As its name implies, the poisoning of Majorana devices by normal electrons is fatal to topological computation, so much effort is now focused on characterizing the degree of poisoning either by the creation of quasiparticle pairs within the device, or by electrons entering the device through the leads.

A recent experiment (see https://arxiv.org/abs/1612.05748), led by Sven Albrecht and carried out at Station Q Copenhagen, demonstrates that when a Majorana device is strongly coupled to normal-metal leads, poisoning has a distinct experimental signature—a set of “shadow” Coulomb blockade diamonds, offset from the main diamonds by one electron charge from the main Coulomb diamonds associated with Cooper-pair tunneling.

Detailed theoretical modeling by Esben Hansen, Jeroen Danon, and Karsten Flensberg, also presented in the paper, was in good quantitative agreement with the experiment, and allowed the strength of the shadow peaks to be converted into a poisoning rate. The rate was measured at, and calculated at, strong tunneling, which is not where one would operate a Majorana device. Extrapolating the theory to the point where the shadows would be invisible allows a bound to be placed on the poisoning rate from the leads, even when they are more closed than in the experiment. The fact that we don’t see shadows in the more closed devices means that poisoning must occur with a characteristic time of around 10 microseconds, given the parameters of these nanowire devices. We expect that the situation is much better than this conservative bound, an assumption we will test in future experiments.

Read the full paper.