“Walking” drops to understand quantum properties

Writing / Irene Vega

When a drop is removed from a vibrating bath of silicone oil, it can bounce on its surface without sinking into the fluid. Each time the drop hits the underlying fluid, it produces traveling waves on its surface, which can affect the movement of the drop in subsequent collisions. If, upon colliding with the fluid, the latter has a certain inclination, the drop can begin to “walk” horizontally, giving rise to a particle with a guide wave accompanying it. From this phenomenon, it is possible to demonstrate that these experimental systems reproduce many quantum phenomena such as the quantization of orbits or tunnel effect.

Researcher Álvaro García López, professor in the area of ​​Applied Physics at the Rey Juan Carlos University, in collaboration with Professor Rahil Valani from the Rudolf Peierls Center for Theoretical Physics at the University of Oxford, has initiated a pioneering line of research related to hydrodynamic quantum analogues. In this sense, a walking drop model has been extended by incorporating an energy potential with two wells separated by a barrier. Using tools from the nonlinear dynamics, the existence of numerous quantized orbits - the only possible ones in which quantum particles can move and which have the given energy as an average value - has been demonstrated, which correspond to sustained oscillations. "Throughout the quantized orbits, the system absorbs on average the same mechanical energy that it emits," says Professor Rahil Valani. Moreover, it has been observed that these orbits can lose stability and become chaotic, allowing the particle to pass unpredictably from one well to another.

Unlike conservative systems of the newtonian mechanics, these acrobat droplets are dissipative structures with memory, since the waves they produce in the medium in past collisions travel through it and affect them in the present time. “Here the memory involves the entire past history of the drop, although it is the most recent past that affects it the most. The dynamics of this active matter is very complex, to such an extent that we have been able, by simplifying the profile of the waves that the drop carries attached to it, to connect the equations that describe its movement with the famous Lorenz system “This gave rise to chaos theory,” says Professor Álvaro García López. “This has allowed us to use this theory to explain phenomenology specific to the atomic world, such as the tunnel effect, which we have unmasked as a process of chaotic intermittence.”

These systems are expected to allow the exploration of other typical phenomena of quantum physics, such as particle entanglement, understood as a synchronization process of chaos, which causes two bodies to tremble in unpredictable unison. results of this work have recently been published in the prestigious journal Chaos, Solitons & Fractals of the Elsevier group.