Researchers report reversal of current flow in a quantum system | Ángel Rubio

Artist's impression of the role of a quantum observer: depending on where the observer is positioned, and what part of the figure is seen, the water will be seen to flow differently. Credit: © K. Aranburu

Oct. 9, 2017 (Phys.org) -- In a classical thermodynamic system, heat current flows from the hotter body to the colder one, or electricity from the higher voltage to the lower one.

The same thing happens in quantum systems, but this state can be changed, and the flow of energy and particles can be reversed if a quantum observer is inserted into the system.

This is the main result obtained by the group led by Professor Ángel Rubio of the UPV/EHU and of the Max Planck Institute PMSD, together with collaborators at the BCCMS centre in Bremen. Their study has been published in npj Quantum Materials.

In macroscopic objects such as a current of water, observing the current does not affect the flow of the water and, in accordance with the laws of classical thermodynamics, this flow would take place from the upper to the lower part of the system.

However, in quantum systems, "the process of observation changes the state of the system, and this makes it more likely that the current will be made to flow in one direction or another," says Ángel Rubio, a professor with the Hamburg-based Max Planck Institute for the Structure and Dynamics of Matter.

Rubio says this does not constitute "an infringement of any fundamental theorem of physics nor is energy created out of nothing. What happens is that inserting an observer into the system acts as an obstacle, as if you were to close off the channel in a pipeline through which the water is flowing.

Obviously, if the load starts to build up, it would end up going in the opposite direction. In other words, the observer projects the state of the system onto a state that transmits the current or energy in opposite directions."

Rubio remembers his surprise when discovering that inserting the quantum observer caused the directions of the current and the transfer of energy to change: "Initially, we thought it was an error. We expected to come across changes and we thought it would be possible to halt the transport, but we didn't expect there was going to be a complete change of flow.

These changes in the direction of the current can also be made in a controlled way. Depending on where the observer is inserted, the flow can be changed, but there are specific areas in the device in which, despite looking, the direction does not change," he says.

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