NTT Demonstrates Quantum Cryptography with Single Photon

6/16/2005 - Nippon Telegraph and Telephone Corp. (NTT; Head Office: Chiyoda-ku, Tokyo; President: Norio Wada)(NTT: 9433)(NYSE: NTT) has successfully demonstrated the quantum cryptography with a single photon, whose state is so fragile against the attacks from the eavesdroppers, can be realized in the photonic network of optical fibers. This result was enabled by combining the original protocol of the quantum cryptography developed by the collaboration of NTT and Stanford University (USA) and the NTT developed optical switch to control the flow of photons. The quantum cryptography is expected to be the last resort of the cryptography protocol, and to enhance enormously the safety of the transmitting information.

In this connection, 2005 is the World Year of Physics, and this experiment utilizes the quantum effect, which had puzzled Prof. Einstein about a century ago: a single photon is like a particle (quanta) but sometimes behaves like a wave and interferes with itself. Although the modern physics still cannot explain why such an interference occurs, the present work clearly demonstrated that this quantum effect is applicable in the technology of cryptography for the first time.

Secure data: a top secret or financial data, for instance, should be back-upped against an emergency. For that purpose, such data are frequently exchanged via the optical fiber for the exclusive use. In the near future, the subject of data backup will be enlarged into general companies and personal users. Since maintaining an exclusive optical fiber between the users and backup center is too expensive, usage of general network, such as the Internet will be unavoidable.

Then it arises the necessity to setup the procedure to protect the secret data by cryptography while being exchanged through the Internet. Currently, the cryptography system called public-key protocol is widely used. In this protocol, although the eavesdropper may try to decipher the data, he/she cannot decipher in practice since it may take hundred's million years. However, the recent increase of computer ability, which may shorten the time to decipher, and corresponding increase of ciphers to make the deciphering difficult, are in a vicious circle.

Quantum cryptography is assumed as a next generation cryptographic system that may replace the public-key protocol. This technology utilizes the property that the quantum state is very delicate to the external environment and is destroyed by observing it. Namely, the transmitted secret key encoded in the quantum state, single photon, is destroyed when the eavesdropper observed it. Since the eavesdropper cannot resend the quantum state that is identical to the original one, the receiver can easily detect if the secret key had been stolen.

Although the quantum cryptography breaks the vicious circle mentioned above, it is required that the very weak signal of single photon and strong light signal currently used in photonic network can transmit in the same network. Moreover, when it is transmitted via a switchboard, the optical signal should not be transferred to the electric signal since the quantum state is destroyed at the moment of transfer. Therefore, the signal path should be controlled without transferred into the electric signal. Fortunately, PLC optical switch can control path with the optical signal as it is. However, it was not clear if such a weak signal as single photons can be controlled similarly to the commonly used light signal.

For details on the experiment and results please go to the NTT website at

About NTT
NTT is a holding company of the Global Information Sharing Enterprise Group and NTT group, which consists more than 430 companies. One of the important missions of NTT group is to contribute the achievement of a Ubiquitous Broadband society. NTT group concentrates on integrating the group on expanding Broadband Service on Photonic Access, Third Generation Cellular Phone, Wireless LAN, are provided for Access means, promoting the structure of distributing the contents of Movies and music, and enhance the providing contents. In November 2002, the Vision for a new optical generation is announced. For further information, please visit the NTT home page at:

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