Phonons go for a quantum walk

The random walk is a useful model in fields from biology to computer science to math (see Physics Today, September 2019, page 18). For a horizontal one-dimensional walker, each step has some probability of going right or left. With the appropriate parameters, that simple picture turns out to be an accurate description of, for example, the paths that particles take during diffusion. The quantum analogue of the random walk replaces stochastic steps with superpositions: The walker steps left and right. With each step, the wavepacket spreads and eventually interferes with itself.

Quantum walks, like their classical counterparts, are versatile. They have applications in fields such as quantum optics, condensed-matter physics, and quantum information. Experimentally, they are typically realized and performed by photons in waveguides. Kenji Toyoda of Osaka University and his colleagues have now for the first time demonstrated quantum walks with phonons, the quanta of vibration. The phonons, which are exchanged between ultracold ions, have the advantage of being easily generated at a desired location and conveniently measured through fluorescence.