Long-distance quantum teleportation using multiplexed quantum memories

Quantum teleportation is a technique that allows quantum information to be sent between two distant quantum objects, a sender and a receiver, using a phenomenon called quantum entanglement as a resource. The most unique feature of this process is that the information is not transmitted by sending quantum bits (qubits) through a communication channel that connects the two parties; rather, information is destroyed in one place and appears in the other without physically traveling between the two. This amazing phenomenon only happens thanks to quantum entanglement.

Today there is considerable interest in quantum teleportation in the field of quantum communications and quantum networks because it would allow the transfer of quantum bits between network nodes over very long distances, using pre-distributed entanglement. Such a technique would facilitate the integration of these quantum technologies in current telecommunications networks and would allow ultra-secure communications to be extended over very long distances.

Quantum teleportation was theoretically proposed in the early 1990s, and experimental demonstrations were carried out by various groups around the world. While the scientific community has gained extensive experience on how to perform these experiments over the years, there is still an open question on how to practically teleport information, enabling fast and reliable quantum communication over a vast network.

Such infrastructure should be compatible with the current telecommunication network. In addition, for the quantum teleportation protocol, a final operation is needed on the qubit with the teleported information, a characteristic called “active feed-forward”, to allow the transmission of information to be done faithfully and longer. speed. For this, the receiver must possess a device known as a quantum memory that can store the qubit without degrading it until the final operation can be implemented. Finally, this quantum memory should be able to operate in a “multiplexed” or “multimodal” manner to maximize the speed of information teleportation when the sender and receiver are far apart. To date, no implementation has incorporated these mentioned requirements into the demo itself.

In a recent study, Dario Lago-Rivera, Jelena V. Rakonjac and Samuele Grandi, from the ICFO (Institute of Photonic Sciences) in the Barcelona town of Castelldefels, led by ICREA Professor Hugues de Riedmatten, have achieved long-term teleportation quantum information distance from a photon to a solid-state qubit, that is, a photon stored in a multiplexed quantum memory. The technique involved the use of active feed-forward (active feed-forward scheme), which, together with the multimodality of memory, has made it possible to maximize the rate of teleportation. The architecture of the experiment has been shown to be compatible with telecommunication channels and would therefore allow future integration and large-scale use for long-distance quantum communication.

From left to right: Hugues de Riedmatten, Jelena Rakonjac, Dario Lago and Samuele Grandi, in an ICFO laboratory. (Photo: ICFO / D. Lago. CC BY-NC)

How to get quantum teleportation

The team built two experiment stations, often called Alice and Bob in the jargon of the quantum community. Both were connected by an optical fiber of 1 kilometer rolled on a reel, to emulate a physical distance between the parties.

Three photons were involved in the experiment. In the first configuration, Alice, the team used a special crystal to create two entangled photons: the first photon, 606 nanometers, called the signal photon (photon 1), and the second photon, 1436 nanometers, called the idle photon (photon 2). compatible with the telecommunications infrastructure. Once created, “we saved the first 606-nanometer photon in Alice and stored it in multiplexed solid-state quantum memory, keeping it in memory for future processing. At the same time, we took the telecommunications photon created in Alice and sent it through the kilometer of fiber optics to reach the second experimental station, called Bob”, sums up Dario Lago.

In this second configuration, Bob, the scientists had another crystal where they created a third photon (photon 3), where they had encoded the quantum bit they wanted to teleport. Once the third photon was created, the second photon of 1436 nanometers reached Bob from Alice, and this is where the main achievement of the teleportation experiment took place.

Teleporting information 1 kilometer away

Photons 2 and 3 interfered with each other through what is known as Bell State Measurement (BSM). The effect of this measurement was to mix the state of photons 2 and 3. Thanks to the fact that photon 1 and photon 2 were quantum entangled from the beginning; that is, their properties were correlated, the result of the BSM was to transfer the information encoded in photon 3 to photon 1, stored by Alice in quantum memory, 1 kilometer away.

As Dario Lago and Jelena Rakonjac mention, “we are able to transfer information between two photons that have never been in contact before, but are connected through a third photon that was entangled with the first. The uniqueness of this experiment lies in the fact that we used a multiplexed quantum memory capable of storing the first photon for long enough that, when the first device, Alice, knew that the interaction had occurred, we could still process the teleported information as and as described in the protocol”.

This “processing” mentioned by Dario and Jelena was the active feed-forward technique discussed above. Depending on the result of the BSM between photons 2 and 3, a phase shift was applied to photon 1 after storage in memory. In this way, the same state would always be encoded in the first photon since, without this, half of the teleportation events would have to be discarded. On the other hand, quantum memory multimodality/multiplexing allowed them to increase the rate of teleportation beyond the limits imposed by the 1 kilometer separation between them, without degrading the quality of the teleported qubit. This resulted in a teleportation rate three times that of single-mode quantum memory, limited primarily by hardware speed.

Integration and use on a large scale

The experiment carried out by this group in the year 2021, where they managed for the first time to entangle two multimodal quantum memories separated by 10 meters, has been the precursor of this experiment.

As Hugues de Riedmatten emphasizes, quantum teleportation will be crucial to enable high-quality long-distance communication in the quantum internet of the future. “Our goal is to implement quantum teleportation in increasingly complex networks, with pre-distributed entanglement. The nature of our quantum nodes (multiplexed and solid state), as well as their compatibility with the telecommunications network, makes them a promising candidate for long-distance deployment in the installed fiber network.”

Despite having these important results, improvements to the experiment are already underway. On the one hand, the team is focusing on developing and improving the technology to extend this setup to much longer distances while maintaining the aforementioned efficiency and teleportation rates. On the other hand, they also plan to study and use this technique for the transfer of information between different types of quantum nodes, in order to establish a future quantum internet that will be able to distribute and process quantum information between remote parties.

Hugues de Riedmatten and his colleagues present the technical details of the progress they have made and the experiments they have carried out in the academic journal Nature Communications under the title “Long distance multiplexed quantum teleportation from a telecom photon to a solid-state qubit”. (Source: ICFO)