Earlier this month, a group of research labs in Chicago revealed an extensive 124-mile quantum network that runs from the suburb of Lemont, through the city of Chicago, to the Hyde Park neighborhood and back. That total length accounts for a newly added 35-mile segment of optical fiber that was recently connected to an 89-mile quantum loop from the U.S. Department of Energy’s Argonne National Laboratory. launched in 2020linking labs from the Chicago Quantum Exchange and the University of Chicago.
The intent behind building such a network is to allow researchers to experiment with new types of quantum communications, security protocols, and algorithms with the aim of moving on to a preliminary quantum internet (which could very well resemble an early version of the classic internet† Currently Toshiba is using it to test their distributed quantum encryption keys in an environment that experiences factors such as noise, weather and temperature fluctuations to understand how robust this method is and what potential problems it may encounter.
The researchers have so far been able to transmit information at a rate of 80,000 quantum bits (or qubits — more on what those are below) per second. These kinds of experimental keys can be useful in a future where: powerful quantum computers threaten to break through classical encryption, a problem that has been highlighted by: legislators in Congress†
As larger quantum computers appear, researchers are actively exploring ways to use the laws of quantum physics to establish a communication channel that would be tamper-proof and hack-resistant. This type of communication channel could also become a method of “wiring” quantum devices together.
“Suppose you have a quantum computer that can handle up to 1000 qubits. And here you have a second computer of 1,000 qubits. You’d like to connect them together the same way we build supercomputers today by making clusters, but you can’t just connect the computers with classic wire. You need a quantum wire to maintain the quantum states of both machines,” said David Awschalom, a professor at the University of Chicago and a senior scientist at Argonne National Laboratory. “So a quantum communication channel is one way to do that — basically building a way for two quantum circuits to talk to each other without ever entering the classical world.”
Exploring the possibilities of quantum communication
Because this is the quantum world, things work a little differently. For starters, in order to exhibit quantum qualities, objects must be either very cold or very small. Chicago chose small.
“Many of today’s commercially available quantum machines are mostly superconductors, so they have to have very low temperatures,” Awschalom says. “Quantum communication uses photons and the polarization of the light encodes the information.” This means that the network can be used at room temperature.
By using photons, they can also use the optical fibers through which today’s classical communication flows. But this is where the problems begin to appear. Optical fibers are made of thin strands of glass and glass has imperfections. When some photons, or pulses of light, travel through it, it can run smoothly for a while, but over time and distance, the signal’s amplitude shrinks as the light is scattered by impurities. For the classic internet, the solution is repeaters. These are thumb-sized devices placed every 50 miles to amplify and forward the signal.
The quantum world has tricky rules. Quantum bits (qubits), unlike classical bits, are not 0 or 1. They are a superposition of the two, meaning they can be either 0, 1, or both at once. You may see a qubit depicted as a sphere with an arrow coming out of the center. You cannot copy a quantum state (see the non-cloning theorem), and looking or observing it takes it out of superposition, so you destroy the qubit. (The benefit this brings is that it makes quantum links tamper proof).
The quantum signal can still span distances in a city through a fiber without a repeater. For the future, however, there are some ideas to expand the range. One is to go through the sky to a satellite and then back (this is what researchers in China are doing† But in the air, light can also be absorbed by moisture, and many of the photons don’t return to Earth (NASA is trying to see if they can improve stability entangled in space). With fiber optic you can tune the signal and see where it is, and you can broadcast multiple frequencies of signals at once. In addition, you can take advantage of the existing infrastructure. Awschalom envisions that a future quantum network will take advantage of both fiber and satellite communications, perhaps fiber for short distances and satellite for longer distances.
Another idea is to apply a trick called entanglement swapping. This is where the various nodes come into play (Chicago’s network currently has six nodes). Nodes don’t refer to a giant quantum computer with hundreds of qubits. In most cases, they are a kind of quantum memory, which Awschalom likens to a small, simple quantum computer. You can put information in it and take it out again.
“Let’s just say I got my [quantum] stands for you. You want to send it to someone else in a different location. But we don’t have a repeater,” he says. “What you could do is take the entangled information without looking at what it is, put it in a memory, and then you can turn it into something else.”
How Quantum Keys Work
Creating quantum keys to encrypt information is a practical application of quantum communication through entanglement. Entangled particles would behave as if they were connected, no matter how far apart they are. That means if you look at one particle, it will change the other, and if you look at both, their measurements will be correlated. Once you’ve identified entanglement, spread the entangled state, and maintain it over distance and time, you can use that property to immediately convey information.
Classic keys, which act like ciphers for information, are generated based on algorithms to encrypt and secure information. These algorithms usually contain a mathematical function that can be easily solved in one direction, but is difficult (but not impossible) to reverse engineer.
“It’s actually hard to make keys that are tamper-proof, that you can’t work backwards and figure out how the keys were generated, or it’s hard to stop people from copying the key,” Awschalom says. “And you don’t know if someone copied it.”
A quantum key is generated through quantum mechanics, and the pair of keys distributed between the sender and receiver are closely linked through quantum entanglement. In the Chicago experiment, the quantum keys are sent via photons whose properties have been modified (by factors such as polarization directions) to encode the bits. No one can copy or intercept the key without destroying the quantum information.
Quantum keys can consist of a series of quantum bits. “The quantum key is a function of the base state. You have a coordinate system to read it,” Awschalom explains. “Your ‘bit’ and my ‘bit’ are correlated. So it is very different from a classic key. If someone messes up your key, he’ll mess up mine too. I can also be sure that you received it, based on the way I received my key.”
A test bed for new technology
Despite all the hype, the quantum field is still in its infancy. That means researchers aren’t sure what will work well and what won’t. Part of how that ambiguity will be explored by this network is the fact that the different nodes in the different labs in Chicago are all experimenting with different strategies. “For example, right now we have a cold atom lab as one of the nodes, so you can actually put quantum communication information into a simple trapped atom and then extract it,” Awschalom says. His lab, another node in the network, integrates magnetic atoms from the periodic table to store and transmit quantum information. Another lab works with superconductors. “Each node is designed to amplify different technology ideas,” he says.
They also plan to open up this network to outside researchers and companies who can come in, plug in, test, and use their prototype devices and detectors.
Quantum keys are just the beginning when it comes to the possibilities of distributed entanglement. “You can do a lot more if you think about disseminating information in a different way,” says Awschalom, taking a global experience of the environment as an example. “Today we mainly research the world with classical sensors, but the world is quantum mechanical. It begs the question: What don’t we see just because we’ve never looked? Between these sensing technologies and a way to bring the sensors together, I’m optimistic we’re going to learn a lot.”