Pan Jianwei’s team set another record, quantum entanglement distance enough to connect two cities

Just now, there’s good news for quantum entanglement technology. Pan Jianwei and Bao Xiaohui’s team at the University of Science and Technology of China achieved long-distance quantum entanglement in the laboratory, and the two experimental schemes achieved 22 km and 50 km of quantum entanglement, respectively, setting a world record. With a distance of 50 km enough to connect the two cities, this experimental results and the related techniques used in the experiment, or paving the way for multi-node, long-range quantum entanglement, will be an important step towards achieving a long-distance quantum communication network.

On February 13, the results were published in the form of a paper on Nature.

Let quantum entanglement go a little further.

From the history of computer and network development, to achieve large-scale network, long-distance communication technology is essential.

In the field of quantum computing and quantum communication, quantum entanglement can achieve not only the characteristics of qubits, multi-state and encrypted information transmission, but also long-distance communication capabilities. The ultimate application of quantum communication is the transmission of information, and after quantum entanglement, scientists have been working on the application of long-distance quantum entanglement and related information transmission, but scientists have struggled to figure out how to get further.

In fact, long-distance entanglement has made significant progress over the past two decades. To achieve long-distance entanglement, it was previously common practice to have entangled photons transmitted between nodes on the fiber or via satellite. However, severe transmission losses limit the success rate of photon distribution and limit the distance of quantum entanglement. In 2015, the Ronald Hanson team at Technische University of Technology validated quantum entanglement between two diamond chromatote systems at a distance of 1.3 km, and also validated for the first time the feasibility of long-distance quantum entanglement. But that distance is not enough to support the creation of a long-distance quantum communication network or quantum Internet.

However, in this experiment, the research team led by Professor Pan Jianwei and Professor Bao Xiaohui of the Chinese University of Science and Technology has innovatively combined several technologies, and has solved the problem of long-distance quantum entanglement in a targeted manner, realizing the quantum entanglement of long distances.

“The main innovation of this experiment is the development of high-efficiency optical and atomic entanglement technologies adapted to low-loss transmission within fiber optics, as well as the remote interference of memory light sources after transmission through long fibers,” Bao Xiaohui, a professor at the University of Science and Technology, one of the study participants, told Deeptech. “

Self-researching critical devices that significantly reduce attenuation

Theoretically, quantum entanglement refers to the interaction of two particles over a period of time, assuming that the quantum state is not destroyed, no matter how far apart, the form they were finally observed in, although based on quantum mechanics is random, but there is always a correlation between the two. But to achieve long-distance quantum entanglement is far from separating two quantum entanglement systems far away.

Today, long-range quantum entanglement is similar to long-range communication, establishing the transmitter, receiver, and connection between them. Mapping to quantum entanglement requires the creation of a pair of quantum entanglement systems, then the establishment of long-distance connections, and finally the correctness of quantum entanglement between them. It may seem simple, but there are many difficulties to achieve.

First, to establish effective quantum entanglement. According to Bao Xiaohui, the research team is using the “entanglement exchange” technology to complete the establishment of quantum entanglement. The idea is to select two separate quantum memorys in the same laboratory to select the niobium atomic group, and then establish two pairs of light and atomic entanglement, that is, each quantum memory emits a photon, which is entangled with the atomic cluster. The photons given by the two memorys are then measured by interference after long-distance transmission, and quantum entanglement is established between the two quantum memorys, and the original state of the quantum memory is stored. It is more similar to the diplomatic relations between the two countries, which send envoys to negotiate in neutral countries.

Pan Jianwei's team set another record, quantum entanglement distance enough to connect two cities

It is worth noting that in this experiment, two separate quantum systems had a linear physical distance of only 0.6 meters — in just one lab, they were connected by two parallel fiber optics from the China University of Science and Technology in Hefei, Anhui province, to the software park. The “Middle Station” in the figure contains superconducting nanowire sensors for interferometry after long-range transmission of photons, a form of presence in quantum relay stations, and this setting is the result of previous research by Pan Jianwei’s team.

Pan Jianwei's team set another record, quantum entanglement distance enough to connect two cities

Then, the research team used two independent research techniques to solve the problem of high loss in the connection.

First, from the source, the team set up cavity enhancement technology in both quantum memorys to improve the coupling between single photons and atomic systems, and to optimize the efficiency of optical transmission, increasing the brightness of previous light and atomic entanglement by an order of magnitude. Among them, cavity-enhanced optical path is the research team’s independent development, the main idea is to improve the coupling between single photon and atomic system to reduce intracavity loss, and finally achieve the intracavity atomic state to photon state conversion efficiency of about 90%.

Second, the research team chose optical fiber as the connecting medium, with light as the carrier for transmitting information. However, the optical wavelength corresponding to the atomic memory is lost in the fiber by approximately 3.5 db/km, and the attenuation of the 50 km fiber will be one billion times (17.5 times of the specific decay multiple of 10), making quantum communication impossible.

In this regard, Bao Xiaohui introduced: “The research team independently developed the cycle polarization lithium niobium acid waveguide, through the nonlinear differential frequency process, the memory of the optical wavelength from near-infrared (795 nm) to the communication band (1342 nm), 50 km of optical fiber attenuation reduced to less than 100 times, compared with the previous increase of 16 orders of magnitude.” “Simply put, the photon frequency to be transmitted was changed to the communication band frequency with a smaller transmission, enabling quantum entanglement over long distances. This variable frequency setting is similar to the transformer we see every day, raising the voltage to reduce the energy lost in the transmission line when transmitting electrical energy.

Convergence of multiple technologies

In general, the experiment combines a variety of technologies to achieve long-distance quantum communication, in which technologies such as “cavity enhancement” and “variable frequency” are creatively used to solve years of unresolved high-loss challenges. Many of the technologies and innovative thinking used in the experiment also reflect the technical precipitation of Pan Jianwei’s team for many years of scientific research.

The field of quantum computing and quantum communication is undoubtedly one of the hottest fields in scientific research in recent years, and it is basically several major breakthroughs in a year. This experiment is of great significance for the realization of quantum communication.

However, the realization of long-distance quantum communication is far more than this step, Pan Jianwei, Bao Xiaohui led the research team at the end of the paper also mentioned that the distance of quantum entanglement further distance and the introduction of a variety of quantum memory may be a new research direction, and this is of great significance for the implementation of advanced quantum communication applications. The form of a quantum relay station used in this experiment can further broaden the span of quantum entanglement.

The paper for this experiment is submitted on March 26, 2019, nearly a year ago, i believe Chinese scientists will bring us new surprises, 2020 quantum information technology is worth looking forward to!