  Quantum interference can profoundly affect charge transport in single
molecules, but experiments can usually measure only the conductance at the
Fermi energy. Because in general the most pronounced features of the quantum
interference are not located at the Fermi energy, it is highly desirable to
probe charge transport in a broader energy range. Here by the method of
electrochemical gating, we measure the conductance and map the transmission
functions of single molecules at and around the Fermi energy, and study
signatures associated with constructive and destructive interference. With the
electrochemical gate control, we tune the quantum interference between the
highest occupied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO), and directly observe anti-resonance, a distinct feature of
destructive interference. By tuning the molecule in and out of anti-resonance,
we achieve continuous control of the conductance over 2 orders of magnitude
with a subthreshold swing of ~17 mV/dec, features relevant to high-speed and
low-power electronics.
