The quantum internet, a network that exploits the unique effects of quantum physics, will be fundamentally different from the classical internet we use today, and research groups around the world are already working in the early stages of their development, and it could find applications long before it reaches Technological Maturity Physicists in China have established a mysterious quantum bond between particles, called entanglement, over tens of kilometers of a standard optical fiber, setting a new record. The progress marks a long step towards a fully quantum mechanical Internet, although such a network is still many years away. But this achievement is not due to one specific breakthrough, but to the careful implementation of many methods. The Quantum Internet is such a super-secure network that can be useful long before it reaches technological maturity. Entanglement links the strange states of tiny quantum mechanical objects. For example, a part can rotate either clockwise or counterclockwise, but an atom can rotate both directions at the same time - at least until it is measured, and this two-sided state collapses to one side or the other. Two atoms can be entangled so that each is in an indeterminate two-sided state, but their rotations are definitely correlated, say in opposite directions. So if physicists measure the first atom and find that it is spinning clockwise, they will immediately know that the other must be spinning counterclockwise, no matter how far away it is. Entanglement would be the key to a fully quantum Internet that would allow the quantum computers of the future to communicate with each other and be immune to hacking. If hackers messed up the communication, they would mess up the entanglement by exposing their presence. Various companies are already selling systems that send messages in quantum states of light, which are basically unhackable. But to use such links, the information must still be decoded at each network node that is potentially vulnerable. In the quantum Internet, any node can be entangled with any other, so messages between them cannot be decoded at intermediate nodes. But developers must first stretch the entanglement over long distances. Previously, researchers have demonstrated the entanglement of two pieces of matter per 1.3 km of optical fiber. Now, Xiao-Hui Bao, Jian-Wei Pan and colleagues at China University of Science and Technology, Beijing, have demonstrated entanglement over fiber optic links up to 50 kilometers, they report in the journal Nature. The details are dizzying, but the basic idea of the experiment is relatively simple. The researchers start with two identical stations in the same lab, each containing a cloud of rubidium atoms. Picking up each cloud with a laser, they generate a photon whose polarization, which can be clockwise or counterclockwise, is entangled with the internal state of the cloud. They then send the photons down two parallel optical fibers to a third station in another lab 11 kilometers away, where the photons interact in such a way that they instantly pass through the initial entangled link with two distant clouds of atoms. Prototype nitrogen vacancy center, hardware used in a quantum network at the QuDelft laboratory in the Netherlands. Credit: Marcel Wogram for Nature. To do this, physicists use the fact that, according to quantum mechanics, a measurement can affect the state of the object being measured. At the destination lab, physicists have set up a photon polarization measurement that, even though it consumes photons, also “projects” them into a certain entangled state with a 25% probability. For these tests, the measurement instantly transmits entanglement back to the atomic clouds. The researchers performed a variant of the experiment that extended the link from 22 kilometers to 50 kilometers, although the fibers were wound on spools. For the experiment to be successful, Peng said, the team needed to pick a few elements correctly. The main obstacle was to prevent the absorption of photons in the optical fiber. To do this, Pan and his colleagues used a different laser pulse and a device called a waveguide to stretch the wavelength of photons by 60% to the sweet spot for transmission over a standard optical fiber. At the same time, the researchers made their lives easier because the atomic clouds were less than 1 meter apart and simply connected by a long optical fiber. This closeness greatly simplified the synchronization of the experiment. So, strictly speaking, the history of entanglement of atomic-scale particles separated by 1.3 kilometers still remains. However, the experiment is important because for a network, the installation connection is about half of the basic element, calledquantum repeater. The repeater will consist of two systems, similar to the one in the experiment, placed back to back. Once physicists had entangled the atomic clouds at the ends of each system, they could perform additional measurements on the clouds in the middle, which would reverse the entanglement with the clouds at the ends, stretching the entanglement in half. This experiment is a big step towards a quantum repeater. Optical fibers can be used to create a quantum internet But some aspects of the work must be improved before they can be used to create a quantum repeater. In particular, atomic clouds do not store their subtle quantum states long enough to provide the multiple binding required in a quantum repeater. Goals clear, targets set The well-known team of quantum internet researchers at the Delft University of Technology in the Netherlands has now released a roadmap outlining the steps to improve the network - and detailing the technological challenges that may arise at each level. Their predictions are described in Science. The researchers argue that the technology, which will complement rather than replace the existing Internet, could eventually become widespread for both large users such as university labs and individual consumers, although they do not give specific timelines for the projects. Researchers aim to create machines that can outperform classical computers. In the field of quantum computing, this is much more than anything or nothing. It is difficult to predict which technology will come first - the widespread quantum internet or useful quantum computers. But quantum networks have a great advantage in that such a network can be created step by step, and different functions can be added at each stage. The roadmap also aims to create a common language for a field that involves researchers with diverse backgrounds, including information technology, computer science, engineering, and physics. A laboratory in QuTech that houses a special crystal capable of storing quantum memory and serving as a network node for quantum communication over long distances. Six Steps to the Quantum Internet Quantum networks and quantum computing share many concepts and techniques. Both take advantage of phenomena that have no analogues in classical physics: for example, a quantum particle such as an electron or a photon can be in one of two well-defined states of rotation, clockwise or counterclockwise - but also in a simultaneous combination of and both are called superpositions. And two particles can be "entangled" in which they have a common quantum state. This causes them to act in seemingly coordinated ways (such as spinning in opposite directions) even when separated by vast distances. The researchers outlined six milestones of excellence that the future quantum internet can achieve and what users can do at each level. 