Microwave quantum communications: new approaches to sensing and mitigation of the bosonic pure-loss channel

With the current availability of microwave quantum technologies, it is imperative to investigate the different methods and techniques that would enhance the performance of currently existing microwave communication systems. There are two particular areas of interest that are considered in this thesis: (1) quantum microwave sensing in the presence of extreme additive white Gaussian noise, and (2) the imperfect propagation and storage of bosonic modes inside lossy transmission media. Due to the small signal powers in the microwave domain, the task of finding the most efficient detection method for the completion of the aforementioned tasks while maintaining the quantum advantage is complicated. In this thesis, novel methods and techniques are proposed that ease the experimental requirements for microwave quantum technologies. The thesis comprizes four main publications that summarize the research investigation. Publications I and II consider the problem of physically realizing microwave quantum illumination without the need for ideal single-photon counters. Firstly, publication I studies the effect of the excess noise and losses induced by the environment on the utilized signal-idler pair. Then, publication II provides a novel solution, a CNOT (controlled not) gate quantum illumination receiver that achieves an optimal performance set for a quantum illumination receiver without the need for single-photon counters. In publications III and IV, the focus is on devising new strategies to mitigate the losses experienced by microwave bosonic modes during propagation or storage. The objective here is to adapt the concept of noiseless linear amplification, earlier demonstrated in the optical domain, to the microwave region. Despite the persistent problem of microwave detection, the novel one-way noiseless linear amplifier based on quantum non-demolition detectors managed to outperform a conventional one based on microwave photon counters. Furthermore, it also offered an uninterrupted performance due to its fault tolerance which could not be replicated by a conventional noiseless linear amplifier. Finally, publication IV considers several future applications of one-way noiseless linear amplifiers in sensing, remote entanglement sharing and secret key generation, where the device demonstrated in this thesis is able to outperform any other conventional noiseless linear amplifier.