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Phononic Waveguide-based Platforms for Quantum Technologies

Periodic Reporting for period 1 - PWAQUTEC (Phononic Waveguide-based Platforms for Quantum Technologies)

Reporting period: 2018-04-01 to 2020-03-31

In the last decades science has witnessed the so-called “Second Quantum Revolution”, by which the now well understood laws of quantum physics, namely the laws of nature at very small scales, are starting to be harnessed for technological applications. Among the most promising of these applications are quantum networks, namely networks, similar to the internet, where information could be transported and processed in much more efficient ways thanks to quantum physics. The design of such networks at a large scale has become a clear objective of global and European research, since achieving this goal would allow, among others, for ultra-fast ad ultra-secure distant communication, or the fabrication of computing systems with much larger computational power.

The most popular approaches to quantum networks and quantum technologies in general are based on light (photons) exchanging quantum information between information nodes such as, for instance, atoms. On the one hand, photons are good information carriers because they propagate fast and lose their quantum properties very slowly. On the other hand, they interact very weakly with atoms and other nodes. This motivates researchers to explore other possible carriers that can be used instead of, or in combination with, photons. Promising but so far unexplored candidates as carriers of quantum information are the “quantum particles of vibration”, called phonons. These phonons are in some aspects similar to photons, as they can propagate relatively fast and lose their quantum properties relatively slowly. However, they can also interact more strongly with other systems, and they could store more quantum information than photons.

In this project, we propose to explore the potential of phonons for quantum technologies. We set three main goals: first, to find, study, and design the “nodes” of a phononic quantum network, equivalent to the atoms in the case of light. Second, to explore the interaction between these nodes and phononic “wires” (waveguides) and how to modify the state of one using the other. Third, to devise particular applications relevant to quantum technologies: for instance, phononic diodes that allow phonons to flow only in one direction, or phononic “bandgaps” where phonons can be stopped at will.
A significant part of our work has been devoted to the identification and optimization of very small nanoparticles as ideal nodes for quantum technologies. These particles can be very well isolated from the outside world, e.g. by levitating them, and thus can hold and protect quantum information for a very long time. We first calculated the kind of phonons that can be sustained by these nanoparticles, and uncovered their unexpected exotic behavior induced by the small particle size. Among the surprising features was the very strong interaction between phonons and the “quantum particles of magnetism” (magnons), which revealed as a powerful tool. Indeed, one can use these magnons to externally control, modify, and read out the information stored in the phonons, establishing these small particles as ideal nodes for a phononic network. As we revealed, this can be done by transferring this information to external electronic defects in diamond, or to the global motion of the particle. Finally, we also studied the so-called decoherence phenomenon, by which the phonons in the quantum nodes could lose their quantum properties. We studied many different decoherence mechanisms including the particle rotation, heat, and interaction with electrons and other impurities. This exhaustive study allowed us to satisfactorily conclude that nanoparticles are ideal nodes for phononic quantum networks.

For our second and third objectives, we first investigated theoretically how to modify the way in which phonons propagate in phononic wires. In particular, we uncovered a way to design phonon pulses that can “self-focus” as they propagate, namely they become more concentrated within a travelling “hot-spot”. We also showed how to use these pulses to deliver energy to a selected small area, thus allowing to transfer and read out information from individual nodes in a quantum network. Second, motivated by levitated nanoparticles as phononic nodes, we studied phononic wires where the nodes are allowed to move along the wire. This system turned out to be very rich and allow for (i) Stopping the phonons in phononic bandgaps; (ii) Generate “bound states” where a node is surrounded by a tight cloud of phonons. These states are very useful in quantum technologies to engineer interactions between different nodes; (iii) devising single-phonon diodes that allow phonons to propagate in only one direction; (iv) modifying the energy of the nodes at will using phonons propagating in the wire. In general, all these properties evidence that phononic networks can be as flexible as photonic ones, and promote phonons as excellent candidates for information carriers in quantum technologies.

Our overall work resulted in the publication of eight scientific articles, plus four more currently in preparation. It was disseminated to the scientific community through research visits, seminar talks, and presentations in 10 international conferences. Our dissemination activities also extended to the general public through several popular science press releases, social networks such as Twitter, Facebook, and Instagram, and the website of the host institution. A collaboration with an international painter from Chicago also resulted in four fine art paintings based on our work.
Both the main results of PWAQUTEC (levitated nanoparticles as phononic nodes and phonon-based wire-node interactions) significantly advanced the state of the art of levitated nanoparticles, nano-magnetoelasticity, quantum magnonics and acoustics. Our results form a solid theoretical basis for further advances in the fields, and pave the way to related experiments. Indeed, two of the multiple experimental collaborations established during PWAQUTEC resulted in the publication of two related experiments.

The results of our publications have been explained to the public in layman’s terms through a dedicated Instagram and Facebook hashtag, advancing the public understanding of the relevance of PWAQUTEC and their awareness toward the importance of global research in quantum technologies.

During PWAQUTEC several master and PhD students have been trained, receiving a solid formation in optomechanics, acoustics, nanophotonics, and others, and acquiring expertise in science presentation and scientific writing, as well as team working and critical scientific thinking. These students are now equipped with the necessary skillset to advance the quantum technology research endeavor of the European Union.