Quantum key distribution (QKD) uses individual light quanta in quantum superposition states to guarantee unconditional communication security between distant parties. In practice, the achievable distance for QKD has been limited to a few hundred kilometers, due to the channel loss of fibers or terrestrial free space that exponentially reduced the photon rate. Satellite-based QKD promises to establish a global-scale quantum network by exploiting the negligible photon loss and decoherence in the empty out space. Here, we develop and launch a low-Earth-orbit satellite to implement decoy-state QKD with over kHz key rate from the satellite to ground over a distance up to 1200 km, which is up to 20 orders of magnitudes more efficient than that expected using an optical fiber (with 0.2 dB/km loss) of the same length. The establishment of a reliable and efficient space-to-ground link for faithful quantum state transmission constitutes a key milestone for global-scale quantum networks.
Private and secure communications are fundamental human needs. Traditional public key cryptography usually relies on the perceived computational intractability of certain mathematical functions. In contrast, quantum key distribution (QKD)1 proposed in the mid-1980s—the best known example of quantum cryptographic tasks—is a radical new way to offer an information-theoretically secure solution to the key exchange problem, ensured by the laws of quantum physics. QKD allows two distant users, who do not share a long secret key initially, to produce a common, random string of secret bits, called a secret key. Using the one-time pad encryption, this key is proven to be secure by Shannon2 to encrypt (and decrypt) a message, which can then be transmitted over a standard communication channel. In the QKD, the information is encoded in the superposition states of physical carriers at single-quantum level, where photons, the fastest flying qubits with their intrinsic robustness to decoherence and ease of control, are usually used. Any eavesdropper on the quantum channel attempting to gain information of the key will inevitably introduce disturbance to the system, and can be detected by the communicating users.
Original article: Satellite-to-Ground Quantum Key Distribution