The cryptography widely used on the Internet currently is guaranteed by "conditional security" based on the limit of computer's ability, that is, "deciphering is difficult because the time necessary for the calculation is quite long." Generally, the strength of cryptography is decided on the presumption that it will not be broken until 10 to 40 years later. However, you cannot tell when an effective calculation method may be developed. To begin with, sending information that you do not want to be deciphered for several decades is not guaranteed to be secure.

 The technology for quantum cryptography communication is a new communication technology that can protect the security of information transmission for a long time. It is actively researched in some countries in the world, and Japan is targeting to realize it around 2020 to 2030. By using this communication technology, a cryptographic key can be safely shared between sender and receiver by using quantum mechanics. From quantum mechanics, that, "the information of a photon cannot be copied by dividing it into two," when eavesdroppers try to steal the photon, it is transmitted as a loss of photon or changed information, so that the eavesdropping can be detected. When it is materialized, it is expected to be used at sites where high quality security is necessary, such as public offices, financial institutions, and medical care facilities. For materialization, however, various problems need to be solved, and the most important one is a single-photon generator that can limit the photons per optical pulse to one.

 With the wavelength band used for practical fiber-optic communication （1.3-1.55 micrometers）, a laser light source with an extremely weakened light had to be used, as there was no technology to generate a single photon in the past. Its transmission speed over a long distance was quite slow, and there was a problem that a long cryptographic key, which was necessary to decipher the cipher of which absolute security quantum cryptography communication guarantees, could not be sent.

 In a series of experiments conducted from 2005 to 2006, a single photon was successfully transmitted on a 1.55 micrometer band by which the loss is the smallest in fiber-optic transmission. It is expected that it will accelerate, making longer distances possible for quantum cryptography communication.

Source provided by：
 Japan Science and Technology Agency