China researchers score a quantum leap

Scientists in Hefei have established the world's first integrated quantum communication network

“Those who are not shocked when they first come across quantum theory cannot possibly have understood it.”
― Niels Bohr

In the strange world of quantum mechanics, the laws of physics as we know them are tossed out the window.

In other words, reality is not exactly what we think it is.

For example …

Today, sensitive data is typically encrypted and then sent across fiber-optic cables and other channels together with the digital “keys” needed to decode the information.

The data and the keys are sent as classical bits – a stream of electrical or optical pulses representing 1s and 0s. And that makes them vulnerable.

Smart hackers can read and copy bits in transit without leaving a trace. Even if that cable is at the bottom of an ocean, it can be tapped into and hacked.

Throughout history, the battle between encryption and decryption never ends.

Enter quantum communication, which takes advantage of the laws of quantum physics to protect data.

These laws allow particles – typically photons of light for transmitting data along optical cables – to take on a state of superposition, which means they can represent multiple combinations of  1  and 0 simultaneously.

The particles are known as quantum bits, or qubits.

The beauty of qubits from a cybersecurity perspective is that if a hacker tries to observe them in transit, their super-fragile quantum state “collapses” to either 1 or 0.

This means a hacker can’t tamper with the qubits without leaving behind a telltale sign of the activity.

In a giant technological step toward this end, Chinese scientists have established the world’s first integrated quantum communication network, combining over 700 optical fibers on the ground with two ground-to-satellite links to achieve quantum key distribution over a total distance of 4,600 kilometers for users across the country, Phys.Org reported.

The team, led by Jianwei Pan, Yuao Chen, and Chengzhi Peng from the University of Science and Technology of China in Hefei, reported in Nature their latest advances toward the global, practical application of such a network for future communications.

Unlike conventional encryption, quantum communication is considered “uncrackable” and therefore the future of secure information transfer for governments, banks, power grids and other sectors.

In the 1980s, researchers developed a theoretical method for generating secure keys using quantum mechanics.

They figured out that secure keys could be encoded into the quantum properties of individual particles, and exchanged secretly back and forth.

The advantage of this “quantum key distribution” (QKD) is that quantum physics dictates that the very act of observing a particle irreparably changes it.

So any spies who tried to intercept the quantum key could be immediately detected by the changes in the particles.

So far, the most common QKD technology uses optical fibers for transmissions over several hundred kilometers, with high stability but considerable channel loss.

Materials in cables can absorb photons, which means they can typically travel for no more than a few tens of kilometers. In a classical network, repeaters at various points along a cable are used to amplify the signal to compensate for this.

QKD networks have come up with a similar solution, creating “trusted nodes” at various points. The Beijing-to-Shanghai network has 32 of them, for instance.

At these waystations, quantum keys are decrypted into bits and then re-encrypted in a fresh quantum state for their journey to the next node.

Another major QKD technology uses the free space between satellites and ground stations for thousand-kilometer-level transmissions.

In 2016, China launched the world’s first quantum communication satellite (QUESS, or Mozi/Micius) and achieved QKD with two ground stations 2,600km apart.

In 2017, a more than 2,000km-long optical-fiber network was completed for QKD between Beijing and Shanghai.

Using trusted relays, the ground-based fiber network and the satellite-to-ground links were integrated to serve more than 150 industrial users across China, including state and local banks, municipal power grids, and e-government websites.

In essence, the achievement indicates that quantum communication technology can be used for future large-scale practical applications.

Similarly, a global quantum communication network can be established if national quantum networks from different countries are combined, and if universities, institutions and companies come together to standardize related protocols.

In the last couple of years, the team extensively tested and improved the performance of different parts of the integrated network.

For instance, with an increased clock rate and more efficient QKD protocol, the satellite-to-ground QKD now has an average key generation rate of 47.8 kilobits per second, which is 40 times as high as the previous rate.

The researchers have also pushed the record for ground-based QKD to beyond 500km using a new technology called twin-field QKD (TF-QKD).

TF-QKD is a new extraordinary QKD protocol, which can overcome the fundamental rate-distance limit without quantum repeaters.

Experimentally, TF-QKD has already been performed over 400km of telecom fibers, as well as more than 1,000km of free space through satellite to ground links.

This result is possible thanks to a different way of encoding and retrieving the information in the quantum carriers used for the protocol. 

In TF-QKD the information is encoded in the phase of the optical pulses prepared by the two users that want to establish the secure communication, and the secret key is retrieved via a single photon interference measurement made by a user in the middle.

Another interesting aspect of TF-QKD is that it is also Measurement Device Independent, which means that it meets the strictest standards of security.

Sources: Phys.Org, Technology Review, Springer Professional, QCALL, University of Science and Technology of China

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