Which result is stronger? | New science popularization, quantum communication papers from China and the United States were published in "Nature" at the same time

Recently, quantum communication papers from the University of Science and Technology of China and Harvard University were published in the same issue of Nature, attracting attention from the industry and society. Relying on Pan Jianwei, Bao Xiaohui, Zhang Qiang and other researchers from the Chinese Academy of Sciences' Institute of Quantum Information and Quantum Technology Innovation established by the University of Science and Technology of China, they built the world's first urban three-node quantum network based on quantum entanglement; Mikhail Lukin of Harvard University The team used telecommunications optical fibers in the Boston area to achieve dual-node long-distance quantum entanglement in the diamond SiV color center system for the first time, with the optical fiber distance reaching about 35 kilometers.

What is the science behind these two scientific breakthroughs? Which outcome has a higher level? What application value do they have? The reporter interviewed an expert team from the University of Science and Technology of China.

Basic Principles: Why Quantum Communication Can Be Eavesdropping-proof

The scientific principles of quantum communication originate from quantum mechanics, a branch of physics that is very different from classical mechanics and the most accurate theory to date describing the microscopic world. According to experts, if a physical quantity cannot change continuously and can only take on some discrete values, this quantity is quantized. Physical quantities in the macro world seem to change continuously, but in the micro world, many physical quantities are quantized, that is, there is a smallest unit that cannot be divided further.

Classical mechanics describes the state of a macroscopic object and gives its clear position. Quantum mechanics describes the state of a microscopic particle and gives a superposition state - this particle may be here or there under certain circumstances, with no definite position. Just like Sun Wukong's clone technique, it can appear in multiple places at the same time, and each clone is like the superposition state of Sun Wukong.

In the field of communication, the signals of classical communication are only 0 and 1, while quantum communication not only has signals 0 and 1, but also quantum superposition states such as 01 and 0-1. This principle of superposition leads to the principle of quantum unclonability: in quantum mechanics, it is impossible to achieve an exact copy of an unknown quantum state. This is the basis for quantum communication to achieve "unconditional security."

Any classical communication has the potential to be eavesdropped. When eavesdropping, the two signals 0 and 1 will not be disturbed, so the two communicating parties cannot detect it. Quantum communication can encode information into the polarization direction of the quantum superposition state of a single photon. A single photon is the smallest unit of light energy and cannot be divided. Its quantum state cannot be accurately copied, so any eavesdropping behavior will disturb it and be detected by both parties in the communication. Through quantum state transmission, the communicating parties negotiate to generate a quantum key, coupled with the "one-time pad" encryption protection of the information, the information can be completely random and undecipherable during transmission, fundamentally ensuring communication security.

Quantum entanglement: USTC has obvious efficiency advantages

"In the field of quantum information, entanglement is a very valuable resource. In quantum communication, quantum key distribution uses the non-locality of entanglement to determine the security of the key by comparing the entanglement measurement results in the hands of the sender and receiver. " Lin Mei, Ph.D. of the University of Science and Technology of China and popular science writer of "Mozi Salon" said that when several microscopic particles interact with each other, the characteristics of each particle will be synthesized into the overall properties, and the properties of a single particle cannot be described. This phenomenon is called "Quantum Entanglement".

The results of the University of Science and Technology of China recently published in "Nature" are the research team's use of cold atom ensemble to establish quantum entanglement between independent storage nodes through single photon interference for the first time, and on this basis, they built the world's first entanglement-based urban multi-sensor system. The node quantum network extends the distance of quantum entanglement network experiments from tens of meters to tens of kilometers.

Schematic diagram of experimental node layout: Alice node is located in the East District of USTC, Bob node is located in Hefei Innovation Industrial Park, and Charlie node is located in Anhui Institute of Optics and Mechanics

Entanglement efficiency is an important indicator of quantum entanglement network experiments. Compared with the results published by the Harvard University team in Nature, the entanglement efficiency of the results of the University of Science and Technology of China is more than two orders of magnitude higher, with obvious advantages.

Of course, the Harvard team also made an important breakthrough - achieving dual-node long-distance entanglement in the SiV color center system for the first time. Similar to USTC’s dozens of kilometers of entangled network, the Harvard team first entangled two quantum storage nodes together, then separated the two nodes through optical fiber links and deployed them across Cambridge, Somerville, and Watertown. and Boston on a roughly 35-kilometer loop. This means that they have also achieved quantum communication at the metropolitan level.

Map of two-node quantum network paths through Cambridge and Boston, USA

Both quantum storage nodes in the Boston area are made from thin sheets of diamond with a defect in their internal atomic structure called a SiV center. Using SiV centers as single-photon quantum memory devices is a technical route that the Harvard team has been studying for many years. It solves a conundrum in quantum internet theory—the inability to boost signals in traditional ways. According to the principle of quantum unclonability, an unknown quantum state cannot be accurately copied, so optical fiber signal repeaters cannot be used in quantum networks, making it difficult to transmit data over long distances.

Quantum network nodes based on SiV centers can not only capture, store and entangle quantum information bits, but also correct signal loss, making long-distance data transmission possible. The Harvard team has installed a demonstration network on existing optical fiber, demonstrating the feasibility of creating a quantum internet with similar network wiring.

Dr. Lin Mei believes that in terms of future feasibility verification, the metropolitan three-node quantum network built by the University of Science and Technology of China team is better than the optical fiber link established by the Harvard team. Because the latter only has two nodes, and the network prototype must contain at least three nodes, only in this way can its communication switching function be verified - when users A and B are communicating using the quantum network, user C can switch in at any time and communicate with User A or B communicates.

Looking to the future, quantum networks, with their extremely high security, can be used in important fields such as government affairs, finance, electricity, and big data, providing strong technological support for confidential communications. Quantum networks are also expected to connect quantum computers to form a quantum Internet in the future, allowing professionals and the public to experience the super computing efficiency of quantum computers.