Particles can be described as waves – these can collide and interfere. Quantum interference, the phenomenon in which a single particle exhibits interference with itself, is a well-known concept in modern physics. An example is the interference of electrons in conductors.
Quantum interference of electrons in Aharonov-Bohm rings can result in oscillations in the conductivity. Aharonov-Bohm rings are metal rings with dimensions of several micrometres. The strength of the magnetism that passes through the ring influences the phase of the electron wave. That in turn determines whether constructive interference occurs, in which case the conductivity increases, or destructive interference, in which case the conductivity decreases. The difference in conductivity is usually in order of several percent. The measurements need to take place at low temperatures.
Quantum interference has been well studied at the micrometre scale but not at the nanoscale. Molecules form superb model systems to change this situation. The energy differences that play a role in molecules are much larger than those in Aharonov-Bohm rings , so that quantum effects can even be expected at room temperature. It can also be expected that the change in conductivity will be much greater than in the case of the Aharonov-Bohm rings.
In this programme, researchers from Delft University of Technology and Leiden University will investigate quantum interference in molecules. Several predictions about how exactly the quantum interference will manifest have been made but these have not yet been confirmed experimentally. A measurement of the conductivity as a function of the magnetic field, as is done for Aharonov-Bohm rings, is not an option for these systems. The molecules are so small that the magnetic fields needed to influence the conductivity are very large (about 1000 Tesla). Researchers will therefore look for alternative methods to demonstrate quantum interference and how this affects the molecular conductivity. Measurements at room temperature and bypassing interference are important aspects of the work.
The programme has a strongly exploratory nature. Collaboration between chemists and theoretical and experimental physicists is vital. Demonstrating quantum interference effects is the first step in the research programme. As soon as it is known how quantum interference can be made visible then these quantum effects can also be used, for example, to make new molecules that can act as sensitive sensors.