Defining quantum networks

In this lesson we'll give you an overview of what quantum networks are and what they're not. Let's start by discussing quantum key distribution and quantum cryptography. These are related but different topics. Quantum key distribution uses the properties of quantum mechanical systems, like photons, to both generate and distribute cryptographic keys. Now, this usually requires special purpose technology. Quantum cryptography, on the other hand, uses the same physics principles and similar technology to communicate over a dedicated communication link. There are lots of theories in physics that allow us to do QKD and QC to detect the presence of an eavesdropper. In other words, it's impossible to intercept, say, the key distribution and not be detected. That's not true of standard classical cryptography and key distribution. Now, related to this is quantum-resistant algorithms. These are simply cryptographic algorithms that are resistant to quantum attacks like Shor's algorithm. The National Institute of Standards and Technology has conducted a rigorous selection process to identify quantum-resistant or what we call post-quantum algorithms. It's currently in round four, which are robust algorithms you could readily use for any cryptographic purpose. Now, the Department of Energy has done a lot of work in this area. You'll also see a link at the bottom. Throughout these lessons, you'll frequently see links that you could consult for additional information. But the Department of Energy has their own definition of quantum networks. They tell us that quantum networks use quantum properties of photons to encode information. Now let me pause there for a little extra information. Currently, photons are the only thing being used for quantum key distribution. Now with traditional quantum computing, where we talk about qubits, there are other things used like trapped ions and electrons and things of that nature. But for quantum networks, we only use photons, least so far. So you could have photons that are polarized in one direction, so the direction that might allow them to pass through polarized sunglasses, and we could associate that with the value of 1. If they're polarized in the opposite direction, so that they don't pass through the sunglasses, we call that the value of 0. There are lots of researchers working on a lot of different quantum communication protocols to formalize this process and allow the quantum state of photons to carry information from a sender to a receiver through a quantum network. We're going to explore many of these within the following lessons. It should also be noted that one reason photons are preferred is because we already have existing fiber optic technology that can be utilized to transmit photons. Now, quantum networks are a uniquely quantum phenomenon. Just like the things in superposition, the no-cloning theorem and entanglement, quantum networks are not something we can do with classical approaches. So before the photon is measured, we know that it exists in a superposition of its various possible quantum states, and each of those states has a particular probability. Measurement will, of course, collapse the wave function and select one of those states. Frankly, you can't measure the photon's quantum state state without causing that collapse, and that betrays the attempt for someone intercepting and attempting to measure the photon. No arbitrary unknown quantum state can be copied. That's the no-cloning theorem. So when we design quantum networks, they have inherent security, not based on the complexity of algorithms or particular protocols we put in place, but based on the fundamental physics of the universe. This makes them fundamentally far more secure. Now currently there are major applications already being done in the quantum communication space and lots more being worked on. So the ability to exchange quantum information gives us a whole new option for trusted communication without eavesdropping or at a minimum without having the eavesdropping go undetected. We could also use quantum sensors that are distributed in different locations using their shared quantum resources, and they have the potential to vastly outperform classical computing sensor technologies. There's also the possibility of simply connecting quantum computers into clusters so that we can exponentially improve the combined computing power. You're probably aware that one of the main thrusts in quantum computing is increasing qubits. As of this recording, there are two major vendors who have exceeded 1,000 qubits, and it's taken a lot of effort to get there. It would be possible to take multiple smaller quantum computers, perhaps one that have 256 qubits, and link them together into clusters, thereby getting the power of literally thousands, perhaps eventually millions of qubits, without having to overcome the issues of putting so many qubits close together. That's yet another way we use the term quantum networking. And you can see at the bottom a nice article from NIST that describes these quantum network options. Now this was a very interesting article that basically described quantum networks and how they work, and they had a really interesting way of describing this that I liked. And And they said the quantum key distribution network works by transmitting an encoded key in the form of quantum bits or qubits between endpoints over a fiber-optic cable. The qubits are typically polarized photons, which can travel easily along fiber-optic cables. Any attempt to intercept the quantum key destroys the qubit's delicate quantum state and the information it holds, alerting the endpoints that an intrusion occurred. The detectability of the intrusions is what ensures the security of the transmission. So let's pause right there and make a comment about this. Classical key distribution methods, and there are many, Diffie-Hellman, MQV, ElGamal, some elliptic curve variations, all of them at least have some possibility of being intercepted. We protect against interception in classical key distribution by simply the mathematical difficulty particular problems, like in the case of Diffie-Hellman, the difficulty of solving the discrete logarithm problem. Well, in quantum key distribution, we're not dependent on the difficulty of solving a mathematical problem. We're depending on the basic physics of the universe. So that tells us that not only is it something that can't be circumvented now, it's not going to be something that can be circumvented in any time in the foreseeable future. Now continuing on, the same article talked about, the challenge of a quantum network is that quantum communications are limited by the distance that photons can travel over fiber optic cable. That's usually about 100 kilometers. So what they often do, and in the case of quantum exchange, they have trusted nodes that basically operate like a traditional repeater in a classical network. At each node, the key is decrypted and then put back into its quantum state to be transmitted to the next node. Now this does bring up a security concern. Despite the fact that QKD is based on physics, we do have these trusted nodes or repeaters, which if they were compromised, would compromise the entire transmission. I alluded to this issue a little earlier in this lesson, but let's just make it crystal clear now. We really have two things that we call a quantum network. One, which is probably the more common use, is when we use quantum networking for communications. We use either quantum key distribution or some form of quantum entanglement. There are lots of variations on this, many of which use existing fiber optic cabling. The DARPA quantum network started using the BB84 protocol, which we'll learn more about later, way back in 2001, so this has been around for a while. The other use of the term quantum network is what I talked about earlier, clustering. I basically link two or more quantum processors together so we create what amounts to a quantum cluster. That's a less common use of the term, but one that's very important. Free space quantum networks. Well think about how you use networking now with classical computing. A lot of it is done over the air. of sight transmissions or radio transmissions, things of that nature. Well, free space quantum networks are using line-of-sight transmission paths through the air or even outer space, perhaps between satellites or satellites and ground stations. Now again, unlike traditional communication networks, these networks are using the principles of quantum mechanics, often things like superposition and entanglement, to really facilitate secure communication and also distributed quantum communication and quantum computing. Finally, in this introductory lesson, let's talk about, in general, an overview of the technology used. One of the most important technologies are single-photon sources and detectors, because if we're going to encode information in, for example, the polarization of a photon, We have to be able to both produce a single photon at a time and detect a single photon at a time. Fortunately, we have such technology already in existence. It's been used for other physics applications. Adaptive optics. We need to be concerned, particularly with free space quantum, about atmospheric distortions. Well, adaptive optics allow us to compensate for that. High precision beam steering. steering. Again, this is particularly important for the open or free space transmission. We need to make sure the light beams are lined. Well, we also need to have the repeaters we talked about before with some type of memory to store the data, at least in before we transmit it on to the next stage. Quantum memories are something that's currently in development. Right now what usually happens is things are measured and the data is stored in classical memory before being produced. Now this is just our first lesson. We're just giving you an overview of the quantum networking space. This should give you a generalized understanding of the topic before we delve into very specific issues in the coming lessons.