Marc Barthelemy and Vincent Verbavatz proposed a new equation to understand the distribution of urban populations in a country and their results are published in the journal Nature (online November 18, 2020)
This equation, constructed from data for several countries, accounts for the first time for the temporal variations of urban populations and their organization. This stochastic equation of a new type (with two multiplicative noises, one gaussian and the other of the Levy type) highlights the importance of “interurban migratory shocks”, rare but significant population movements, and makes it possible to understand the hierarchical structure of cities and statistical regularities such as Zipf's law.
An IPhT researcher, Nicolas Sangouard, and his partners from the universities of Geneva and Basel have succeeded for the first time in "intriguing" the outputs of two optical fibers sharing a single photon at a distance of 2 km. By this performance, they show how a form of quantum entanglement that is simple to produce can be distributed and detected over long distances.
An important step towards the construction of a secure quantum the internet!
Quantum entanglement or entanglement refers to a very surprising phenomenon in which two systems form a linked state, with the state of each individual system remaining undefined. Regardless of the distance between them, these systems have correlated physical properties. Without stopping being amazed, physicists have learned to use entanglement and to imagine technological applications that have no classical equivalent. One of them consists in using entanglement to develop secure communication networks, a kind of quantum the internet with unprecedented security guarantees.
While the most classical scheme is based on a pair of photons whose polarization states are correlated, researchers have chosen to work with only one photon. To produce the single-photon entanglement, they "only" need a single-photon source, a semi-reflective plate and two optical fibers. Where two photons share a polarization state, two "optical paths" share a single photon. While the production of the single-photon entanglement is much easier than that a two-photon entanglement, the detection is quite different. How can we demonstrate the correlation between the properties of the two optical fibers, presence, absence or presence-and-absence of a photon at the same time? Physicists show that by adding a little light into the two optical fibers, it becomes possible to detect these three configurations that sign the desired correlation.
This method applies not only locally, in the proximity of the separating blade, but also at the end of the fibers, two kilometers away! The experiment reproduces a "complete" elementary quantum lattice link, including the entanglement "announcement" device. At this stage, it does not appear impossible to extend this link to a few hundred kilometers, thanks to the use of quantum repeaters.
Compared to its two-photon counterpart, the single-photon entanglement has a much higher loss resistance. For a 100 km link, the probability that the entanglement is preserved reaches 10% for one photon and only 1% for two. In addition, the announcement of the availability of the entanglement is easier to generate in the one-photon case. So many advantages for this brand new modality!
How do we protect the privacy of a communication if we cannot trust the devices used to communicate?
Researchers from IPhT, the University of Basel and ETH Zurich have provided innovative answers to this question, which is at the heart of research in quantum cryptography.
Hackers in possession of quantum computers pose a serious threat to some of today's crypto-systems. An interesting solution uses encryption methods based on keys produced by quantum principles. However, current quantum encryption protocols assume that the devices used to communicate are known and trustworthy. Otherwise, a door is opened for eavesdropping.
A team of physicists around Nicolas Sangouard from IPhT and the University of Basel, as well as Professor Renato Renner from ETH Zurich, have developed the theoretical bases of a communication protocol that offers ultimate protection of the privacy and can be implemented experimentally. This protocol guarantees security against hackers having a quantum computer with communication devices related to "black boxes", the reliability of which is unknown.
The researchers published their results in the journal Physical Review Letters and filed for a patent.
Physical Review Letters Abstract:
Device-independent quantum key distribution provides security even when the equipment used to communicate over the quantum channel is largely uncharacterized. An experimental demonstration of device-independent quantum key distribution is however challenging. A central obstacle in photonic implementations is that the global detection efficiency, i.e., the probability that the signals sent over the quantum channel are successfully received, must be above a certain threshold.
We here propose a method to significantly relax this threshold, while maintaining provable device-independent security. This is achieved with a protocol that adds artificial noise, which cannot be known or controlled by an adversary, to the initial measurement data (the raw key). Focusing on a realistic photonic setup using a source based on spontaneous parametric down conversion, we give explicit bounds on the minimal required global detection efficiency.