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 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 that the information is destroyed in one place and appears in the other without physically traveling between the two. This incredible phenomenon only occurs thanks to quantum entanglement and together with the transmission of what we now know as classical bits.
Quantum teleportation allows quantum information to be sent between two distant quantum objects, a sender and a receiver, using quantum entanglement.
Currently, there is great interest in quantum teleportation in the field of communications and quantum networks, as 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 into current telecommunications networks and would allow ultrasecure communications to be extended over very long distances.
Quantum teleportation was theoretically proposed in the early 1990s, and experimental demonstrations have been carried out by various groups around the world. Although the scientific community has gained vast experience on how to perform these experiments over the years, there is still an open question on how to practically teleport information, allowing fast and reliable quantum communication over a vast network.
Such infrastructure must be compatible with the current telecommunications network. In addition, for the quantum teleportation protocol, a final operation on the qubit with the teleported information is required, a characteristic called “active feed-forward”, to allow the transmission of information to be carried out with fidelity and greater speed.
For this, the receiver must have 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 must be able to operate in a “multiplexed or multimodal” way to maximize the speed of information teleportation when the sender and receiver are far apart. So far, no implementation has incorporated these requirements mentioned in the demo itself.
Long-distance teleportation of quantum information from a photon to a solid-state qubit, i.e. a photon stored in a multiplexed quantum memory, has been achieved
In this context, ICFO researchers Dario Lago-Rivera, Jelena V. Rakonjac and Samuele Grandi, led by ICREA Professor Hugues de Riedmatten, have achieved long-distance teleportation of quantum information from a photon to a solid-state qubit, or that is, a photon stored in a multiplexed quantum memory. The results are published in Nature Communications.
The technique involved the use of active feed-forward (active feed-forward scheme), which, together with memory multimodality, allowed to maximize the teleportation rate. The architecture of the proposed experiment proved to be compatible with telecommunication channels and, therefore, would allow future integration and scalability for long-distance quantum communication.
How to get quantum teleportation
The team built two experimental stations, often called Alice and Bob in community parlance. Both were connected by a 1 km optical fiber wound on a spool, to simulate a physical distance between the parts.
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 at 606 nm, called the signal photon (photon 1), and the second photon at 1436 nm called the idle photon (photon 2), consistent with the telecommunications infrastructure.
Scheme of the experimental assembly of the quantum teleportation platform. /ICFO
Once created, “we save the first 606 nm photon in Alice and store 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 optical fiber to the second experimental station, called Bob”, recalls Dario Lago.
In this second configuration, Bob, the scientists had another crystal where they created a third photon (photon 3) where they encoded the quantum bit they wanted to teleport. Once the third photon was created, the second 1436 nm photon reached Alice’s Bob, and this is where the bulk of the teleportation experiment takes place.
Teleporting information 1 km 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 up the state of photons 2 and 3.
Thanks to the fact that Photon 1 and Photon 2 were entangled from the start; 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 the quantum memory, 1 km away.
As Dario Lago and Jelena Rakonjac mention, “we are able to transfer information between two photons that were never in contact before, but connected through a third photon that was entangled with the first”.
Furthermore, they add: “The uniqueness of this experiment lies in the fact that we used a multiplexed quantum memory capable of storing the first photon long enough so that when the first device, Alice, knew that the interaction had taken place, we could still process the teleported information as described in the protocol”.
We are able to transfer information between two photons that have never been in contact before, but connected through a third photon entangled with the first.
Dario Lago and Jelena Rakonjac (ICFO)
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, because without it, half of the teleportation events would have to be discarded.
On the other hand, the multimodality/multiplexing of quantum memory allowed them to increase the teleportation rate beyond the limits imposed by the 1 km separation between them, without degrading the quality of the teleported qubit. This resulted in a teleportation rate three times that of a single-mode quantum memory, limited mainly by hardware speed.
Scalability and Integration
The experiment carried out by this group in the year 2021, where for the first time they were able to intertwine two multimodal quantum memories separated by 10 meters and announced by a photon at the wavelength of telecommunications, was the precursor of this experiment.
As Hugues de Riedmatten emphasizes, “Quantum teleportation will be crucial in enabling high-quality long-distance communication for 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 (both multiplexed and solid-state), as well as their compatibility with the telecommunications network, makes them a promising candidate for long-haul 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 focused on developing and improving the technology to extend this setup to much greater distances while maintaining the previously mentioned efficiency and teleportation rates.
On the other hand, they also intend to study and use this technique for transferring 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.
Dario Lago-Rivera, et al. “Long-distance multiplexed quantum teleportation of a telecommunications photon to a solid-state qubit”. Nature Communications 2023