then transmit entanglement data. Third, cell towers should demonstrate the ability to exchange this data between nodes in a network in a predictable manner that is not subject to the whims of circumstances.

Researchers have now developed for the first time a quantum repeater with all these capabilities combined in one system. Pieces of a full-fledged quantum repeater node have been shown separately before, but they hadn’t all been combined together, says Lanyon.

The new booster has a pair of calcium ions captured in a ion trap used as two quantum memories. When illuminated by violet laser pulses, they each emit a single photon which remains entangled with its ion. Another device then converts each of these photons into Light with a telecommunication wavelength of 1,550 nanometers. One photon is then sent down a 25 kilometer long coil of optical fiber to one node, while the other photon is guided through another coil to a different node. The repeater then exchanges ion entanglement data, trapping the photons and their nodes over a combined distance of 50 kilometers.

Scientists found that the repeater could help transmit entangled photons at a rate of 9.2 per second. Conversely, in experiments where they directly transmitted entangled photons from one point to another over 50 kilometers without a repeater, they achieved a rate of about 6.7 per second. While the repeater may provide only a small advantage at 50 kilometers, the researchers calculated that without a repeater, transmission speeds drop significantly at distances above 100 kilometers.

Furthermore, Lanyon and his colleagues calculated that, with minor modifications, using 17 copies of this repeater in a chain could transmit entangled photons over distances of 800 kilometers, albeit with a tenfold drop in transmission rate. The system wouldn’t need to be improved so much to allow 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 quite large and unwieldy. Others have studied quantum boosters based on solid-state systems such as nitrogen vacancy centers, microscopic man-made diamonds with defects in which one carbon atom is replaced with a nitrogen atom and the adjacent carbon atom is missing, he says.

However, while solid-state quantum boosters may turn out to be leaner and more 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 continue to develop and explore a range of different systems for future quantum technologies and perhaps even combine the best parts of them.

In the future, in addition to creating repeater chains, scientists want to experiment with sending large numbers of photons in parallel. This multi-mode quantum network is where accelerations may come in the future, Lanyon says.

The detailed scientists their discoveries May 22 in the diary Physical Review Letters.

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