Quantum Repeaters: Advances in Entangled Memory Pave the Way for Quantum Internet

Efforts to build a global quantum internet are being boosted by two developments in quantum information storage, which could one day enable secure communications over hundreds or even thousands of kilometers.

The Internet, as it currently exists, transmits information by sending strings of digital bits, or 0s and 1s, in the form of electrical or optical signals. The quantum internet, which can be used to send unhackable communications and link quantum computers, uses qubits instead. These rely on a quantum property called particle entanglement, a phenomenon in which particles can bind and a measurement of one particle can immediately affect the state of another particle, no matter how far apart they are. I’ll give it.

To send these entangled quantum bits (qubits) over very long distances, we need quantum repeaters, hardware that can store the entangled state in memory, reproduce it, and send it further downstream. These must be placed at various points on the long-distance network to ensure that the signal gets from A to B without degradation.

Although quantum repeaters do not yet exist, two groups of researchers have demonstrated long-lasting entangled memory in quantum networks spanning tens of kilometers. This is an important characteristic required for such devices.

Kankunaut Harvard University and its colleagues Set up a quantum network It consists of two nodes separated by a 35-kilometer fiber optic loop throughout the city of Boston. Each node contains both a communication qubit, which is used to transmit information, and a memory qubit, which can store quantum states for up to one second. “Our experiments bring us really close to demonstrating a quantum repeater,” says Knauth.

To set up the link, Knauth and his team entangled the photons they sent to the first node, which contained a type of diamond with an atomic-sized hole, and the second node, which contained a similar diamond. When the photon arrives at his second diamond, it intertwines with both nodes. Diamond can save this state for a moment. Knauth said a fully functional quantum repeater using similar technology could be demonstrated within the next few years, enabling quantum networks that connect cities and countries.

In another work, Xiao Hui Bao University of Science and Technology of China and colleagues intertwine three nodes, in Hefei city, are each about 10 kilometers apart. Bao and his team’s nodes used a supercooled cloud of hundreds of millions of rubidium atoms to generate entangled photons, which he sent to three nodes. The three nodal centers coordinate these photons to link the atomic cloud and act as a form of memory.

The main advance in Bao and his team’s network is to match the frequencies of the photons that meet at the central node, which is crucial for quantum repeaters that connect different nodes. Although his storage time was shorter than his Knaut team at 100 microseconds, it was still long enough to perform useful operations on the transmitted information.

He said these demonstrations of quantum entangled memory are a major accomplishment compared to quantum internet technology from a decade ago. Mohsen Razavi At the University of Leeds, UK. But a fully functional network with quantum repeaters would require higher entanglement production rates, he says.

“This points to a quantum network that is highly scalable and has a large number of users,” he says. alex clarke At the University of Bristol, UK. “Current entanglement speeds are very slow and limited by various efficiencies within the system, so reducing these losses and increasing efficiency will require a lot of quantum and classical network engineering. Masu.”