Luca Dellantonio

A thesis for the degree of Doctor of Philosophy defended November 2018.

The PhD School of Science, Faculty of Science, Quantum Optics, Niels Bohr Institute, University of Copenhagen

Supervisor:
Anders S. Sørensen

Looking for mechanical hiccups & High dimensional mdi–QKD

The fields of opto- and electromechanics have facilitated numerous advances in the areas of precision measurement and sensing, ultimately driving the studies of mechanical systems into the quantum regime. To date, however, the quantization of the mechanical motion and the associated quantum jumps between phonon states remains elusive. For optomechanical systems, the coupling to the environment was shown to preclude the detection of the mechanical mode occupation, unless strong single photon optomechanical coupling is achieved. Here, we propose and analyse different setups, which allow us to overcome this limitation and resolve the energy levels of a mechanical oscillator. We find that the heating of the membrane, caused by the interaction with the environment and unwanted couplings, can be suppressed for carefully designed electromechanical systems. The results suggest that phonon number measurement is within reach for modern technology.

Quantum key distribution (QKD) provides ultimate cryptographic security based on the laws of quantum mechanics. For point–to–point QKD protocols, the security of the generated key is compromised by detector side channel attacks. This problem can be solved with measurement device independent QKD (mdi–QKD). However, mdi–QKD has shown limited performances in terms of the secret key generation rate, due to post–selection in the Bell measurements. We show that high dimensional (Hi–D) encoding (qudits) improves the performance of current mdi–QKD implementations. The scheme is proven to be unconditionally secure even for weak coherent pulses with decoy states, while the secret key rate is derived in the single photon case. Our analysis includes phase errors, imperfect sources and dark Counts to mimic real systems. Compared to the standard bidimensional case, we show an improvement in the key generation rate.