The approach they’re using is called quantum teleportation. This may sound like science fiction, but it’s an actual method of transmitting data. It relies on a phenomenon known as quantum entanglement.
Entanglement means creating a pair of qubits—photons of light, for this purpose—in a single quantum state, so that even if they travel off in opposite directions, they retain a quantum connection. Changing the state of one photon will instantaneously change the state of the other one in a predictable way, no matter how far apart they are. Albert Einstein called this “spooky action at a distance.”
Quantum teleportation, then, requires first sending a pair of entangled photons to two people—call them Alice and Bob. Alice receives her entangled photon and lets it interact with a “memory qubit” that holds data she wants to transmit to Bob. This interaction changes the state of her photon, and thus changes the state of Bob’s photon too. In effect, this “teleports” the data in Alice’s memory qubit from Alice’s photon to Bob’s. The illustration below lays out the process in a little more detail.
Another way to think of it: the entangled pair of photons are like the two ends of a virtual, one-time-only data cable. Each time Alice and Bob want to send data, they first receive a new cable, and because each of them holds one end, only they can use it. That’s what makes it secure from eavesdropping.