A two-dimensional electronic system (2DES) under strong magnetic fields manifests quantum Hall effects (QHE), which accommodate current-carrying edge modes at the sample's periphery.  These edge modes are one of the ideal arenas to study the properties of chiral one-dimensional electronic systems.  In the so-called integer QHEs, for a certain magnetic field range near filling factor 2, there are two co-propagating electron channels (modes 1 & 2).  A recent prediction by Berg et al. suggested that the two channels, as they flow in extreme proximity, interact coulombically, giving rise to two new eigen-modes (instead of the non-interacting modes 1 & 2): a charge mode and a neutral mode, both traveling in the same chirality ("downstream"), however, at different velocities (the charge mode faster than the neutral mode). It is instructive to view the neutral mode as a dipole-like excitation carrying energy without net charge.  Because of its chargeless nature, simple conductance measurements have been ineffective for its electrical detection. However, impinging the dipole-like excitation on a narrow constriction (QPC - quantum point contact) was found to break stochastically  the dipoles into particle-hole excitations, leading to shot noise without net current. Only recently, similar but "upstream" neutral modes were detected in unique fractional QHE states also via shot noise measurements.  

In the present talk, we report the observation of a "downstream" chiral neutral mode via sensitive shot noise measurement in a 2DES embedded in a GaAs/AlGaAs heterostructure.  Since the charge and neutral modes are expected to flow in the same direction, their spatial separation is a key to our observation. Our strategy consisted of three steps: (1) Inject electrons via a QPC into mode 1 alone (while mode 2 remains cold), in order to excite the charge and the neutral modes; (2) Let the two new modes propagate in order to allow spatial separation; (3) Detect shot noise that is being generated by another QPC that partitions the cold mode 2. By ensuring that no inter-channel tunneling takes place in the step 2, we found severe shot noise, proportional to the injected current into mode 1, without a net current reaching the detector. Our result exemplifies a finite shot noise without net current observed in interacting chiral one-dimensional modes.