An expanded, 124-mile quantum network connecting a number of research facilities in Chicago was presented earlier this month. It runs from the Lemont suburb through the city of Chicago to the Hyde Park district and back. The 89-mile quantum loop that the US Department of Energy's Argonne National Laboratory established in 2020 connecting labs from the Chicago Quantum Exchange and the University of Chicago accounts for a newly linked, 35-mile-long stretch of optical fiber in the overall length.
Building such a network is intended to allow researchers to test out novel quantum communications, security protocols, and algorithms in order to move closer to a prototype quantum internet (which could very well look like an early version of the classical internet). In order to determine how reliable this technology is and what possible problems may arise, Toshiba is currently utilizing it to test its distributed quantum encryption keys in an environment that encounters elements like noise, weather, and temperature variations.
Up until this point, the researchers had been able to transmit data at a rate of 80,000 quantum bits (also known as qubits; more on what they are below) per second. These kind of experimental keys might be helpful in the future when potent quantum computers threaten to defeat conventional encryption, a problem that has been raised by Congressmen.
Researchers are currently looking for methods to employ quantum physics to create a communication channel that would be impenetrable to tampering and hacking when larger quantum computers start to appear. This kind of channel for communication may potentially develop into a way of "wiring" quantum devices.
“Let’s say you have a quantum computer that’s up to 1,000 qubits. And here you have a second computer that’s 1,000 qubits. You’d like to wire them together in the same way we build supercomputers today by making clusters, but you can’t just wire the computers using classical wire. You need a quantum wire to keep the quantum states of both machines,” says David Awschalom, a senior scientist at Argonne National Laboratory and a professor at the University of Chicago. “So, a quantum communications channel is a way to do that—basically building a way for two quantum circuits to talk to one another without ever entering the classical world.”
Probing at the possibilities of quantum communications
Things function a bit differently in this world since it is a quantum one. To begin with, either very cold or very tiny things can display quantum properties. Chicago went with tiny.
“Many of today’s commercially available quantum machines are usually superconductors, so they have to have very low temperatures,” Awschalom explains. “Quantum communications use photons, and the polarization of the light encodes the information.” The network can therefore be used at room temperature.
Since they would be using photons, they could also make use of the optical fibers that are now used for traditional communication. But this is when issues begin to surface. Glass contains flaws, and optical fibers are constructed of tiny glass strands. Single photons or pulses of light may initially pass through them without incident, but as they move further away and over time, contaminants cause the signal's amplitude to diminish. Repeaters are the answer for the traditional internet. The signal is amplified and sent on by these thumb-sized transmitters, which are placed roughly every 50 kilometers.
The rules of the quantum realm are complex. Unlike conventional bits, quantum bits (qubits) are neither a 0 nor a 1. They can be either 0, 1, or both at once since they are a superposition of the two. A qubit may be shown as a sphere with an arrow pointing out from the center. A quantum state cannot be duplicated (see the no-cloning theorem), and viewing or watching one causes it to lose its superposition, which destroys the qubit.
Without a repeater, the quantum signal can still travel a distance across a fiber in a metropolis. There are, however, some plans to widen its scope in the future. One is to fly to a satellite and then return (this is what researchers in China are doing). However, moisture in air may absorb light, which prevents many photons from returning to Earth (NASA is trying to see if they can improve the stability of entangled in space). With optical fiber, you may simultaneously send out numerous frequencies of signals, adjust the transmission, and observe where it is.Additionally, you may benefit from current infrastructure. According to Awschalom, a future quantum network may use satellite and fiber for communication, perhaps using fiber for short distances and satellite for larger ones.
A different approach is to use a technique called entanglement switching. This is when the various nodes—network Chicago's now has six nodes—come into play. Nodes don't mean a massive quantum computer with a large number of qubits. They often take the form of a quantum memory, which Awschalom compares to a small, straightforward quantum computer. You have the ability to add and remove information.
“Let’s say I can barely get my [quantum] state to you. You would like to send it to somebody else in another location. But we don’t have a repeater,” he claims. “What you might be able to do is take the entangled information without looking at what it is, put it into a memory and then you can swap it into something else.”
How quantum keys work
A real-world use for quantum communication via entanglement is the development of quantum keys for data encryption. No matter how far away they are, entangled particles would act as though they are coupled. This implies that observing one particle will affect the other, and observing both will result in a correlation in the measurements of the two particles. You may utilize this characteristic to instantly transmit information after entanglement has been established, distributed, and maintained throughout time and space.
To encrypt and safeguard information, traditional keys—which function as ciphers for data—are produced using algorithms. These algorithms often incorporate a mathematical function that is straightforward to solve in one direction but challenging to reverse engineer, but not impossible.
“It’s actually hard to make keys that are tamper-proof, that you can’t either work backwards and figure out how the keys were generated, or it’s hard to keep people from copying the key,” according to Awschalom. “And you don’t know if someone copied it.”
Quantum physics is used to create a quantum key, and quantum entanglement is used to create an intimate connection between the pair of keys sent to the sender and recipient. In the Chicago experiment, the quantum keys are sent by photons whose characteristics have been altered to encode the bits (for example, by varying their polarization orientations). The key cannot be duplicated or intercepted without erasing the quantum information.
A string of quantum bits can be used to create quantum keys. “The quantum key is a function of the basis state. You have a coordinate system to read it,” says Awschalom. “Your ‘bit’ and my ‘bit’ are correlated. So it’s very different from a classical key. If somebody scrambles your key it will scramble mine. I can also be sure that you’ve received it, based on the way that I received my key.”
A testbed for new tech
Despite all the excitement, the quantum sector is still developing. In other words, scientists are unsure of what will be successful and what won't. The fact that the many nodes at the various laboratories across Chicago are all experimenting with various tactics is one way in which that uncertainty will be probed at by this network. “For example, right now we have a cold atom lab as one of the nodes, so you can actually take quantum communications information, and put it into a simple trapped atom, and then extract it,” Awschalom adds.
His laboratory is combining magnetic atoms from the periodic table to store and transmit quantum information. It is another node in the network. Superconductors are being used in another lab.
“Each node is designed to amplify different technology ideas,” according to him. Additionally, they intend to make this network accessible to independent academics and businesses so that they may run and test their prototype detectors and devices on it.
The potential applications of distributed entanglement go well beyond quantum keys.
“There’s a lot more you can do when you think about distributing information differently,” Awschalom claims, with global sensing of the environment as one example. “Today we’re probing the world with classical sensors mostly, but the world is quantum mechanical. It does beg the question—what are we not seeing only because we’ve never looked? Between these sensing technologies and a way to bring the sensors together, I’m optimistic that we’re going to learn a lot.”
