A classical computer with N bits (transistors), has $2^N$ possible states, and the computation process consists in switching on or off certain transistors according to a prescribed program. In contrast, the quantum computer with N qubits (spins) is characterized by $2^N$ quantum amplitudes, which are continuous complex variables. Thus, a toy quantum computer with, say, 50 qubits will be characterized at any given moment by $2^{50} = 1125899906842624$ continuous parameters, which we should be able to manipulate at will. However the number of qubits needed to have a useful machine (i.e. one that can compete with your laptop in factoring very large numbers) is estimated to be $10^3$−$10^5$. The number of continuous variables describing the state of such an apparatus at any given moment is at least $2^{1000}$ (~$10^{300}$) which is much, much greater than the number of particles in the entire Universe (only ~$10^{80}$)! Thus the answer to the question in the title is: As soon as the physicists and engineers will learn to control this quantity of continuous parameters!In recent years the trend is to advertise as quantum computers circuits made from truly quantum devices based on Josephson junctions at liquid helium temperatures, an approach that was initially started by the D-wave company, followed and developed by IBM, Google, Microsoft, and others. This is not going to be the quantum computer everyone was talking about during the last 20 years, as it will not be able to factor large numbers by Shor's algorithm. It will not and cannot compute anything at all, except its own ground state, a process labeled as "quantum annealing". After initial preparation, any system, whether classical or quantum, at low temperature will relax to its ground state. Calculating the ground state of a more or less complex quantum system, either analytically or numerically, is usually impossible – this is what originally inspired Feynman's idea of quantum computing. However, we can measure some properties of this ground state. (The full characterization of the ground state requires measuring $2^N$ parameters).It is not entirely clear what will be the possible practical use of such systems. However, such modeling might provide some additional knowledge on the behavior of large and complicated quantum systems.

The spatial distribution of suitable environmental conditions defines a species habitat, and colonization-extinction dynamics within this habitat determine the distribution of species populations. In addition, the success of colonization and the risk of extinction are expected to be influenced by the proximity to habitat boundary. We address here the influence of boundary vicinity for a contact-process model of population dynamics in a percolating habitat lattice. To separate the influence of boundary vicinity from that of fragment area, we investigate population dynamics in the very large spanning cluster of the percolating habitat. The geometry of the spanning cluster varies when habitat density in the lattice is tuned away from the percolation threshold. We expect that the colonization success decreases closer to the boundary, leading to depleted site occupancy in the cluster, and that this effect is even more pronounced near the extinction threshold of the contact process. For the set of suitable sites of the cluster and unsuitable sites next to its boundary, we quantify the boundary density, σ, as the probability to draw a pair of suitable and unsuitable sites. The cluster boundary is most rugged and σ is maximal close to the percolation threshold of the habitat. We expect that the global influence of boundary on population dynamics in the cluster is greater when the boundary is more rugged. We thus investigate population dynamics in the spanning cluster for varying values of σ. We determine the stationary properties of population dynamics in the spanning cluster according to σ and to the species-specific ratio of extinction and colonization rates, denoted r. Using both numerical simulations and a pair-approximation model, we assess global species persistence and site occupancy patterns in the spanning cluster. We show that the extinction threshold rc depends crucially on σ, i.e., increasing σ limits global species persistence. Furthermore, increasing σ decreases the probability of site occupancy in the spanning cluster. A key result is that this influence extends to sites far from the boundary when r gets close to rc. Therefore, species that are at risk of extinction are more sensitive to the influence of habitat boundary. These results are of both theoretical and practical interest for understanding and forecasting species viability in heterogeneous and fragmented habitats.

Metasurfaces have been used for light manipulation and control, from the point of view of planar optics or polarization control, or non-linear light extraction. The properties of metasurfaces is rooted in the presence of resonant basic elements which are responsible for strongly confined electromagnetic surface modes. We propose to use these modes to enhance light-matter interaction in the vicinity of the surface by considering hybrid structure made of a thin layer, such as a 2D quantum material or a thin film with embedded quantum dots. A semi-classical theory will be proposed along with some functionalities attainable with the device.