0 Trusted Node Network: Users can receive quantized codes, but cannot send or receive quantized states. Any two end users can share the encryption key (but the service provider will also know it). It is kind of stage 0 in that it doesn't describe a true quantum internet, it's a network that allows users to set up a shared encryption key so they can share their (classic) data securely. Quantum physics only happens behind the scenes: the service provider uses it to create the key. But the provider also knows the key, which means users must trust it. This type of network already exists, especially in China, where it spans 2,000 kilometers and connects major cities including Beijing and Shanghai. Lab at QuTech In the first phase, users will start participating in a quantum game in which the sender creates quantum states, usually for photons. They will be sent to the receiver, either via an optical fiber or via a laser pulse emitted through outer space. At this point, any two users will be able to create a private encryption key that only they know. The technology will also allow users to send a quantum password, for example, to a machine such as an ATM. The machine will be able to check the password without knowing what it is and will not be able to steal it. 1 Prepare and Measure: End users receive and measure quantum states (but the quantum entanglement phenomenon is not necessarily involved). Two end users can share the private key only they know. Also, users can check their password without revealing it. Stage 1 has not been tested on a large scale, but it is already technologically feasible at the scale of small towns, although it will be very slow. Group in gLave with Pan Jian-Wei of the China University of Science and Technology in Hefei set the world record for this kind of transmission in 2017 when they used satellite to link two laboratories more than 1,200 kilometers apart. Scientists want to build quantum networks that are fully quantum, where information is created, stored and moved in such a way as to reflect the paradoxical behavior of the quantum world 2 Entangling distribution networks: any two end users can receive entangled states (but not store them). They provide the highest possible quantum encryption. In the second stage, the quantum Internet will use the powerful phenomenon of entanglement. His first goal will be to make quantum encryption virtually indestructible. Most of the methods required for this step already exist, at least as rudimentary laboratory demonstrations. 3 Quantum memory networks: any two end users to receive and store entangled qubits (quantum unit of information) and can teleport quantum information to each other. Networks enable cloud quantum computing. 4 and 5 Quantum computing networks. Devices on the network are full-fledged quantum computers (capable of correcting errors in data transmission). These steps would allow varying degrees of distributed quantum computing and quantum sensors to be used with applications for scientific experimentation. Stages 3 to 5 will allow any two users to store and exchange quantum bits or qubits for the first time. These are units of quantum information, similar to the classical 1 and 0, but they can be in the superposition of 1 and 0 at the same time. Qubits are also the basis for quantum computing. (A number of labs—both in academia and in large corporations such as IBM or Google—are building ever more complex quantum computers; the most advanced have memories that can hold dozens of qubits.) A single qubit is all it takes to operation of a quantum communication network. The Quantum Internet will build on and make the most of the existing classical Internet. It will take several breakthroughs to get to the final stage. The avant-garde team is working on building the first “quantum repeater,” a device that could help entangle qubits over greater and greater distances. To solve complex problems Early users of the networks of the highest level of the quantum Internet are likely to be scientists themselves. Laboratories will be able to remotely connect to the first advanced quantum computers or connect such machines to work as a single computer. They could then use these systems to perform experiments that are not possible with classical machines, such as simulating the quantum physics of molecules or materials. Networks of quantum clocks can greatly improve measurement accuracy for phenomena such as gravitational waves, and distant optical telescopes can link their qubits to sharpen images. But there may be applications outside of science. Quantum Internet is real In the near future, the quantum Internet may become a specialized branch of the ordinary Internet. Research teams around the world are currently developing chips that could allow a classical computer to connect to a quantum network. Humans will use classical computing most of the time and only connect to the quantum network for specific tasks. For example, you can connect a classical quantum computer to a quantum network to send a message using quantum cryptography, perhaps the most mature quantum technology. In quantum cryptography, the sender uses a cryptographic key encoded in a quantum signal to encrypt a message. According to the laws of quantum mechanics, if someone tries to intercept the key, they will destroy it. Like quantum computing, quantum communication records information in so-called qubits, similar to how digital systems use bits and bytes. While a bit can only take on the value of zero or one, a qubit can also use the principles of quantum physics to take on the value of zero and one at the same time. This is what allows quantum computers to perform certain calculations very quickly. Instead of solving multiple versions of a problem one after the other, a quantum computer can process them all at the same time.