Scott Kelly/NASA via AP
The first entangled pair of photons has been sent from an orbiting satellite to Earth, scientists reported today. It's a step toward sending quantum keys from satellites — an approach that has been heralded as a secure way to encrypt communications because they can't be cracked by ever-improving computer algorithms.
What they did: A laser on China's Micius satellite, which launched last year and is dedicated to studying quantum satellite communications, spit out pairs of entangled photons from its position 500 km above Earth. Two telescopes on Earth - about 1200 km apart — then had 5 minutes each day to look for them as the satellite passed over both telescopes. They found that paired photons survived the journey through Earth's atmosphere and detected 1 entangled pair per second out of the 6 million sent in that time.
How this all works: Two people looking to communicate would first generate a quantum key. One person receives one of the entangled photons in the pair and the other person gets the other. When they measure the photons they receive, they obtain bits of information that are strung together to create a key that they then both have and can use to encrypt and decrypt a message. A portion of the key can be shared publicly to check if it has been compromised - if someone tries to intercept the communication at any point, they would then see a difference between their strings.
The real problem: The feat, while an important proof of concept, doesn't address one of the biggest problems with quantum communications, says Caltech's John Preskill. These messages currently can't be sent long distances.
Using an optical fiber, photons carrying a quantum signal can only make it about 100 km before the light dissipates. Similar to optical telecommunications here on Earth, quantum systems need repeaters that can amplify the message so it can be passed long distances. But amplifying a quantum message in the same way we do optical ones would effectively destroy it. That's why researchers are eyeing satellite-based communication. The reported 500 km from space is an improvement over optical and another study published today measured quantum signals from a satellite 38,000 km away to a single point but in deploying a global network, which would likely combine optical fiber and satellites, the repeater problem stands.
A prediction: Preskill says it is more likely we will first come up with another form of encryption for communication. "There will be other ways of doing classical public key crypto systems that we won't know how to break with quantum computers."