QUROPE Quantum Information Processing and Communication in Europe

Devices that harness the laws of quantum physics hold the promise for information processing that outperforms their classical counterparts, and for unconditionally secure communication. However, in particular, implementations based on condensed-matter systems face the challenge of short coherence times. Carbon materials, particularly diamond however, are suitable for hosting robust solid-state quantum registers, owing to their spin-free lattice and weak spin–orbit coupling. In this paper J.

In this work, the SOLID researchers G. Johannsson, C. M. Wilson and E. Solano propose a microwave photon detector that successfully reaches 100% efficiency with only one absorber. Their design consists of a metastable quantum circuit coupled to a semi-infinite transmission line that performs highly efficient photodetection in a simplified manner as compared to previous proposals.

The estimation of parameters characterizing dynamical processes is central to science and technology. The estimation error changes with the number N of resources employed in the experiment (which could quantify, for instance, the number of probes or the probing energy). Typically, it scales as . Quantum strategies may improve the precision, for noiseless processes, by an extra factor . For noisy processes, it is not known in general if and when this improvement can be achieved.

Device-independent quantum key distribution (QKD) aims to provide key distribution schemes, the security of which is based on the laws of quantum physics, but which does not require any assumptions about the internal working of the devices used in the protocol. This strong form of security is possible only when using correlations that violate a Bell inequality. Here, we provide a general security proof for a large class of protocols in a model in which the raw key is generated by independent measurements.

Metropolis algorithm is the standard method for the simulation of interacting particles on classical computers. In this work, the authors demonstrate how to implement a quantum version of the Metropolis algorithm on a quantum computer. This algorithm permits to sample directly from the eigenstates of the Hamiltonian and thus evades the sign problem present in classical simulations. A small scale implementation of this algorithm can already be achieved with today's technology.

Quantum state tomography—deducing quantum states from measured data—is the gold standard for verification and benchmarking of quantum devices. It has been realized in systems with few components, but for larger systems it becomes unfeasible because the number of measurements and the amount of computation required to process them grows exponentially in the system size. Here, we present two tomography schemes that scale much more favourably than direct tomography with system size.