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Promising an unrivaled level of security, the quantum internet is slowly approaching reality. Recently, researchers have made important progress in maintaining quantum entanglement, which could result in the first quantum repeater, essential for the operation of a quantum network.
Also called quantum entanglement, quantum entanglement refers to a type of connection that allows particles to remain connected in such a way that the quantum state of one instantly affects the state of the other, regardless of the distance that separates them. This phenomenon is the basis of the quantum internet, whose development is currently at the center of many researches. Among the main challenges of developing this Internet of the future is the quantum repeater. It is an essential device for maintaining the integrity of information transmitted over the network.
You should know that in a traditional network, signals traveling long distances must be regularly amplified to compensate for transmission losses. However, in a quantum network, this amplification process would destroy the state of the particles. This is where the quantum repeater has to come into play. However, so far no research team has been able to come up with even a viable theoretical model. Two recent studies, published in the journal
Naturefinally propose a plausible architecture for such a device.
Towards the development of a quantum repeater
Quantum repeaters theoretically function as intermediate relays placed between communication cells in a quantum network. Their way of working involves several steps, starting with receiving quantum signals, usually photons that serve as qubits (quantum bits). This information is then temporarily stored in quantum memory, which allows the state of the qubit to be preserved during its transmission.
The repeater is also responsible for performing operations aimed at improving signal quality and detecting any interception attempts. After storage and processing, the signals are finally transmitted to the next quantum communication station.
Recent progress has enabled a significant improvement in the degree of quantum information storage. Although storage times remain on the order of a second or less, this advance still represents a further step towards the design of quantum repeaters.
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Memorizing quantum states with diamonds and rubidium atoms
The first study, conducted by Harvard researchers, focused on building an experimental quantum network with two nodes 35 kilometers apart and connected by optical fiber. Each node included a diamond with an atomic-level cavity to store quantum states. The goal of the experiment was to maintain quantum entanglement between these two points. To achieve this, the researchers entangled the first node with a photon before sending it to the second node where it was supposed to interact with the second diamond. This interaction made it possible to maintain the two diamonds in an intertwined state for a second, a period sufficient to perform additional operations or transfers, according to the researchers.
Another study, conducted at the University of Science and Technology in China, this time involved three nodes several tens of kilometers apart. Instead of diamonds, each node consisted of a cloud of supercooled rubidium atoms (a state achieved after being subjected to temperatures near absolute zero) and served as both a storage memory and a generator of entangled photons. These photons alternated between the nodes, thus maintaining their entanglement for 100 microseconds. To optimize the preservation of entanglement in the network, a central node is dedicated to synchronizing the frequencies of all photons.