British researchers have successfully tested a method of quantum cryptography that that could make it a more practical system for large-scale communication.

Quantum cryptography works by sending each unit of binary data as a light particle (photon) in one of two states, representing a 0 or 1. Not only can this binary information be encrypted in the same way as other Internet data, but the use of quantum physics means that if anyone intercepts and reads the stream of data is physically changed, allowing the intended recipient to know something is amiss.

One limitation to the technique is that to work perfectly it needs to be over a direct connection: once the data read by a router or other relaying device, it’s no longer useful.

Earlier this year, Los Alamos National Labs revealed it’s been running a local quantum network for a couple of years that gets round this with a spoke and hub model, meaning every message goes from the sender direct to a central hub and then directly on to the recipient. The data has to be decrypted and re-encrypted by the hub, which means relying on the hub being electronically and physically secure. That’s not a major problem within the walls of a government research lab, but might not be practical on a larger scale, particularly as it creates the type of single-point-of-failure that the Internet is designed to avoid.

Now researchers at the Cambridge Research Laboratory, backed by Toshiba, have looked at another limitation to quantum networking: the need for each user to have their own photon detector (the equivalent of a modem) and solo access to a fiber cable.

They’ve found it’s possible for up to 64 users to be connected on a local network to a single photon detector. They’ve found a way to label each stream of photons such that they can be individually identified, making it possible to carry information from multiple users securely and simultaneously through the same fiber cable to the destination.

That still wouldn’t allow a quantum Internet as such but could, for example, allow a direct connection between multiple users in two different facilities such as different government departments or branches of a financial trading firm.

In its current form, the cable-sharing technique doesn’t allow high transfer speeds: even by cutting down to eight simultaneous streams, each user can only achieve a maximum speed of 250kbps. Of course, that could be viable for text-based data: the researchers noted it’s the equivalent of eight users being able to send a million encrypted text-only e-mails a month.

even by cutting down to eight simultaneous streams only allowed each user a maximum speed of 250kbps.

(Image credit: Nature)