Quantum communication using phonons: Towards a quantum network using high frequency mechanical oscillators

Quantum communication refers to the field of science that studies the ability to connect separated quantum devices via coherent channels, i.e. via buses that maintain coherently the information encoded. The importance of the task is on multiple levels: from secure communications to scaling quantum computers. The first one can be of fundamental importance in moments like government elections, or banks transaction, but even to secure the right of privacy of individuals. The latter could open the way to, for example, faster and more precise solutions to problems in chemistry (for drug development), or material science (more environmentally friendly solar cells). At this moment, quantum communication and computing are in a similar stage as of the early computers: the machines are with very little connectivity and require large spaces and specialists to be operated. In a few years, we can expect these systems to be interconnected more and more, scaled, and made easier to use. In this thesis, we present work done to create quantum channels using high frequency mechanical oscillators. In chapter 1 we present recent progress in the field of quantum communication done with several types of systems, both in the long and short distance. We also introduce how high frequency mechanical oscillators could play an important role in this research area. We discuss the current challenges and limitations and possible future developments. In chapter 2 we perform a optomechanical quantum teleportation. In this work, we teleport a polarization encoded telecom photon onto a quantum memory, made by two single mode mechanical oscillators in a dual-rail configuration. This work is a step towards entanglement swapping (also referred to as ’teleportation of entangled state’) and represents a proof of principle towards quantum repeaters (using the scheme proposed by Duan, Lukin, Cirac, and Zoller - DLCZ scheme).
In chapter 3 we report the first experiment done with the multimode mechanical
devices. These devices are formed by a single mode optomechanical cavity coupled to a single-mode mechanical waveguide (ended with a phononic mirror). We show that the non-classical information created in the optomechanical cavity can be guided on chip in the mechanical waveguide, using as witness the cross-correlation between the scattered photons. However, the non-uniform spacing between the mechanical modes severely lowers the maximum value of the non-classical correlation measured. This was greatly improved with the new design of the device. With this design, we are able to entangle two traveling phonons in the mechanical waveguide, shown in chapter 4. In this way, we show that the traveling phonons can be used to distribute quantum entanglement on-chip, a first step towards connecting quantum devices on a short scale. In chapter 5, we measure in time the frequency jitter of two spectrally close mechanical modes of the same device. We demonstrate that the frequency diffusion of the modes
is not correlated in time, and so the coherence length of the traveling information will ultimately be limited by the jitter. This result shows the importance of performing a detailed study on the surface defects.
Lastly, in chapter 6 we summarize the findings of these experiments and we discuss the future developments of the field.