番外篇----通訊系統介紹（採自Wikipedia）純手打，道德搬運，請勿噴

Communications system

In telecommunication,a communication system is a collection of individual communications networks,transmission systems,relay stations,tributary stations,and data terminal equipment(DTE) usually capable of interconnection and interoperation to form an integrated whole.The components of a communications system serve a common purpose,are technically compatible,use common procedures,respond to controls,and operate in union.Telecommunications is a method of communications(e.g.,for sports broadcasting,mass media,journalism,etc.).A communications subsystems is a functional unit or operational assembly that is smaller than the larger assembly under consideration...

Russian Scientists Develop Long-Range Secure Quantum Communication System

Newswise - A group of scientists from ITMO University in Saint Petersburg,Russia has develped a novel approach to the construction of quantum communication systems for secure data exchange.The experimantal device based on the result of the research is capable of transmitting single-photon quantum signals across distance of 250 kilometers or more,which is on par with other cutting edge analogues.The research paper was published in the Optics Express journal.
Information security is becomming more and more of a critical issue not only for large companies,banks and defense enterprises,but even for small business and individual users.However,the data encryption algorithms we currently use for protecting our data are imprefect in the long-term,their logic can be cracked.Regardless of how complex and intricate the algorithms is,getting round it is matter of time.
Contrary to algorithms-based sncryption,systerms that protect information by making use of the fundamental laws of
quantum physics,can make data transmission completely immmune to hacker attracts in the future.Information in a quantum channel is carried by single photons that change irreversibly once an eavesdropper attempts to intercept them.Therefore,the legitimate users will instantly know about any kind of intervention.
Researchers from the Quantum Information Centre of the International Institute of Photonics and Optical Information
Technology at ITMO university along with colleagues from Heriot-Watt University in Edinburgh have divised a new way to effectively generate and distribute quantum bits.This is the first system in Russian,which can compete with the best existing analogues and make it possible to share quantum signals via optical fiber across 250 kilometers in distance.

"To transmit quamtum signals ,we use the so-called side frequencies,"says Artur Gleim,head of the Quantum Information Center in ITMO University."This unique approach gives us a number of advantages,such as considerable simplification of the device architecture and large pass-through capacity of the quantum channel.In terms of bit rate operating distance our system is comparable to absolute champions in the field of quamtum communications."
The very possibility of stable transmission of quantum signals through fiber optical channels is instrumental to subsequent integration of quantum key distribution systems that will be used to secure the useful data.
According to Robert Collins,research associate at the Institute of Photonics and Quantum Science at Heriot-Watt University and one of the authors of the study,the work may become a big pivot-point for the whole field of quantum communication and cyptography:"Down the track,this new approach can enable smooth coexistence of numerous data streams with different wavelengths in one single optical cable.On top of it,these quantum streams can be fed into the already existing fiber optic lines along with conventional communications."
In order to encode quantum bits in the system,laser radiation is directed into a special device called the electro-optical phase modulator.Inside the modulator the central carrier wave emitted by the laser is split into several independent waves.After the signal is transmitted through the cable,the same spliting occurs on the reciver end.Depending on the relative phase shift of the waves generated by the sender and the receiver,the waves either enhance or cancel each other.This pattern generated by overlapping wave phases is then converted into the combinaton of binary digits, 1 and 0,which serves compile a quantum key.
Importantly,the scientists have achieved high stability of the relative phase shifts of the signal in the system."All waves undergo random changes while passing through the fiber,"explains Oleg Bannik,one of the authors of the study and research at Quantum Information Centre,"But these changes are always identical and get smoothed over during the additional run through the receiver's modulator.In the end,the receiver observes the same combination as the sender."
Now the researchers are on the mission to create a full-ileaged quantum crytographic system,which will genetrate and distribute quantum keys and transmit useful data simultaneously.