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Science

Phone home: how photon entanglement could secure our interplanetary internet

Think we're on the back foot with cybersecurity? Well the deeper we explore space the more we are going to need an interplanetary internet, and the more we'll need it to be secure. Could photon entanglement help?

When E.T. said he wanted to phone home in 1982 he probably wasn't bothered about using a secure line. He just wanted to call his people back home.

In 2082, millions of humans in colonies on the moon or Mars might also want to phone home - if our dearly beloved planet still exists. And they probably will want a secure line.

In fact, the deeper we explore space the more important secure communications networks will get. All the threats we face on Earth, be they hacktivists, rogue states, terrorists, or teenagers noodling with ransomware, we will want a secure interplanetary internet.

"If we launch a spacecraft to Mars, we'll want to remain in control of that spacecraft," says Professor Nicolas Gisin, who heads the Applied Physics Group at the University of Geneva. "And you keep control by securing the communication."

To achieve secure communications you need a form of cryptography. And in space, as on Earth, this is where a complex field in physics known as quantum physics could help - specifically, it's the area of photon or "strange" entanglement. Albert Einstein doubted the phenomenon and called it "spooky actions at a distance." But it's technically known as quantum nonlocality.

Twenty years ago, Gisin and his team performed the first demonstration of photon entanglement outside a lab. 

"Before it was always done on optical tables in laboratories over maybe 10 meters, and we did it using telecom optical fiber over a bit more than 10 kilometers," recalls Gisin. "What the Chinese have done over a thousand kilometers is a quantum leap."

He's referring to a Chinese experiment, which has just demonstrated photon entanglement over a new record of 1,203 kilometers.

Located about 500 kilometers above Earth - that's higher than the International Space Station - the Micius satellite successfully beamed entangled pairs of photons to three optical telescopes: in Delingha in Tibet, and Lijiang and Nanshan in northwest China.

Photon entanglement is an extremely fragile state. Only about one pair out of six million pairs beamed each second of transmission survived the trip through Earth's atmosphere and other interference.

But they made it all right. And the team has its sights set on future computing and communications.

Jian-Wei Pan, who leads Quantum Experiments at Space Scale, the team behind this study, says the technology is "the only way to establish secure [encryption] keys between two distant locations on Earth without relying on trustful relay." That is, when you can't trust all parties in a communication network.

But this is getting deep. So let's roll back a little. 

It takes two to … entangle

A photon is a particle of light. Entangled photons are pairs of photons that are fundamentally linked - fundamentally in the sense that they could be thought of as identical twins.

"A photon is generated when an electron drops down from a higher-level orbital to a lower one in an atom, and that energy is given off as a photon. That's how most light is produced," says Brian Clegg, author of "The God Effect - Quantum Entanglement, Science's Strangest Phenomenon."

And sometimes, says Clegg, two photons are produced instead of one. "Their combined energy is the same and they're in an entangled state," he says. "But unfortunately it's almost impossible to describe in any meaningful way how they get entangled."  

Cryptography in space

What's clear is that they share a "quantum state." And this is where entangled photons get interesting for cryptographers, because as soon as you interfere with their entangled state, the entanglement is effectively broken. So, if you'll excuse this tiny transgression, it definitely takes two to entangle.

"We've been able to do totally unbreakable encryption since the 1920s," says Clegg.

It started Gilbert S. Vernam's idea of a one-time pad, "a random set of data on a pad which you add to your information, and that's your key. And because it's totally random, the output is also random and there is nothing to crack, effectively."

The only trouble is that to decode your data you need another copy of the pad and passing pads around is insecure - they can get intercepted.

China Start des ersten Quantenkommunikationssatelliten Micius (Getty Images/AFP/STR)

Launch 2016: the US Pentagon calls China's quantum communications satellite a "notable advance in cryptography research"

"Similarly the encryption used on the internet, RSA, is breakable, but you can't see the key, which makes it difficult to break and you can pass it around easily," says Clegg, "whereas the one-time pad is impossible to break but you can't pass it around easily without the risk of being intercepted."

It's like … totally random

With photon entanglement, however, it's possible to use a measurement on the particles as the key, which is a purely random thing.

If you measure the spin of a particle, there's a 50-50 chance it'll be either up or down. And if you have an entangled pair, and you measure the spin of one as up, you know then the other one will be down.

"But until you do the measurement," says Clegg, "it's totally random and you have no idea what's going to come out. That's ideal for a [encryption] key, because the key doesn't exist until you examine the particle, so you can't intercept it."

Get it? By looking at the particle - measuring it for the encryption key - you break the entanglement. So, theoretically, by making sure the entanglement is intact, you know your data is safe.  

Entangled-tinted specs    

The future for quantum communications is, as Jian-Wei Pan puts it, "bright."

But will quantum entanglement provide the key for secure space-based communication?

"Cryptography will play an important role in communication in space, that's certain, and I guess it's already being done," says Gisin, "but whether they'll use quantum means, I'm not sure. A thousand kilometers is a huge distance on Earth, but it's not a lot in space. So we'll see. But I'm very optimistic."

That said, American and European teams are considering sending quantum-based equipment to the International Space Station to test, for instance, whether gravitational fields influence photon entanglement. So there may be movement there.

On Earth, photon entanglement is being considered for teleporting properties in quantum computers - the next generation after your contemporary supercomputer.

And the next step looks like a collaboration between Pan's Chinese team and colleagues in Vienna.

Renowned Austrian physicist Anton Zeilinger, for one, says he's "impressed." But he would say that - Zeilinger was Pan's PhD advisor in the 1990s.

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