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Quantum Microwave Communcation and Sensing

Periodic Reporting for period 1 - QMiCS (Quantum Microwave Communcation and Sensing)

Reporting period: 2018-10-01 to 2020-03-31

The mission of QMiCS is to lead the European efforts in exploiting the quantum properties of propagating microwaves to trigger applications in distributed quantum computing, quantum communication, and quantum sensing. Based on the substantial experience of the partners, QMiCS mostly focuses on the regime of continuous variables (CV), developing novel components, experimental techniques, and theory models. QMiCS’ long-term visions are:
• Distributed quantum computing & communication via microwave quantum local area networks (QLANs).
• Sensing applications based on the illumination of an object with microwaves (radar) in the quantum regime.
In the important task of interfacing the quickly advancing solid-state quantum processors, microwaves intrinsically allow for zero frequency conversion loss since they are the natural frequency scale. Furthermore, they can be distributed via superconducting cables with surprisingly little losses. Radar also works at gigahertz frequencies because of the atmospheric transparency windows. Therefore, QMiCS’ disruptive developments eventually also lead to an implementation of quantum communication and cryptography techniques in the microwave domain. Within the 3-year horizon, QMiCS has three scientific grand goals:
(1) QLAN demonstration: Remotely prepare and quantum teleport squeezed microwave states between two dilution refrigerators connected by a 6m long cryogenic link (“QLAN cable”).
(2) Demonstrate a quantum advantage in microwave illumination via a proof-of-principle experiment.
(3) Establish a roadmap to real-life applications for the second/third phase of the QT Flagship by substantially advancing the theory of microwave communication and sensing.
Subordinate objectives: Microwave single photon detectors/counters; Improved superconducting parametric devices; Theory of microwave quantum communication and sensing under real-life conditions; Improved semiconductor-based cryogenic HEMT amplifiers; Industry-compatible packaging of superconducting devices; Realize a 6m long cryogenic “QLAN cable”; Foster awareness in industry about the revolutionary business potential of quantum microwave technologies. Via all these measures, QMiCS eventually helps to place Europe at the forefront of the second quantum revolution, kick-starting a competitive European industry in quantum compuing/communication/sensing and in enabling technologies.
QMiCS has reached all relevant milestones and deliverables in the first period. The main highlights are:

Novel number-resolving photon counter. Microwave photon counter are a highly active branch in the field of superconducting quantum circuits. In QMiCS, we consider photon counting as an indispensable prerequisite to demonstrate a quantum advantage in microwave illumination. 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)]. The detection sequence works as follows. (i) Emit a propagating microwave mode in which we want to count the number of photons. (ii) Catch the incoming microwave packet into a memory resonator by using a pumped parametric superconducting circuit. (iii) Count the number of photons that are stored in the memory using an ancillary transmon qubit and its readout resonator. (iv) Release the memory state using the same parametric circuit into the transmission line.

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)]. The experiment has required the independent control of three Josephson parametric amplifiers. By employing propagating two-mode squeezed microwave states and feedforward, we achieve the remote preparation of squeezed states with up to 1.6 dB of squeezing below the vacuum level. 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.

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. All components have arrived at the installation site, where they will be connected with another cryogenic link of the same length to an existing dilution refrigerator. Importantly, the device developed within QMiCS is a fully commercial solution from a single source readily available to potential customers in the community.

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. The quantum state shall be broadcast from the fridge via a cryogenic linear antenna to open air at room temperature. We take into account losses due to refraction, diffraction, and the addition of thermal noise. Finally, the receiver is modelled by another linear antenna. As a result, we predict that entanglement should not collapse completely over free-space propagating distances of several hundreds of meters under ambient conditions. This result is very encouraging for future quantum illumination and free-space quantum communication with microwaves and has to be verified experimentally in the future.
All main highlights describes in the previous section present a significant step beyond the state of the art at project start. In general, QMiCS has greatly improved the visibility of quantum microwaves in the scientific community, industry, and society as a whole. Especially a potential for actual applications is now significantly better recognized.This statement refers to both quantum microwaves in general and continuous-variable quantum microwaves in particular. Some minor delays, mostly CoViD-19-related, have been mitigated by the amendment AMD-820505-3. Experiments on quantum microwave teleportation are ongoing with promising preliminary results. We expect to have the microwave QLAN cable between two dilution refrigerators ready soon. Along with theory progress, also our goals in quantum microwave illumination still appear feasible. Finally, as an originally unintended branch, discussions with industry have triggered the active exploration of microwave QKD.
"EU Quantum Flagship project ""Quantum Microwaves for Communication and Sensing"" (QMiCS)"