“Achieving device independent quantum key distribution over such a long distance is an important step forward towards quantum networks. QKD has been achieved over such distances before but this work demonstrates the gold standard device independent version which is guaranteed secure even if your measuring apparatus
isn’t trustworthy.

"At the heart of this achievement is a quantum memory for light using trapped neutral atoms. In New Zealand, among other quantum technologies, solid state versions of
these memories are being  developed based on rare earths.

"This work is taking place in the context of ever improving quantum computers, computers that could eventually break many of the cryptographic codes currently in
use. It's widely believed that post-quantum cryptography will be required from around 2030 when quantum computers may be less error-prone and able to operate with a high number of qubits. Governments and companies in the USA, Australia and the UK are all working to meet this milestone or earlier.”

"In a future quantum communications network, a sufficiently large and reliable quantum computer (far removed from those we have today) could break the cryptographic keys used to protect our data and operations. The solution is to use new cryptographic keys based on the properties of quantum physics.

"These new keys are also vulnerable to certain types of attacks, but a particularly secure way to generate them is to have the receiver and sender share a quantum system with the famous property of quantum entanglement. Attacks would destroy the entanglement, and this would be easily detectable by measuring the so-called Bell inequalities. This is the principle behind device-independent quantum key distribution (DI-QKD).

"If we really want it to have practical application in a future communications network, we must be able to perform DI-QKD between network nodes that are separated by sufficiently large distances. In this article in Science, researchers manage to perform DI-QKD between parties separated by distances of between 10 and 100 kilometres. The quantum systems that become entangled are neutral rubidium atoms.

"The method for entangling them was proposed, among others, by Ignacio Cirac in 2001 and consists of detecting the light emitted halfway between the atoms. Since that light can come from either of the two atoms indiscriminately, the rules of quantum physics tell us that, when a photon of light is measured, the atoms become entangled. Although in this experiment the atoms are not actually separated by tens of kilometres, as they are in the same laboratory, the distance is simulated by circulating the light through coiled fibre optic cables of those lengths.

"Quantum entanglement is a very fragile property: as light travels through the fibre, small losses accumulate and the entanglement generated is of poorer quality, which translates into higher error rates in the cryptographic keys generated. The results of the experiment show that errors in the key range from 3% when the distance is 11 kilometres to more than 7% for 100 kilometres. Therefore, although this is an important step in the right direction, we are still a long way from being able to perform completely secure and error-free quantum key distribution on a scale of distances between cities."

1. The most important one: Alice and Bob are not separated by tens of kilometres. They are in the same laboratory, but connected by a fibre that is tens of kilometres long. In principle, this configuration simulates the situation where Alice and Bob are in separate locations tens of kilometres apart and connected by a fibre, but in practice it is not the same.
2. To be a little more technical: the experiment reports that for fibre distances of hundreds of kilometres, the conditions that would allow the protocol to be successfully carried out are observed, but full implementation has not been achieved."