Presented as the future of telecom networks, the quantum internet offers many advantages, especially in terms of security. Thanks to an encryption method based on the quantum properties of information carriers (typically photons) and a corresponding infrastructure, it is then impossible for third parties to intercept and decrypt a message. But the implementation of the quantum internet is still a major technical challenge, some aspects of which are still being studied in the laboratory. For example, when two users are very far apart, the main problem is the loss of photons along the way. To date, the systems proposed to compensate for this signal loss have introduced security flaws. Quantum teleportation offers a reliable solution, but only simplified and limited versions have been implemented. However, the team led by Ronald Hanson of Delft University has just developed a quantum teleportation device that would work for users far apart.

In quantum computers and tomorrow’s Internet, the unit of information is the qubit, a generalized form of the classical bit that takes on the values ”0″ or “1”. Intuitively, the qubit has more degrees of freedom because quantum superposition of the “0” state and the “1” state is allowed. This qubit is therefore very useful for performing quantum calculations in computers. In addition, when transmitting messages, it is impossible to extract information from a qubit, i.e. to copy it, without changing it. Quantum cryptography uses this property to its advantage, since it is then quite easy to determine whether a third person has intercepted the encryption key carried by the qubits that two interlocutors (Alice and Bob) send to each other. If these two interlocutors notice anomalies in the exchanged qubits, they conclude that their key has been read. They then transmit another until they are sure there hasn’t been a break-in, and they can then encrypt their messages with this new key.

This advantage is also a weakness: from the point of view of Alice and Bob, the disturbances in the environment, such as the loss of the photon in the glass fiber (which corresponds to signal attenuation) are equivalent to proof that a third party was trying to read their message. This fragility severely limits technological applications, with the risk of losing interest in the quantum system or creating security vulnerabilities. For example, to deliver a message to Bob, Alice must send qubits in the form of photons down an optical fiber. The problem with this support is that when the distance between Alice and Bob is typically more than a hundred kilometers, the signal will be attenuated fairly quickly and lost entirely. In a typical fiber optic communication system, the solution is to install repeaters in the fiber path. A repeater receives a weakened signal from the transmitter, measures it, copies it and then amplifies it and sends it on to the receiver. You can add as many repeaters as you like to the path.

The downside with a quantum network is that the effect of a repeater would destroy the interesting properties of the qubits. In effect, the repeater performs a “measurement” of the qubit. Consequently, the use of a classic repeater is impossible in a quantum network. One solution remains to decrypt and re-encrypt Alice’s message to send it to Bob. For example, the experimental quantum network connecting Shanghai to Beijing uses 32 transponders. This then boils down to dividing the line into multiple segments and setting up a quantum cryptographic protocol for each. But in this case one must be sure that no attacker penetrates at the level of the repeaters (we speak of “trusted nodes”) that connect two consecutive segments. This limitation cannot be guaranteed on a global network scale and would constitute a security breach.

However, quantum mechanics offers a solution thanks to the phenomenon of entanglement. Two particles are entangled when their properties are more strongly correlated than classical physics allows. “Concretely, explains Marc-Olivier Renou from the Institute of Photonic Sciences in Barcelona, these correlations cannot be explained by an information transfer that would be carried by information carriers moving at almost finite speed. Regardless of the distance between the two particles, they must be interpreted as a single system. This causes a measurement made on one of the particles to appear immediately on the two particles, even if they are very far apart. We can then set up a quantum teleportation protocol. This works in two stages. In the first step, an entangled pair is distributed between Alice and Bob, taking fiber losses into account. In the second step, Alice prepares a qubit that contains the useful information she wants to convey to Bob. She then performs a combined measurement of this information qubit and its entangled qubit with Bobs. The result of this pure quantum operation is then that the information disappears from Alice’s side and ends up with Bob. This second step is not subject to any quantum losses.

In 1997, Anton Zeilinger’s team from the University of Innsbruck performed this variant of quantum teleportation called “between two neighboring nodes”, i.e. directly between Alice and Bob, fed directly from a source of entangled photons. Many teams have performed variations on this experiment. In particular, the team led by Nicolas Gisin from the University of Geneva implemented this protocol in 2007 over long distances using infrared photons (the fibers have minimal loss for these wavelengths) in Swisscom’s fiber optic network. The team has thus shown that the idea is not completely unfeasible.

The device described so far with a teleporter between Alice and Bob does not seem very interesting since it is still necessary to propagate photons in the fiber to set up a teleporter, a stage that suffers from a risk of photon loss. And the method is much harder than directly sending photons that carry information.

But when the distance between Alice and Bob is very large, the strategy is to divide the distance into two segments! Let’s place Charlie halfway between Alice and Bob and equip them all with quantum memory systems. Alice has a source that emits entangled pairs of photons, and Bob has another source. Each sends out a pair of these particles, stores one photon in memory, and sends the other to Charlie. The latter thus receives a photon from Alice and a photon from Bob. Charlie stores photons in quantum memory until he has both. This is particularly useful when a photon is lost on a branch, say between Alice and Charlie; it is then enough for Alice to re-emit an entangled photon, but not for Bob. The method saves time because the photons only travel half the total distance at a time and it takes half the time to see if they are lost.

Once Charlie has the two photons, he performs a simultaneous measurement on those photons, which are then entangled. But according to the laws of quantum physics, the photon that Alice kept entangles in turn with the photon that stayed with Bob.*exchange entanglement*). The state of Alice’s qubit is then directly linked to that of Bob. This “teleporter” is then ready to transmit information.

One of the major technical difficulties in teleporting to non-neighboring nodes, i.e. with Charlie as an intermediary, is the realization of a quantum memory. It is in fact necessary to store the photon without measuring it and triggering its restitution when needed without changing its state. But thanks to advances in this field, Ronald Hanson’s team has implemented the quantum teleportation method between two non-neighboring nodes. A year earlier, the team had designed a three-node quantum network, but its properties were not sufficient to reliably operate quantum teleportation: fidelity (the probability that the state Bob finally gets is the same as the report Alice wanted to send) was insufficient. In this system, the qubits are physically supported by the spin of a crystalline defect (specifically, a nitrogen vacancy center, denoted as NV) in a diamond. At each node, the physicists placed an information qubit and a storage qubit (carried by a carbon atom next to the defect). The information is transmitted from one diamond to another via optical fibers with a protocol adapted to this device. The Delft team then optimized the performance of their system, for example by protecting the memory with a magnetic field to avoid interactions with neighboring atoms. The researchers thus doubled the number of entangled qubits between Alice and Bob, achieving an accuracy rate of 71% (a classical transmission has a maximum limit of 66%, confirming that teleportation worked well).

This new finding is a crucial step in the long-distance transmission of information through a quantum cryptographic protocol. Nevertheless, for a specific application it will be necessary to further improve the performance of the various components. “If it can be said that the war lies mainly in improving the properties of quantum memories,” emphasizes Julien Laurat, professor at the Kastler-Brossel laboratory at the Sorbonne University in Paris and a specialist in these systems. Today there are several techniques for designing quantum memories. They are based on single entities (an ion or NV center) or groups of elements (ions or cold atoms). The challenge then is to have a long enough storage time to synchronize the quantum connections and high writing and reading efficiency to store and recover the photons with high success rates, which requires good quality light-material coupling. Added to this is the possible possibility of multiplexing these memories, which remains an experimental challenge. The results of the team of Ronald Hanson and others, like those of Julien Laurat and his colleagues on very efficient cold nuclear storage, are a sign of accelerating progress in the field and point to future applications.

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