Tuesday, August 16, 2016

China - Quantum Physics in Space


This morning at 01:40 local time, China successfully launched a "Long March - 2D" rocket from its XiYuan launching site that was carrying a satellite for scientific research, named Micius after an ancient Chinese philosopher and scientist (Chinese: Mozi 墨子) whose direction of thought was dedicated to the well-being of the people.

The satellite is operating on a sun-synchronous orbit (inclination 97,37°) at some 500 km distance from Earth where Micius is expected to remain active for two years. Its goal is testing a new technology that could one day offer "hack-proof" digital communications.




中国成功发射全球首颗量子科学实验卫星

China successfully launched first satellite
globally for quantum science experiments.


The new technology tested is based on the entangled particle theory of quantum physics dating back to the 1930ies when Albert Einstein and Erwin Schrödinger published relevant papers. That theory predicts the existence of pairs of similar elementary particles whose physical states (position, momentum, spin, polarization) are interrelated or entangled, no matter how far both particles are away from each other. Modern physicists have already gained experimental evidence for that theory while observing pairs of photons, neutrinos, electrons and even molecules.

The satellite will then create pairs of so-called entangled photons - tiny sub-atomic particles of light whose properties are dependent on each other - beaming one half of each pair down to base stations in China and Austria.



Spontaneous parametric down-conversion process can split photons
into type II photon pairs with mutually perpendicular polarization.


The experiment is using a special kind of laser that has several curious properties, one of which is known as "the observer effect" - i.e. the quantum state of any photon cannot be observed without changing it.

So, if the satellite were to encode an encryption key in that quantum state, any interception would be obvious. It would also change the key, making it useless.

If that mechanism works in technical practice, it will solve the central problem of encrypted communications - how to distribute keys without interception - promising hack-proof communications. The encrypted message itself can be transmitted normally after the key exchange.

其过程如下:事先构建一对具有纠缠态的光量子,将这对光量子分别放在通信双方,把具有未知量子态的光量子与发送方的光量子进行联合测量,则接收方的光量子瞬间变化为某种状态,这个状态与发送方的光量子变化后的状态是对称的,然后把联合测量的信息通过经典信道传送给接收方,接收方根据接收到的信息对变化的光量子进行幺正变换(相当于逆转变换),就可得到与发送方完全相同的未知量子态。

The proceedings are as follows: After the generation of one pair of photons that are in an entangled state, both photons are separately sent to two places communicating with each other. Take one of the photons bearing unknown quantum state [and participating as] the transmitted photon [and make it subject of] joint measuring, [when] the received photon instantaneously changes into a certain kind of state that is symmetrical to the state of the transmitted photon after [being subject to] change. Thereafter, a [trustworthy] script of information transport [is] jointly passed to the receiving site [obtained by joint measurement]. The received photon on the basis of received information change, undergoes one change (corresponding to a deteriorating change). Such it is possible to obtain at the transmitting side a completely identical, yet unknown quantum state.


[Source: FengHuang 凤凰 on August 16, 2016]




我国预计2030年建成量子通信网 每户配一个“密钥机”

China estimates that by 2030 a quantum communications network
will be created where each site connection uses one secret key.


[Source: RenMin Wang 人民网 on August 17, 2016]


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London visitor to "blueprint news" and who is interested in quantum physics.


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