Many people have played the megaphone game when they were children: A whispers the news to B, and then B whispers what he heard to C, and so on. Anyone who has played it knows that the final message is often completely different from the real message.
In a sense, this is closely related to the importance of repeater technology. A repeater is a device designed to avoid distortion in the transmission of information. Without a repeater, the data sent over the connection might become useless. Essentially, without repeaters, we wouldn't have large-scale computer networks.
While we have a variety of conventional repeaters, there is not yet a fully functional quantum repeater. With the development of quantum computers , scientists begin to need to connect these computers together, and quantum repeaters will also become a necessity. Paul Kwiat, director of the Kwiat Quantum Information Group and professor at the University of Illinois at Urbana-Champaign, is also a leader in quantum communications efforts at Q-NEXT, a U.S. Department of Energy (DOE) National Quantum Information Science Research Center led by the U.S. Department of Energy's Argonne National Laboratory. Q-NEXT, an organization composed of approximately 100 experts from three national laboratories, 10 universities, and 14 companies, is developing the science and technology needed to control and distribute quantum information. "Our goal is to implement and develop long-distance quantum communication links in a better way than existing systems, which requires the creation of quantum repeaters," said
Kwiat.
Copy Question
The quantum world is a strange realm and difficult for humans to understand. One of the differences between our experience in the realm of and : is the qubit of and cannot replicate .
Traditional repeaters are experts in the megaphone game. Instead of obfuscating the information children are passing around in whispers, many classic repeaters effectively take the information they are told in the form of some data, copy it exactly a few times, and then send those copies to the next node.
For quantum computing experts, the process is not simple when you use qubits (the basic units of quantum information) instead of the classical bits used in conventional computers. Like Schrödinger's cat , quantum systems have no definite state until they are measured, and the very act of measuring them can change the state of these quantum objects. In fact, Erwin Schrödinger conceptualized his cat problem by stating that we cannot understand the quantum world in the same way we understand the human world. "You can copy classical bits , but if you have a qubit and you don't know what its state is, you can't faithfully copy it because there is noise,"
Kwiat said. The "noise" Kwiat mentioned is one of the biggest challenges facing the field of quantum computing. To simplify this complex issue, here's an analogy: quantum noise is a bit like the noise we hear at parties. It can be difficult to hear our friends over the music and the sounds of other people talking.
In quantum computing, this noise is not audible to humans. It can be an electromagnetic signal from nearby Wi-Fi or a tiny interference in the earth's magnetic field.
So, if scientists can't replicate what they've done with classical systems, how are they going to create a quantum repeater to enable a long-distance quantum network?
While we don’t yet have a fully functional quantum repeater, we can make some claims about how they might work. One promising avenue is entanglement exchange.
Entanglement Swap Solution
Entanglement occurs when two or more quanta interact so that they are no longer independent of each other. Each quantum has certain properties, such as momentum, position, or polarization, that can be strongly coupled to the same properties of the other particle it is entangled with.
Entanglement exchange occurs when entanglement is transplanted to another particle. Entanglement swapping will form the basis of future quantum repeaters as it links otherwise unconnected nodes together.(Picture source: Internet)
A special case of entangled state is the Bell state, which is the simplest and largest entangled quantum state of two qubits. If two particles are measured independently in the same way, they will produce the same result, even if each result is itself random. It's like two coins are tossed in different cities but always give the same result.
One such application is quantum teleportation, where the concept of entanglement can be used to transfer unknown quantum states between parties that share the entanglement. If a transported particle is itself entangled with another particle, we have a process of entanglement exchange. For ease of explanation, we first introduce Alice, Bob and Christine.
(Picture source: Internet)
Imagine that each of them controls quantum. Christine and Alice share a pair of entangled quanta, as do Christine and Bob. The goal is to have Bob's quantum entangled with Alice's quantum, but they are not directly connected.
