Carlos Sabín
Ramón y Cajal Researcher Department of Theoretical Physics UAM Madrid
In a future quantum communication network, a sufficiently large and reliable quantum computer (far beyond anything we currently have) 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 for the sender and receiver to share a quantum system with the celebrated property of quantum entanglement. Any attack would destroy the entanglement, which 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 truly want DI-QKD to have practical applications in a future communication network, we must be able to perform it between network nodes separated by sufficiently large distances.
In this Science article, the researchers succeed in implementing DI-QKD between parties separated by distances ranging from 10 to 100 kilometers. The quantum systems that are entangled are neutral rubidium atoms.
The method used to entangle them was proposed, among others, by Ignacio Cirac in 2001 and consists of detecting the light emitted at a point halfway between the atoms. Since this light can originate from either atom indistinguishably, the rules of quantum physics tell us that, upon measuring a photon, the atoms become entangled. Although in this experiment the atoms are not actually separated by tens of kilometers—because they are located in the same laboratory—the distance is simulated by sending the light through coiled optical fiber cables of those lengths.
Quantum entanglement is a very fragile property: as light travels through the fiber, small losses accumulate, and the resulting entanglement is of lower quality, which translates into higher error rates in the generated cryptographic keys. The experimental results show that the key error rate ranges from 3% at a distance of 11 kilometers to more than 7% at 100 kilometers. Therefore, although this represents an important step in the right direction, we are still far from being able to achieve a completely secure, error-free quantum key distribution over intercity distances.