After quitting hacking, Wehner went to university in the Netherlands to study computer science and physics. She heard the quantum information theorist John Preskill give a talk in Leiden describing the advantages of quantum bits for communication. A few years later, after earning her doctorate, she left classical bits behind and joined Preskill’s group at the California Institute of Technology as a postdoc.

At Caltech, in addition to proving several notable theorems about quantum information, quantum cryptography and the nature of quantum mechanics itself, Wehner emerged as “a natural leader,” Preskill said, who “was often the glue that bound people together.” In 2014, after a professorship in Singapore, she moved to Delft, where she began collaborating with experimentalists to lay the groundwork for the quantum internet.

Quanta Magazine spoke with Wehner over two days in August. The interview has been condensed and edited for clarity.

The quantum internet is a network for transmitting qubits between distant locations. Why do we need to do that?

The idea is not to replace the internet we have today but really to add new and special functionality. There are all kinds of applications of quantum networks that will be discovered in the future, but we already know quite a number of them. Of course the most famous application is secure communication: the fact that one can use quantum communication to perform what is called quantum key distribution, where the security holds even if the attacker has a quantum computer. A quantum computer would be able to break a lot of the security protocols that exist today.

What makes quantum keys so secure?

A good way to understand what a quantum internet can do is to think about “quantum entanglement,” a special property that two quantum bits can have that makes all of this possible. The first property of entanglement is that it’s “maximally coordinated”: I would have a quantum bit here and you would have a quantum bit in New York, and we would use the quantum internet to entangle these two qubits. And then, if I make a measurement on my qubit here and you make the same measurement in New York, we will always get the same outcome even though the outcome wasn’t determined ahead of time. So you can intuitively think that a quantum internet is very good for tasks that require coordination, due to that first property of quantum entanglement.

Now, given that this is so maximally coordinated, you might say, “Hey, wouldn’t it be great if this entanglement could be shared with hundreds of people?” But that’s actually not possible. So the second property of entanglement is that it’s inherently private. If my qubit here is entangled with your qubit in New York, then we know that nothing else can have any share of that entanglement. And this is the reason why quantum communication is so good for problems that require security.

As one of the simplest applications of quantum communication, quantum key distribution could be available as soon as the early 2020s on the demonstration network you’re building. What are some of the more advanced applications that will become possible later?

New kinds of remote computing will become possible. Say you have a proprietary material design and you want to test its properties in a simulation. A quantum computer promises to be much better at that than a classical computer. But you can imagine that not everybody in the world will have a large quantum computer in their living room anytime soon — possibly not in our lifetime. One way of doing that is you send your material design to me, and I run a simulation for you on my quantum computer and tell you the outcome. That’s great, but now I also know your proprietary material design. So one thing the quantum network makes possible is that you can use a very simple quantum device — in fact, it can make only one qubit at a time — and the quantum network can transfer qubits from your device to my powerful quantum computer. And you can use that quantum computer in such a way that it cannot learn what your material design is while performing the computation.