City-Sized Ultrasecure Quantum Network

SciAm reports:

Quantum cryptography promises a future in which computers communicate with one another over ultrasecure links using the razzle-dazzle of quantum physics. But scaling up the breakthroughs in research labs to networks with a large number of nodes has proved difficult. Now an international team of researchers has built a scalable city-wide quantum network to share keys for encrypting messages. ...

Instead of building a network in which each of the eight nodes is physically connected to all the other nodes, the researchers created one with a central source that sends entangled photons to the eight nodes, named Alice, Bob, Chloe, Dave, Feng, Gopi, Heidi and Ivan. Each node is only connected via a single optical fiber link to the source, making a total of eight links—far less than the 28 that would be required for traditional QKD with no trusted nodes.

So even though the nodes are not physically connected, the protocol the researchers developed establishes a virtual link between each pair of them via the magic of quantum entanglement such that each pair can create a private key.

The central source has a so-called nonlinear crystal that spits out a pair of photons that are entangled in their polarization. ...

Adding a new node is simple: just connect it to the central source, which only has to modify its channel-splitting-and-multiplexing scheme. ...

Future large-scale quantum networks will have to solve at least two major problems: One is that they must interconnect an arbitrarily large number of users. Secondly, such networks have to span vast intracontinental and intercontinental distances—something that requires using either quantum repeaters to extend the range over which one can distribute quantum states or satellites to beam down qubits or entangled particles to nodes on the ground.

So they can create a central source to share a key over fiber optic lines to 8 nodes.

Of course it still has to use conventional cryptography to send any messages. And there is no way to authenticate the sender. And there is no way to scale up to more users or more cities. And there is no way to protect against your hardware from leaking information.

Underlying all this is a myth that analog hardware cryptography is inherently more secure than digital software cryptography. It is not, and it cannot be. You can trust your PC or phone to add digits correctly, but you can never trust it to send an analog signal perfectly.