The approach is based on quantum physics, which offers the ability to exchange information in a way that the act of eavesdropping on the communication would be immediately apparent. The achievement requires the ability to reliably measure a remarkably small window of time to capture a pulse of light, in this case lasting just 50 picoseconds — the time it takes light to travel 15 millimeters.
The secure exchange of encryption keys used to scramble and unscramble data is one of the most vexing aspects of modern cryptography.
Public key cryptography uses a key that is publicly distributed and a related secret key that is held privately, allowing two people who have never met physically to securely exchange information. But such systems have a number of vulnerabilities, including potentially to computers powerful enough to decode data protected by mathematical formulas.
If it is possible to reliably exchange secret keys, it is possible to use an encryption system known as a one-time pad, one of the most secure forms. Several commercially available quantum key distribution systems exist, but they rely on the necessity of transmitting the quantum key separately from communication data, frequently in a separate optical fiber, according to Andrew J. Shields, one of the authors of the paper and the assistant managing director for Toshiba Research Europe. This adds cost and complexity to the cryptography systems used to protect the high-speed information that flows over fiber optic networks.
Weaving quantum information into conventional networking data will lower the cost and simplify the task of coding and decoding the data, making quantum key distribution systems more attractive for commercial data networks, the authors said.
Modern optical data networking systems increase capacity by transmitting multiple data streams simultaneously in different colors of light. The Toshiba-Cambridge system sends the quantum information over the same fiber, but isolates it in its own frequency.
"We can pick out the quantum photons from the scattered light using their expected arrival time at the detector," Dr. Shields said. "The quantum signals hit the detector at precisely known times — every one nanosecond, while the arrival time of the scattered light is random."
Despite their ability to carry prodigious amounts of data, fiber-optic cables are also highly insecure. An eavesdropper needs only to bend a cable and expose the fiber, Dr. Shields said. It is then possible to capture light that leaks from the cable and convert it into digital ones and zeros.
"The laws of quantum physics tell us that if someone tries to measure those single photons, that measurement disturbs their state and it causes errors in the information carried by the single photon," he said. "By measuring the error rate in the secret key, we can determine whether there has been any eavesdropping in the fiber and in that way directly test the secrecy of each key."