China's quantum communication satellite is the world's first. The best noted feature of such a satellite is that it's uncrackable. So why is it absolutely secure? 

Cryptography of traditional satellites operates through the use of an encryption key. When the key is applied to an encryption algorithm, it can be used to decrypt or encrypt a message.

But a quantum communication satellite relies on "quantum entanglement", which is the act of fusing two or more particles into complementary “quantum states.”

In such states, no particle can be independently described. Eavesdroppers will cause quantum states to collapse and reveal their spying to legitimate parties. The feature makes quantum keys theoretically impossible to crack.