Scientists propose new quantum invisible transfer state
Quantum invisible transfer technology allows quantum information to be sent between two distant quantum objects (the sender and the receiver), during which the information is transferred using quantum entanglement properties.
The uniqueness of this process is that instead of transmitting information through a communication channel that connects the two parties by sending quantum bits, the information is destroyed in one place and appears in another, with no physical "travel process" in this process. This amazing phenomenon occurs only because of the combination of quantum entanglement and the transmission of classical bits as we know them today.
Quantum invisibility currently holds great promise in the field of quantum communication and networking, as it will allow quantum bits to be transmitted over very long distances between network nodes, using distributed entanglement as previously described. This technology will facilitate the integration of these quantum technologies with current telecommunication networks and allow ultra-secure communication, extending them over very long distances.
Proposed theoretically in the early 1990s, experimental demonstrations of quantum invisible transfer have been performed by different groups around the world. While the scientific community has gained a wealth of experience over the years in how to conduct these experiments, there is still an open question in how to actually transmit information to enable fast, reliable quantum communication over a vast network.
--Such an infrastructure should be compatible with existing telecommunication networks; moreover, quantum stealth transfer protocols require a final operation on quantum bits with remote transmission of information, a feature known as "active feed-forward ", which allows the transmission of information with high fidelity over long periods of time.
For this purpose, the receiver must have a device called "quantum memory" that can store the quantum bits without distorting them until the last operation is applied. Finally, this quantum memory must be able to operate in a "multiplexed or multimodal" manner in order to maximize the speed of information transmission when the sender and receiver are far apart. To date, no implementation has included these established requirements in the demonstration itself.
In this context, ICFO researchers Dario Lago-Rivera, Jelena V. Rakonjac and Samuel Grandy, led by ICREA professor Hugues de Ridmaton, have achieved long-range invisible transmission of quantum information from photons to solid-state quantum bits. The results were published in Nature Communications.
The technique involves the use of active feedforward (active feedforward scheme), combined with the polymorphism of the memory, making it possible to maximize the rate of long-distance transmission. The proposed experimental architecture has been shown to be compatible with telecommunication channels and will therefore enable the integration and scaling of future long-range quantum communications.
Specifically, the team built two experimental stations: commonly referred to as Alice and Bob. the two are connected by a 1 km long optical fiber wound on a spool to simulate the physical distance between the two sides.
Three photons are involved in the experiment. The entangled photon pair is generated at Alice, the signal photon is delivered to a Pr-doped crystal, and the twin photon (idler photon) is sent through a 5 m or 1 km fiber to Bob, where an arbitrary quantum of 1436 nm is generated and interferes with the twin photon. The detection results are sent back to Alice, where they are processed for the correct application of the feedforward.
Schematic of the experimental setup of the quantum invisible state transfer platform / ICFO
Once created, "we save the first 606 nm photon in Alice and store it in a multiplexed solid-state quantum memory, keeping it in memory for future processing. At the same time, we took the signal photons created in Alice and sent them through several kilometers of optical fiber to another experimental station, called Bob," recalls Dario Lago.
In this second configuration, the scientists had another crystal - Bob - where they created a third photon (photon 3), where they encoded the quantum bits they wanted to transmit. After the third photon was created, a second 1436-nm photon was transmitted from Alice to Bob, which was the top priority of the stealthy state transmission experiment.
Photons 2 and 3 interfere with each other in a "Bell State Measurement (BSM)". The effect of this measurement is to combine the states of photons 2 and 3.
Since photon 1 and photon 2 are entangled from the beginning, that is, their properties are correlated, the BSM results in the transfer of the information encoded in photon 3 to photon 1, which is stored by Alice in a quantum memory 1 km away.
As Dario Lago and Jelena Rakonjac point out, "we were able to transfer information between two photons that had never been in contact before, connected by a third photon entangled with the first one."
In addition, he says, "What is unique about this experiment is that we used a multiple quantum memory that was able to store the first photon long after the first device (Alice) interaction occurred. We can still process the transmitted information as described in the protocol."
This "processing" is the active feed-forward technique mentioned by Dario and Jelena. Based on the results of the BSM between photons 2 and 3, photon 1 is phase-shifted after being stored in memory. In this way, the first photon will always encode the same state: because without it, half of the invisible transfer events would have to fail.
On the other hand, the multimodal/multiplexing of quantum memories allows them to increase the rate of stealthy transmission over distances of more than 1 km without degrading the quality of the quantum bits transmitted over long distances. This leads to a tripling of the teleportation rate compared to single-mode quantum memories, which are mainly limited by hardware speed.
Multiplexing capability and long-range stealthy transfer
The precursor to this experiment dates back to 2021: an experiment conducted by the same group that year successfully connected two multimode quantum memories separated by 10 meters for the first time.
As Hugues de Riedmaaton emphasizes, "quantum invisible transfer will be the key to achieving high-quality, long-range communication in the future quantum Internet. Our goal is to achieve quantum invisibility in increasingly complex networks with pre-distributed entanglement. The nature of our quantum nodes (multiplexed and solid-state), and their compatibility with telecommunication networks, make them promising candidates for long-range deployment in established fiber-optic networks."
Despite these important results, improvements to the experiment are already underway. On the one hand, the team is focusing on developing and improving techniques to extend this setup to longer distances while maintaining the efficiency and long-range transmission rates described above.
On the other hand, their goal is to study and use this technology to transfer information between different types of quantum nodes in order to create a future quantum Internet capable of distributing and processing quantum information between distant parties.
Reference links:
[1] https://www.nature.com/articles/s41467-023-37518-5
[2] https://worldnationnews.com/new-long-distance-quantum-teleportation-technique/
