Tie: China, the US and Europe reach the quantum internet milestone at the same time

The ability to harness entanglement is seen as a crucial step toward a quantum internet, which promises a way to generate random cryptographic keys for encrypted information so quickly that they become nearly uncrackable. It can also be used to connect quantum computers, expanding their operational capabilities.

The achievements of the Chinese and American teams were detailed in Nature, while the Dutch researchers’ paper was uploaded as a preprint to the repository website arXiv in April and has not undergone peer review.

The Chinese investigators – led by those from the country “father of quantum” Pan Jianwei – from the University of Science and Technology of China (USTC), described the achievement as “a crucial milestone” in the transition to larger-scale experiments.

Lead author of the Dutch paper Ronald Hanson, a physicist at Delft University of Technology, said: “The step has now really been taken from the laboratory to the field,” according to a press release from Nature.

The Harvard University team, led by physicist Mikhail Lukin, said a “key challenge” in realizing practical long-distance quantum communications “involves robust entanglement between quantum memory nodes connected by fiber optic infrastructure.”

Each node contains qubits – the quantum version of computer bits, the basic unit of information represented by 0 or 1 and which can also exist in a third, combined state – that communicate via the ‘photonic channels’, the US researchers said.

Although each team demonstrated quantum entanglement by using fiber optic cables to create secure connections between receiving node devices, their approaches differed.

The US researchers connected two side-by-side nodes at the Harvard laboratory in Cambridge, Massachusetts using a 35 km fiber optic loop that extended into Boston.

The Chinese team set up three nodes – named Alice, Bob and Charlie – in a triangular network around Hefei, the capital of Anhui province and home to USTC, with a central server lab at the center, all at a distance of about 10 km (6 miles). ).

In the Netherlands, a total of 25 km of fiber optic stretched from Delft to The Hague, with two nodes connected to a server in the middle.

Pan and his researchers used a single-photon scheme, using qubits encoded in an ensemble of rubidium atoms, to send one photon from each node to the server for entanglement, the paper said.

If two photons arrive at the server at exactly the same time, an entangled state is achieved, the Chinese researchers said.

“Our work provides a metropolitan-scale testing ground for the evaluation and exploration of multi-node quantum network protocols and initiates a phase of quantum Internet research,” the paper said.

Pan told Nature that his team expects to be able to achieve entanglement across 1,000 kilometers of fiber using about 10 nodes by the end of the decade.

Instead of relying on an ensemble of atoms, the American team used diamond devices with the carbon atoms replaced by a silicon atom. “We’ve essentially entangled two small quantum computers,” says Lukin.

In the US experiment, a single photon was sent to the first node where it became entangled with a silicon atom before being sent around the fiber loop to graze a second silicon atom at the other node, allowing entanglement.

China’s ‘father of quantum’ Pan Jianwei, who led the USTC research team that created a quantum-based communications network in Hefei, the capital of eastern China’s Anhui province. Photo: Xinhua

In a similar approach to their American counterparts, the Dutch researchers used nitrogen atoms embedded in diamond crystals.

The Nature report noted that the Chinese and Dutch methods relied on extremely precise timing for the arrival of photons at a central server, which required a level of fine-tuning that the American researchers’ method did not require.

According to Nature, the single-atom method is less efficient than the Chinese team’s ensemble approach, but is more adaptable because it can be used for basic calculations.

A USTC press release states that the Chinese researchers’ method achieved an entanglement efficiency “two orders of magnitude” higher than that of their American counterparts.

While demonstrating entanglement between nodes in a city is a major achievement, Hanson told Nature that “it doesn’t mean it’s commercially useful, but it is a big step.”

Physicist Tracy Northup of the University of Innsbruck, Austria, who was not involved in any of the research, said the experiments were “the most advanced demonstrations yet” of the technology needed to develop the quantum internet, according to Nature.

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