Securing quantum networks with light emitted by artificial atoms


A strong collaboration between researchers from the Universities of Vienna, Stuttgart and Bayreuth have identified how to excite artificial atoms to enhance the security of quantum communication and cryptography. The results, which bridge the gap between two large fields of quantum photonics, were published in Nature Portfolio’s quantum journal, npj Quantum Information:

Modern communication networks are continuously expanding, with the increase in the number of users and available online resources. On a daily basis, users must inevitably trust local network nodes and transmission channels, which can be compromised by malicious parties and hackers with large computational power.
Remarkably, quantum cryptography can protect users against these malicious parties by harnessing the fragile quantum properties of light particles, known as photons. For instance, measuring the properties of a single photon alter its very state, which implies that cloning quantum light is fundamentally impossible. This striking quantum law has fascinating cryptographic applications, such as the generation of unforgeable quantum money, secure communication, and digital signatures.
In order to create the highest quality single photons for secure quantum applications, artificial atoms, known as quantum dots, are remarkable candidates: when excited with a laser of a given frequency, they can in principle emit very fast and on-demand, while keeping the probability of generating more than one photon very low.
However, the most secure way of exciting the quantum dot remained undefined: do the emitted single photons leak more information to malicious eavesdroppers present in the network when excited on resonance, off-resonance, or through the absorption of two photons? This is exactly what the researchers have managed to identify, and surprisingly found that different applications require different optical excitation schemes for security to be preserved: when exciting the dots on resonance for instance, a fixed phase relationship appears between the vacuum and single photons, while this relationship disappears off-resonance. This novel quantum feature has benefits for some applications, but actually compromises the security of other applications. Exciting the quantum dot with two photons instead of one, furthermore, shows other interesting benefits in terms of non-clonability.
These results will enhance the security of future quantum networks, since they allow the two involved scientific communities to communicate better, which is a challenging mission: “Solid-state physicists can develop quantum dot sources which emit light with remarkable properties, but the challenge is to specifically tune these sources to the needs of the quantum cryptography community, who uses very different vocabulary and mathematical tools”, explains Mathieu Bozzio, one of the lead scientists on the project. Many resulting new works are currently in progress, experimentally investigating the potential and drawbacks of quantum dot sources for the secure quantum networks.

The lower half displays a vacuum chamber in which a planar sample, containing many quantum dots, is carefully placed. The upper half shows an objective through which the laser excites the quantum dots. The resulting single photons are collected through the very same apparatus, and launched in an optical fiber to analyze their properties for quantum cryptography.