TOKYO -- A new method of securing communications is set to make life harder for would-be data thieves.
Researchers in Japan last year announced what they called the first breakthrough in quantum cryptography in 30 years. The original method, which has already been commercialized, is designed to protect information by detecting the presence of an eavesdropper. The new technology promises to keep intruders out in the first place.
Degrees of secrecy
When two parties want to exchange confidential information, such as financial data, they encrypt it. A random number table is used as a cryptographic key; only those with access to the key can read the message. But this is not necessarily foolproof. If the random number table is somehow compromised, it's game over.
Quantum cryptography ensures more secure communication by exploiting the properties of quantum mechanics to safeguard random number tables. This level of protection, which hinges on the laws of physics, has been drawing more attention since the Edward Snowden scandal. The former CIA contractor revealed the extent of U.S. government surveillance of the Internet and phone networks.
Masato Koashi, a University of Tokyo professor and member of the team that announced the breakthrough, said the new method of quantum cryptography "uses a different principle of quantum mechanics" than the old way.
In first-generation quantum cryptography, bits of information are carried by individual photons. If they are tampered with, a trace is left. It is possible to detect an intruder by monitoring the amount of noise in the communication channel.
Koashi and his colleagues, including Toshihiko Sasaki, a project researcher at the University of Tokyo, say their method obviates the need for monitoring. It also promises to be more efficient: When there is a lot of noise in a channel, the number of photons that can be used declines, making communication more difficult. With second-generation quantum cryptography, fewer photons are abandoned.
This second method is still in the theory-verification stage. But according to Koashi's team, it can be achieved with a laser source and an interferometer -- a device that separates beams of light.
Who said it would be simple?
It would work like this: The receiver superposes arbitrary pairs of optical pulses using a variable delay and measures them with photon detectors. In most pairs of pulses, the receiver will not see any photons; when a photon is detected, the receiver will know whether the two pulses are in the same phase, or have the same bit value.
The transmitter already knows the phase of each pulse. The receiver would tell the transmitter that, say, a photon was detected in pulse pairs No. 2 and No. 6. This way, both sides would know whether the bit value is 0 or 1. These values represent bits of digital information.
This method relies on a phenomenon called the "reduction of the wave packet." An elementary particle like photon is at once a particle and wave, and can exist simultaneously in various places. But when observed by chance in a specific place, it is seen only as a particle. So even if intruders detect a photon, they will not be able to steal any data -- the bit information is decided only after the receiver observes a photon.
Using this system for commercial purposes will require, among other things, the development of a device that allows the receiver to measure optical pulses at the desired interval.
Yoshihisa Yamamoto, a program manager for the Japanese government's ImPACT program for spurring innovation, is optimistic about the future of this type of cryptography. "The new method," he said, "will eventually replace the old one."