QMiCS has reached all relevant milestones and deliverables in the first period. The main highlights are:
(i) Novel number-resolving photon counter. QMiCS has developed a qubit-based superconducting device with a 96% efficiency in detecting single microwave photons and the ability to resolve up to three photons. [R. Dassonneville et al., arXiv:2004.05114 (2020)].
(ii) Remote state preparation of propagating squeezed microwaves. Quantum communication protocols based on nonclassical correlations can be more efficient than known classical methods and offer intrinsic security over direct state transfer. In QMiCS, we have demonstrated an experimental realization of deterministic continuous-variable remote state preparation in the microwave regime over a distance of 35 cm [S. Pogorzalek et al., Nature Communications 10, 2604 (2019)]. In addition, the security aspect has been investigated by measuring von Neumann entropies. We find nearly identical values for the entropy of the remotely prepared state and the respective conditional entropy given the classically communicated information, demonstrating close-to-perfect security.
(iii) Millikelvin cryogenic link for a microwave QLAN cable for quantum microwave communication between two two dilution refrigerators at millikelvin temperatures in different laboratory rooms. As a significant step towards this goal, a commercial dilution refrigerator and a so-called cryogenic networking node (CNN) have been connected by a millikelvin cryogenic link over a distance of 3.3 m. The CNN ensures scalability to larger distances and allows for the connection of additional fridges to create a truly interconnected local area quantum network. Temperatures at the center of the link are 35 mK at the warmest spot, i.e. well within the target specification.
(iv) Theory of non-guided quantum microwave state transfer. Quantum state transfer is the key challenge in non-guided quantum teleportation because it is used for distributing the resource state between the communicating parties. In this spirit, we have focused on modeling the transfer of QMiCS’ continuous-variable quantum resource, the two-mode squeezed state. As a result, we predict that entanglement should not collapse completely over free-space propagating distances of several hundreds of meters under ambient conditions.
QMiCS has reached all mandatory milestrones and deliverables in the second project period. In particular:
(i) A quantum advantage in radar-type sensing has been achieved using the qubit-based photon counter from the first project period.
(ii) The microwave QLAN cable between two dilution refrigerators has been installed and taken into operation. The innermost shield has a maximum temperature of 52mK at the center location inside the CNN. CV microwave state transfer and quantum teleportation have been demonstrated over that cryogenic link.
(iii) The roadmap to real-life applications has been drafted in a collaborative effort between all partners. It has the form of a review article and publication is planned in near future. The roadmap includes perspectives based on the theory results and takes into account practical boundary conditions from the experimental findings.
(iv) In addition, the technology milestones in WP1 (parametric devices, HEMT amplifiers, microwave detectors in an industrial environment) have been reached. Several devices have been used to achieve some of the project results.
Exploitation and dissemination have occurred through one patent, three products, commercially available parametric devices, and academic channels such as publications, conferences, and training.
