Researchers used innovative techniques and overcame previous limitations to create a new speed record of 7.1 qubits per second, advancing the development of quantum long-distance transmission technology and marking a critical step towards an efficient and far-reaching quantum internet.

Quantum teleportation uses quantum entanglement and classical communication to transmit quantum information to distant locations. This concept has been implemented in a variety of quantum light systems, ranging from laboratory experiments to practical real-world tests. It is worth noting that by using the low-Earth orbiting Micius satellite, scientists have successfully transmitted quantum information to a distance of more than 1,200 kilometers. However, there is currently no quantum transmission system with transmission rates on the order of Hertz. This hinders future applications of quantum internet.

In a paper published in "Light Science & Application", a team of scientists led by Professor Guo Guangcan and Professor Zhou Qiang of the University of Electronic Science and Technology of China collaborated with Professor You Lixing of the Shanghai Institute of Microsystems and Information Technology, Chinese Academy of Sciences. Based on the "UESTC First City Quantum Internet", they increased the teleportation rate to 7.1 qubits per second for the first time. This set a new record for a quantum teleportation system within a city area.

a, Aerial view of the conveyor system. Alice 'A' is located in the network switching room, Bob 'B' and Charlie 'C' are located in two different laboratories. All optical fibers connecting the three nodes belong to the UESTC backbone network. During the experiment, only the signals created by Alice, Bob, and Charlie were transmitted through these "dark" fibers. Alice prepares the initial state with a weakly coherent single-photon source and sends it to Charlie through the quantum channel. Bob's entangled source generates a pair of entangled photons, which then sends the lazy photon to Charlie through another quantum channel. Charlie performs a joint Bell state measurement (BSM) on the qubits sent by Alice and Bob, projecting them into one of four Bell states. Then, the BSM result is sent to Bob through the classical channel, and Bob performs unit (U) transformation on the signal photon to restore the initial state.

"Demonstrating high-speed quantum teleportation outside the laboratory involves a series of challenges. This experiment shows how to overcome these challenges, thereby setting an important milestone for the future quantum internet," said Professor Zhou Qiang, the corresponding author of this work. A major experimental challenge for real-world quantum teleportation systems is performing Bell state measurements (BSM).

To ensure the success of quantum teleportation and improve the efficiency of Bell State Measurement (BSM), Alice's and Bob's photons need to be indistinguishable at Charlie after being transmitted over long distances in optical fibers. The research team developed a fully operational feedback system that enabled rapid stabilization of photon path length differences and polarization.

The team, on the other hand, used a periodically polarized lithium niobate waveguide at the end of a single optical fiber to generate entangled photon pairs. On this basis, they developed a high-quality quantum entangled light source with a 500MHz repetition rate for long-distance transmission systems.

The red bar is the fidelity measured using QST. The blue bar is the fidelity obtained using DSM. The fidelity of both methods exceeds the classical limit of 2/3, the gray dashed line.

Such high-speed quantum teleportation based on quantum optics requires the most sensitive photon sensors to collect as many events as possible. The team led by Professor You Lixing, together with colleagues from Photon Technology Co., Ltd., provided high-performance superconducting nanowire single-photon detectors for this experiment. Because the detector is extremely efficient and virtually noise-free, high-efficiency BSM and quantum state analysis are achieved.

The research team used two methods, quantum state tomography and decoy state, to calculate the fidelity of remote transmission, which is much higher than the classical limit (66.7%), confirming that high-speed urban quantum remote transmission has been realized.

In the future, the "University of Electronic Science and Technology of China No. 1 Metropolis Quantum Internet" is expected to combine integrated quantum light sources, quantum repeaters and quantum information nodes to develop "high-speed, high-fidelity, multi-user, long-distance" quantum Internet infrastructure. The team also predicts that this infrastructure will further promote the practical application of quantum internet.