  More than one hundred years ago Heinrich Hertz succeeded in transmitting
signals over a few meters to a receiving antenna using an electromagnetic
oscillator and thus proving the electromagnetic theory developed by James C.
Maxwell[1]. Since then, technology has developed, and today a variety of
oscillators is available at the quantum mechanical level. For quantized
electromagnetic oscillations atoms in cavities can be used to couple electric
fields[2, 3]. For mechanical oscillators realized, for example, with
cantilevers[4, 5] or vibrational modes of trapped atoms[6] or ions[7, 8], a
quantum mechanical link between two such oscillators has, to date, been
demonstrated in very few cases and has only been achieved in indirect ways.
Examples of this include the mechanical transport of atoms carrying the quantum
information[9] or the use of spontaneously emitted photons[10]. In this work,
direct coupling between the motional dipoles of separately trapped ions is
achieved over a distance of 54 {\mu}m, using the dipole-dipole interaction as a
quantum-mechanical transmission line[11]. This interaction is small between
single trapped ions, but the coupling is amplified by using additional trapped
ions as antennae. With three ions in each well the interaction is increased by
a factor of seven as compared to the singleion case. This enhancement
facilitates bridging of larger distances and relaxes the constraints on the
miniaturization of trap electrodes. This represents a new building block for
quantum computation and also offers new opportunities to couple quantum systems
of different natures.
