In recent years, nanomechanical oscillators in thin films of superfluid helium have attracted attention in the field of optomechanics due to their exceptionally low mechanical dissipation and optical scattering. Mechanical excitations in superfluid thin films - so-called third sound waves - can interact with the optical mode of an optical microresonator by modulation of its effective refractive index enabling optomechanical coupling. Strong confinement of third sound modes enhances their intrinsic mechanical nonlinearity paving the way for strong phonon-phonon interactions with applications in quantum optomechanics. Here, we realize a phononic crystal cavity confining third sound modes in a superfluid helium film to length scales close to the third sound wavelength. A few-nanometer-thick superfluid film is self-assembled on top of a silicon nanobeam optical resonator. The periodic patterning of the silicon material creates a periodic modulation of the superfluid film leading to the formation of a phononic band gap. By engineering the geometry of the silicon nanobeam, the phononic band gap allows the confinement of a localized phononic mode.

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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.@en

The ability to create, manipulate and detect non-classical states of light has been key for many recent achievements in quantum physics and for developing quantum technologies. Achieving the same level of control over phonons, the quanta of vibrations, could have a similar impact, in particular on the fields of quantum sensing and quantum information processing. Here we present a crucial step towards this level of control and realize a single-mode waveguide for individual phonons in a suspended silicon microstructure. We use a cavity–waveguide architecture, where the cavity is used as a source and detector for the mechanical excitations while the waveguide has a free-standing end to reflect the phonons. This enables us to observe multiple round trips of phonons between the source and the reflector. The long mechanical lifetime of almost 100 μs demonstrates the possibility of nearly lossless transmission of single phonons over, in principle, tens of centimetres. Our experiment demonstrates full on-chip control over travelling single phonons strongly confined in the directions transverse to the propagation axis, potentially enabling a time-encoded multimode quantum memory at telecommunications wavelength and advanced quantum acoustics experiments.

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Distributing quantum entanglement on a chip is a crucial step toward realizing scalable quantum processors. Using traveling phonons-quantized guided mechanical wave packets-as a medium to transmit quantum states is now gaining substantial attention due to their small size and low propagation speed compared to other carriers, such as electrons or photons. Moreover, phonons are highly promising candidates to connect heterogeneous quantum systems on a chip, such as microwave and optical photons for long-distance transmission of quantum states via optical fibers. Here, we experimentally demonstrate the feasibility of distributing quantum information using phonons by realizing quantum entanglement between two traveling phonons and creating a time-bin-encoded traveling phononic qubit. The mechanical quantum state is generated in an optomechanical cavity and then launched into a phononic waveguide in which it propagates for around 200 micrometers. We further show how the phononic, together with a photonic qubit, can be used to violate a Bell-type inequality.

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Quantum teleportation is a key component in long distance quantum communication protocols. Here we demonstrate quantum teleportation of a polarization-encoded optical input state onto the joint state of a pair of nanomechanical resonators.

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Quantum teleportation, the faithful transfer of an unknown input state onto a remote quantum system¹, is a key component in long-distance quantum communication protocols² and distributed quantum computing^3,4. At the same time, high-frequency nano-optomechanical systems⁵ hold great promise as nodes in a future quantum network⁶, operating on-chip at low-loss optical telecom wavelengths with long mechanical lifetimes. Recent demonstrations include entanglement between two resonators⁷, a quantum memory⁸ and microwave-to-optics transduction^9–11. Despite these successes, quantum teleportation of an optical input state onto a long-lived optomechanical memory is an outstanding challenge. Here we demonstrate quantum teleportation of a polarization-encoded optical input state onto the joint state of a pair of nanomechanical resonators. Our protocol also allows to store and retrieve an arbitrary qubit state onto a dual-rail encoded optomechanical quantum memory. This work demonstrates the full functionality of a single quantum repeater node and presents a key milestone towards applications of optomechanical systems as quantum network nodes.

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We present a discrete-variable quantum teleportation scheme using pulsed optomechanics. In our proposal, we demonstrate how an unknown optical input state can be transferred onto the joint state of a pair of mechanical oscillators, without physically interacting with one another. We further analyze how experimental imperfections will affect the fidelity of the teleportation and highlight how our scheme can be realized in current state-of-the-art optomechanical systems.

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