Public defence in Communications Engineering, M.Sc. Hany Khalifa

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

Doctoral student: Hany Khalifa
Opponents: Prof. Matti Silveri, University of Oulu, Finland
Custos: Prof. Riku Jäntti, Aalto University School of Electrical Engineering, Department of Information and Communications Engineering 

Quantum entanglement is now the impetus for many enabling technologies that are unrivalled by their classical counterparts. These include quantum illumination (QI), long-distance quantum communications, and quantum key distribution (QKD), all of which are critical tasks for the eagerly sought quantum internet. 

However, quantum states are fragile, and the effects of noise and losses would eradicate their original entanglement. In spite of this fact, the uniquely improved signal-to-noise ratio (SNR) of the entanglement source persists under the condition that the energy of the generated entangled systems is significantly low. Given this, the microwave domain is more suitable for the realization of entanglement-enhanced quantum technologies. This is due to the fact that microwave photons have small energies and the environmental noise is large. Nonetheless, ideal microwave single-photon detectors (SPDs), which are key components of the aforementioned protocols, remain elusive. 

This thesis proposes novel methods and techniques to realize microwave quantum communications in the presence of noise and losses, and without the need for ideal SPDs. The focus of this work is on QI-enhanced detection and single-photon protection against passive bosonic losses. 

By advancing the novel controlled-not (CNOT) QI receiver, this thesis demonstrates that microwave QI can be realized by utilizing a detection chain comprizing double-port homodyne detectors and spectrum analyzers. Furthermore, compared to other QI receivers, CNOT exhibits notably enhanced performance and, ideally, saturates the optimum quantum advantage for any QI receiver. 

To mitigate passive bosonic losses, this thesis proposes another novel device, the one-way noiseless linear amplifier (NLA). Unlike its optical counterpart, the microwave one-way NLA doesn't rely on ideal SPDs, instead, it utilizes quantum non-demolition detectors (QNDs) which are currently available in the microwave domain. Moreover, the one-way NLA is fault-tolerant, a feature not present in conventional optical NLAs. The device demonstrates a great ability to generate remote entanglement, prepare resources offline for QI, and generate remote secret keys. 

Finally, both devices can be realized by the available hardware in circuit quantum electrodynamics (cQED). 

Although this thesis focuses on microwave quantum communications, the ideas presented here are abstract and can also be deployed in the optical domain.

Keywords: Quantum illumination (QI), noiseless linear amplifier (NLA), controlled-not (CNOT) gate, CNOT QI-receiver, one-way NLA.

Thesis available for public display 10 days prior to the defence at: https://aaltodoc.aalto.fi/doc_public/eonly/riiputus/

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Doctoral theses in the School of Electrical Engineering: https://aaltodoc.aalto.fi/handle/123456789/53