Seeking Key Materials for Quantum Communications

Neil Savage
2017 ACS Central Science  
I n June, Chinese researchers announced a remarkable feat. With the help of an engineered crystal aboard a satellite orbiting Earth, they had beamed a pair of quantum entangled photons from the satellite to receiving stations on two Tibetan mountaintopslocated 1,200 km apartand successfully measured the photons' quantum properties. This 1,200-km separation was more than ten times the previous record, and it marked a major advance in the quest to use the quantum properties of photons to encode
more » ... information. This field, quantum communications, holds the promise of sending information under unbreakable encryption. Quantum communication relies on the quantum entanglement of photons, in which the photons' spins or polarizations complement each other and stay correlated even if the photons are separated. The idea that entangled photons could be used to encode data has been around since the 1960s, when researchers came up with the idea of using the laws of nature to make banknotes that could not be counterfeited. By 1984, scientists had proven that quantum communication could send invincibly encrypted information. The idea is called quantum key distribution (QKD). In essence, the bits of the key that unlocks the encrypted data are encoded on some quantum property of a photon. Because measuring a quantum property changes it, if someone were to intercept the encoded photons and read the values of the key, the intended recipient would see changes in the photons and know there was an eavesdropper. The integrity of the message is protected by the laws of physics; there's no higher level of security. What this technology needs, then, is a way to entangle photons in high enough quantities to be useful. But one of the stumbling blocks to quantum communication becoming practical is coming up with materials that will have just the right combination of properties to produce such photons. The Chinese satellite, called Micius, generated
doi:10.1021/acscentsci.7b00359 pmid:28852692 pmcid:PMC5571455 fatcat:2urnaxj7nfh6bokltyrj7ti2gu