A Qubit.

Detecting Electrons from a Long Distance: 100+ Nanometers. 
A team of physicists at Leiden University in the Netherlands has detected a single electron, using a single molecule, with enough sensitivity to detect an electron from “hundreds of nanometers” apart.

Physicists have been able to manipulate single electrons; but, they can only see them as part of an electric current.  Scientists believe if they could detect an electron optically using one single molecule then you could indirectly see individual electrons that are actually on their own.

It is thought “a quantum computer could use this method to locate its qubits with light.”  Otherwise, at the current point in the evolution of quantum computing, a quantum computer will upset the spin-quantum state – a major challenge to moving quantum computing forward in earnest.

“Leiden physicist Michel Orrit and his group have now taken a first step towards developing this technique, by identifying a molecular system that is sensitive enough to detect an electron from as far as hundreds of nanometers away.”

Reference is found at University of Leiden…

Full study text is found here…