A method known as quantum teleportation makes use of the quantum entanglement phenomenon to transport quantum information between two distant quantum objects, a sender and a receiver.
The distinctive aspect of this method is that the information is not physically moved from one location to the other; rather, it is destroyed at one location and appears at the other without moving through a communication channel between the two parties. Entanglement in quantum mechanics, which is accompanied by the transmission of classical bits, enables this unexpected behavior.
The ability to move quantum bits between network nodes over extremely long distances using previously shared entanglement has sparked intense interest in quantum teleportation in the fields of quantum communications and quantum networks. This would facilitate the incorporation of quantum technologies into existing telecommunication networks and increase the range of ultra-secure communications made possible by these systems.
In addition, quantum teleportation permits the transfer of quantum information between different kinds of quantum systems, e.g. between light and matter or between different kinds of quantum nodes.
In the early 1990s, quantum teleportation was theoretically postulated, and numerous groups around the world conducted experimental demonstrations. Although the scientific community has accumulated substantial knowledge on how to conduct these experiments, there is still uncertainty over the practicality of information teleportation, which would enable dependable and quick quantum communication over a wide network. It would seem reasonable for such infrastructure to work with the current telecoms network.
In order to transfer information more accurately and quickly, a feature known as active feed-forward is required by the quantum teleportation protocol to be applied to the teleported qubit, conditional on the outcome of the teleportation measurement (transmitted by classical bits). This means that the receiver requires a device known as a quantum memory that can store the qubit without degrading it until the final operation can be implemented.
Quantum teleportation will be crucial for enabling high-quality long-distance communication for the future quantum internet. Our goal is to implement quantum teleportation in more and more complex networks, with previously distributed entanglement. The solid-state and multiplexed nature of our quantum nodes, as well as their compatibility with the telecom network, make them a promising approach to deploy the technology over long distance in the installed fiber network.
Hugues de Riedmatten
When the sender and the recipient are far apart, this quantum memory should be able to function in a multiplexed manner to maximize the speed of information teleportation. These three needs have not yet been combined in a single demonstration by any implementation.
In a recent study published in Nature Communications, ICFO researchers Dario Lago-Rivera, Jelena V. Rakonjac, Samuele Grandi, led by ICREA Prof. at ICFO Hugues de Riedmatten have reported achieving long-distance teleportation of quantum information from a photon to a solid-state qubit, a photon stored in a multiplexed quantum memory.
The method used an active feed-forward system, which when combined with the memory’s multimodality allowed for maximization of the teleportation rate. Long-distance quantum communication was made possible by the suggested architecture’s compatibility with the telecommunications channels, which also allowed for future integration and scalability.
How to achieve quantum teleportation
The team built two experimental setups, that in the jargon of the community are usually called Alice and Bob. The two setups were connected by a 1km optical fiber spun up in a spool, to emulate a physical distance between the parties.
Three photons were involved in the experiment. In the first setup, Alice, and the team used a special crystal to create two entangled photons: the first photon at 606 nm, called the signal photon, and the second photon called the idler photon, compatible with the telecommunications infrastructure.
Once created, “we kept the first 606 nm photon at Alice and stored it in a multiplexed solid-state quantum memory, holding it in the memory for future processing. At the same time, we took the telecom photon created at Alice and sent it through the 1km of optical fiber to reach the second experimental setup, called Bob,” Dario Lago recalls.
The quantum bit that they wished to transport was encoded in a third photon, which was produced in the second setup, Bob, by the scientists. The fundamental event of the teleportation experiment occurs when the third photon is created and the second photon has already reached Bob from Alice.
Teleporting information over 1km
The second and third photons interfered with each other through what is known as a Bell State measurement (BSM). The effect of this measurement was to mix the state of the second and third photon. The first and second photons’ initial entanglement, or highly correlated joint state, allowed for the BSM to transfer the information contained in the third photon to the first photon, which Alice had stored in her quantum memory at a distance of one kilometer.
As Dario Lago and Jelena Rakonjac mention, “we are capable of transferring information between two photons that were never in contact before but connected through a third photon that was indeed entangled with the first. The uniqueness of this experiment lies in the fact that we employed a multiplexed quantum memory capable of storing the first photon for long enough such that by the time Alice found out that the interaction had happened, we were still able to process the teleported information as the protocol requires.”
This processing that Dario and Jelena mention was the active feed-forward technique mentioned earlier. Depending on the outcome of the BSM, a phase shift was applied to the first photon after storage in the memory. In this way, the same state would always be encoded in the first photon.
Without this, half of the teleportation events would have to be discarded. Furthermore, they were able to boost the teleportation rate past the restrictions set by their 1 km spacing without sacrificing the quality of the transported qubit thanks to the multimodality of the quantum memory. Overall, this resulted in a teleportation rate three times higher than for a single-mode quantum memory, only limited by the speed of the classical hardware.
Scalability and Integration
The experiment carried out by this group in 2021, where they achieved for the first time entanglement of two multimode quantum memories separated by 10 meters and heralded by a photon at the telecommunication wavelength, has been the precursor of this experiment.
As Hugues de Riedmatten emphasizes, “Quantum teleportation will be crucial for enabling high-quality long-distance communication for the future quantum internet. Our goal is to implement quantum teleportation in more and more complex networks, with previously distributed entanglement. The solid-state and multiplexed nature of our quantum nodes, as well as their compatibility with the telecom network, make them a promising approach to deploy the technology over long distance in the installed fiber network.”
Further improvements are already being planned. One the one hand, the team’s major goal is to advance technology and maintain rates and efficiency while extending the setup over considerably greater distances. For a future quantum internet that will be able to transmit and process quantum information between distant parties, they also plan to research and use this approach to transport information across various kinds of quantum nodes.
