By David Graff, AAU Communication and Public Affairs

There is a worldwide race to develop quantum computers. In the new Center of Excellence called CLASSIQUE at Aalborg University, work has begun to calculate how quantum computers can be used for faster, better, and more secure communication.

“The starting point is that at some point, we will have quantum computers that can form the basis for communication via a quantum internet. This requires that we are able to get the quantum computers to communicate with each other. That's what we're working on,” explains Professor Thomas Garm Pedersen from the Department of Materials and Production.

The photons must be forced together

Quantum computers can communicate using light in optical fibers, for example. The key is to get photons or light particles to entangle, i.e., become entangled with each other. Entangled photons are what will carry the information in quantum communication. 

“But it's a challenge because photons have no electrical charge and will typically just pass straight through each other without interacting. Like two light cones shining through each other without any mutual influence. One solution could be to find new materials with special “non-linear” optical properties. Through such materials, photons can be made to interact and thus entangle,” says Thomas Garm Pedersen.

In ordinary materials with small nonlinearities, photons do entangle, but only with a small probability. And according to Horia Cornean from the Department of Mathematical Sciences, it's just hard to work with when that entanglement can occur anytime and anywhere in such materials between two out of billions of photons.

“So part of our task is to identify or develop a material where the probability of two photons entangling is as close as possible to 1,” he explains.

Communication via teleportation

If a material can be found or developed in which photons can be entangled with a high enough probability, the next step is to use the photons for communication between quantum computers.

“When you need to send quantum information in the form of photons, it can be done via so-called quantum teleportation. This involves emitting two entangled photons from a source to two quantum computers,” explains Thomas Garm Hansen and elaborates:

“And since those photons are entangled, you have to think of them as a quantum system rather than as individual particles.”

The next step is to entangle the two photons of the quantum system with electrons in the computers they are sent to.

“An important element of quantum teleportation is that the sender makes a measurement on its entangled quantum system. The result of the measurement can then be sent classically as ordinary information, typically via fiber, but in principle it could also be via a copper wire,” says Horia Cornean, widening his eyes:

“And that's where the magic happens: The fact that information is sent between quantum computers actually means that the state of the computer that sends the information is transferred to the receiving computer. The information doesn't actually travel; there isn't an electron that has moved. That's why we call it teleportation.”

Can disappear into thin air

There is another remarkable characteristic of quantum communication, which is both an advantage and a disadvantage: when you read the information, it disappears.

“According to quantum mechanical theory, quantum computers can transmit communication by virtue of photon entanglement. But the moment the state of the photons is measured to extract information from them, their entanglement ceases, and thus the communication also disappears into thin air,” explains Thomas Garm Pedersen.

This means that it will always be detected if someone tries to infiltrate or steal the information. Because it will change the entire system. But it also means that it's a challenge for those who actually receive the information to get it out.

“That's why a very important part of our work is to develop theoretical calculations and simulations of how photon entanglement can be maintained for as long as possible,” concludes Horia Cornean.