Public defence, Communications Engineering and Networking Technology, MSc Uesli Alushi

Title of the thesis: Continuous-Variable Quantum Sensing: Criticality-Enhanced Parameter Estimation and Private Distributed Sensing

Thesis defender: Uesli Alushi
Opponent: Dr. Martí Perarnau-Llobet, Universitat Autònoma de Barcelona, Spain
Custos: Prof. Riku Jäntti, Aalto University School of Electrical Engineering  

Quantum sensing exploits quantum phenomena, such as superposition and entanglement, to measure physical quantities with greater precision than classical instruments. It is among the most mature quantum technologies, with applications in gravitational-wave detection, medical imaging, navigation, and clock synchronization.
The first part of this thesis focuses on critical quantum sensing, where the diverging susceptibility near a phase transition is turned into a usable resource to estimate unknown parameters. The thesis investigates when criticality enhances sensing precision and derives fundamental precision bounds. The results show that, counterintuitively, criticality provides a genuine advantage in the presence of dissipation. Critical sensors also outperform conventional sensors when sensor preparation and measurement require non-negligible time. Furthermore, coupling critical resonators into a chain provides a collective enhancement: precision scales quadratically with the number of resonators, compared with the linear scaling of independent sensors, and this advantage persists in the presence of noise.
The second part addresses privacy in distributed quantum sensing (DQS), where distant sensors share an entangled state to estimate a global quantity, such as the average of their local signals. Perfect privacy is achieved when each user has access only to the global result and their own data, but no information about other users. This work addresses the question: Can ultimate estimation precision and perfect privacy be achieved simultaneously? Focusing on continuous-variable states, it shows that specially tailored quantum states can achieve both, although they are challenging to produce experimentally. Gaussian states, which require only linear optics and single-mode squeezing, achieve perfect privacy in networks with more than two nodes only at large photon numbers, while maintaining near-optimal precision.
Overall, the results pave the way towards experimental implementations of controllable critical quantum sensors and private distributed sensing architectures on continuous-variable platforms.

Key words: quantum information, quantum sensing, quantum parameter estimation

Contact: uesli.alushi@aalto.fi 

Thesis available for public display 7 days prior to the defence at Aalto University's public display page.