Take a lattice, poke it at a regular frequency and it will start oscillating. Have you ever thought of how exactly its components are moving? If everything moves harmonically then the dynamics are not extremely interesting, but if they move non-linearly then the process is creating new frequencies. This idea is the basis of non-linear optics. Instead of looking at what happens to light, Kang Cai, Zi-Wen Pan, and colleagues concentrated on the vibrations and realized that they could use the defects in a diamond lattice to generate single quantum vibrational excitations by hitting the lattice with a suitable combination of laser light and microwaves. These are called phonons and they are the "material" cousins of photons. The team not only managed to find a controlled way of generating single phonons, but the process had zero linear response, which means that none of the energy that they were injecting in the lattice was wasted (high efficiency in non-linear processes is a very rare thing). This work is a new milestone in the quest for controlling light-matter interactions and it is of particular interest because at the quantum level phonons are related to heat and noise, and the more we understand how to manage the noise in a quantum device the closer we get to fully functional quantum technology.
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