The fast train from Paris to Rotterdam was an hour late leaving the Gare du Nord. When it finally deposited me in the Dutch city, I discovered that the onward train to Delft had been suspended because of maintenance work on the tracks. It took two circuitous bus journeys and a taxi ride before I finally made it to my destination.

Given that I was there to learn about the future of communications, this seemed appropriate. My trip was a reminder that while shipping people from place to place is still fraught with unforeseen glitches, gargantuan amounts of data flow smoothly and swiftly all day, every day through the fiber-optic cables connecting cities, countries, and entire continents.

And yet these data networks have a weakness: they can be hacked. Among the secret documents leaked a few years ago by US National Security Agency contractor Edward Snowden were ones showing that Western intelligence agencies had managed to tap into communication cables and spy on the vast amounts of traffic flowing through them.

The research institute I was visiting in Delft, QuTech, is working on a system that could make this kind of surveillance impossible. The idea is to harness quantum mechanics to create a flawlessly secure communications network between Delft and three other cities in the Netherlands by the end of 2020 (see map below for the planned links).

The QuTech researchers, led by Stephanie Wehner and Ronald Hanson, still face a number of daunting technical challenges. But if they succeed, their project could catalyze a future quantum internet—in much the same way that Arpanet, which the US Department of Defense created in the late 1960s, inspired the creation of the internet as we know it today.

Inimitable qubits

The internet is vulnerable to the kind of hacking revealed by Snowden because data still travels over cables in the form of classical bits—a stream of electrical or optical pulses representing 1s and 0s. A hacker who manages to tap into the cables can read and copy those bits in transit.

The laws of quantum physics, on the other hand, allow a particle—for example, an atom, an electron, or (for transmitting along optical cables) a photon of light—to occupy a quantum state that represents a combination of 1 and 0 simultaneously. Such a particle is called a quantum bit, or qubit. When you try to observe a qubit, its state "collapses" to either 1 or 0. This, explains Wehner, means that if a hacker taps into a stream of qubits, the intruder both destroys the quantum information in that stream and leaves a clear signal that it's been tampered with.

Because of this property, qubits have been used for quite some time to generate encryption keys in a process known as quantum key distribution (QKD). This involves sending data in classical form over a network, while the keys needed to decrypt the data are transmitted separately in a quantum state.