崔迪

【Abstract】Quantum transmission is based on quantum entanglement， which is a kind of the phenomenon of quantum mechanics.Quantum object refers to two or more between the localized， the classic strong correlation. When two object quantum entanglement in the quantum state is not independent， but related， and the correlation distance， a pair of electronic of entanglement state， no matter how far apart， they spin direction will remain an up and a down. If one of the electronic spin direction is changed， another of the electron spin direction will follow to change immediately.

【Key words】quantum transmission ； quantum entanglement ； polarization direction ； quantum state

【中圖分類號】G64 【文獻標識碼】B 【文章編號】2095-3089（2015）7-0028-02

When a laser is used to inspire calcium atoms， they can be inspired to two level higher than the ground state of calcium atoms.Then， they will be by radiation energy back to the ground state， and in the process， radiation out energy is likely to be a pair of entangled photons. In the moment， the two photon polarization direction is different， such as a horizontal polarization， the other a vertical polarization， the experiment can validate the existence of the phenomenon of quantum entanglement.

By using quantum entanglement， it is a clever idea that quantum communication is a quantum state contact transmission. This method can transfer quantum state information， without having to transfer quantum object itself.

Assume that there are two people， party a and party b which pass the quantum state of particle X to b. Although X quantum state is to be transmitted， according to the uncertainty principle， it is impossible to accurate the condition of quantum state of X without changing state of X directly. So A will adopt a clever way to use another pair entanglement of quantum particles A and B to transmit state of X. First， by entanglement source produces A pair of entangled A and B， respectively， to A and B. Because A and B is entangled， i.e. if A quantum state changes， B quantum states will follow to change immediately. A is going to use them to be used as a tool of information.

Next， X and A of the armour do a bell measurement. Bell is a quantum mechanical measurement， after the measurement， X and A entanglement， that is， the quantum state X produced associated with A， which cause A quantum state to change because A and B are entangled， the quantum state of B also corresponding changes at the same time. At this point， most of the information associated with the quantum state of X have passed it on to B.endprint

Then， the results of a bell measurement is told to B through the classical channel. B again according to the results of particle B make a corresponding transformation， the final processing make B have exactly the same quantum of the original X. Thus a completed transfer the quantum state of the X to the task of b. And after bell measurement， the quantum state of particle X would be destroyed.

This process is called quantum state. For measuring particles after X the original quantum state has been destroyed， so the process is not copying， and can be regarded as a transmission. Due to particle X itself has not been transmitted to b， the transmission is the quantum state of particle X， so called the transmission， and is called stealth， because although is to transfer the quantum state of particle X， but in the transmission channel， particle X does not appear.In fact， to complete the transfer needs to have two channels： quantum channels and classic. In the quantum channel transmission is quantum entanglement information of particles A and B， classic is the result of the bell measurement of channel transmission. Interestingly， in the process of the whole transmission， no one needs to know the transmit and receive the details of the quantum state.

One big characteristics of quantum communication is that its confidentiality is very high.The process of quantum transmission is through two channels： quantum and classical channel to transmit information

Quantum channel information cannot be stolen， according to the uncertainty principle of quantum mechanics. Quantum states cannot be accurately measured， and may not be cloned. Any eavesdropper interception or measurement in the process of information transmission， can change their status. So， in theory， quantum communication is absolutely safe.

For classical channel transmission of information， it does not matter even if you are stolen. In fact classical channel information can open completely transmission， because it does not contain information about quantum states.

We usually refer to Bit， a Binary digit acronym refers to binary one， which is the smallest unit of information. Its value can only be 0 or 1. The bits can use some system with two states to represent. For example logic value is wrong， mathematical symbols， and so positive and negative. Usually we can lighting switch off the transistor， such as positive and negative voltage to represent bits 0 and 1.

In quantum communication， quantum bits of information unit transfer is not in the classical sense， but Oubit. Qubit is not a simple 0 and 1， but two logic states superposition. For example， when used to represent the polarization state of the photon qubit， its polarization state is not necessarily horizontal or vertical， but may be different proportions of horizontal polarization and vertical polarization superposition. In the experiment， any two？鄄state quantum systems can be used to prepare a qubit， e.g.， orthogonal polarization states of photons， electron spins of atomic nuclei.endprint

Study a few years ago， the middle distance quantum teleportation is mainly achieved through the optical fiber， but there will be photon loss during optical fiber transmission process and decoherence effects will make entanglement between particles weakened with increasing large propagation distance. To solve this problem， to achieve long？鄄distance quantum communication， you need to use “quantum repeater”， which can store information in quantum entanglement， purification and extraction.

Academician of Guo Guangcan and Pan Jianwei of University of Science and Technology of China use different methods to developed quantum repeater. The former uses a solid？鄄state quantum memory technology， which uses a cold atomic systems technology. Generally photonic quantum communication， because it spread fast. But the photon is not static， so you want to store information， you should transfer the information to the other photons and other particles in atoms， which is a quantum storage. Stored information can also be completely extracted. Although in recent years the study of quantum repeater have made some progress， its difficult to achieve completely practical level.

Another way is not transmitted through the fiber， but in free space as transmitting radio signals that direct transmission of quantum signals. Because the free？鄄space transmission signal loss is relatively small， so you can transmit longer distances. In long？鄄distance free？鄄space quantum transport， which also solve many problems， such as to overcome the interference of the atmosphere， and to achieve high？鄄precision targeting and so on.

At an altitude of over 3000 meters of Qinghai Lake experimental base， in a total of four hours of time， the use of methods of quantum teleportation through free space can be successfully transferred to 97 kilometers outside of quantum states of light over 1100. Because atmospheric density decreases with increasing altitude， the whole atmosphere of light attenuation signal attenuation is only about five kilometers of the atmosphere near the ground nearby. Theoretically， if we can achieve 10 km invisible quantum teleportation in the ground， it is possible to achieve global scale quantum communication via satellite. Therefore， The research progress in recent years makes the practical world of quantum communication networks closer to reality.

References

[1]P.G. Clem： submitted to Journal of Materials Research （2003）

[2]Information on http：//www.weld.labs.gov.cn

[3]Newton（2012）endprint