Bob and Alice will each start by preparing a known Bell pair, which is an entangled quantum state of two qubits. Alice will send a prepared qubit to Christine and keep one, and Bob will send a qubit to Christine and keep one. Christine performs a Bell projection between her newly acquired qubits and performs error correction, causing the qubits Bob sent to Christine to be sent to Alice and vice versa. The net effect is that Bob's and Alice's qubits are now entangled with each other, creating entanglement over longer links and laying the foundation for large-scale quantum networks.
Entanglement swaps like this will form the basis of future quantum repeaters, as they link nodes together that would otherwise be unconnected. Think of it like playing megaphone at a loud party. If a person does not hear the correct message, the message cannot be transmitted correctly, and the same is true for quantum repeaters. Entanglement swapping is by far the most efficient way to transmit quantum information over long, lossy channels without losing or destroying fragile quantum states. Future quantum repeaters will rely on entanglement switching, and Q-NEXT is working to better understand how to build these repeaters. What is the value of
?
Quantum computing is an inherently difficult topic to understand, so people often ask what the actual value of this technology is. To understand why quantum repeaters are needed, we need to discuss the value of transmitting information over quantum networks.
One of the applications of quantum networks is cryptography . Moving data across a network carries the risk of attackers stealing or altering it, so security measures must be taken.
Quantum Key Distribution (QKD) is a promising technology that relies on quantum repeaters. QKD is a secure communication method that utilizes the unique properties of quantum physics to protect data from attackers. If we want QKD to be efficient and effective, we need to spread the network over large distances. Therefore, powerful quantum repeaters will be used for large-scale QKD deployment.
The second application of quantum networks involves quantum computers. The only way to program such a processor securely and remotely is through a quantum link. Additionally, high-speed quantum networks can be used to directly connect two or more quantum processes to create a giant distributed quantum processor. For example, two quantum computers as a whole are more powerful than if they acted independently. If each quantum processor is a million times more powerful than a classical computer, then their entanglement contribution is a million times more powerful.
Finally, quantum networks can enable very sensitive distributed quantum sensors. For example, Kwiat noted that telescopes and cosmic research will advance rapidly as quantum networks are realized. We currently rely on methods that take a series of physical telescopes and combine their input data to simulate a giant telescope, but these methods only work with radio waves or short distances. Quantum repeaters could help us connect telescopes together more efficiently.
Kwiat said: "If you use a quantum network to connect two telescopes together, you can transmit signals from one telescope to the other. If the quantum network has been efficiently up and running, the transmission of quantum information is lossless. In principle, you can have telescopes that are further apart, achieving higher resolutions."
Of course, all these developments require a functioning quantum repeater. Q-NEXT hopes to be a leader in the development of these devices. Q-NEXT scientists are pursuing multiple hardware platforms to implement repeaters, including ion traps , neutral atoms, and superconducting qubits, as well as means of interconnection between these devices.
Q-NEXT also contributes to the global development of this field. For example, Q-NEXT and the Chicago Quantum Exchange co-organized the 3rd Quantum Repeater and Networking Workshop locally. The symposium aims to bring the quantum research community together to discuss the opportunities and challenges of developing quantum repeaters. This includes a tutorial called "Quantum Repeater Networks from Scratch," which requires no prior quantum experience and they hope to spread this knowledge to as many people as possible.
Considering that approximately 100 researchers from 37 institutions and 9 countries attended the workshop, it was clearly a success for Q-NEXT and the entire quantum community. With the development of quantum communication technology, Q-NEXT will continue to work hard to bring quantum repeaters to the world.
This work was supported by the U.S. Department of Energy's National Quantum Information Science Research Center.
About Argonne National Laboratory
Argonne National Laboratory is dedicated to solving pressing national science and technology problems. As the nation's first national laboratory, Argonne conducts cutting-edge basic and applied scientific research in nearly every scientific discipline. Argonne researchers work closely with researchers from hundreds of companies, universities, and federal, state, and municipal agencies to help them solve specific problems, advance America's scientific leadership, and prepare the nation for a better future. Argonne has employees from more than 60 countries and is managed by UChicago Argonne, LLC for the U.S. Department of Energy's Office of Science.
Compiler: Huike
Editor: Muyi
