Fractionally charged quasiparticles, which obey non-abelian statistics, were predicted to exist in the ν=8/3, ν=5/2, and ν=7/3 fractional quantum Hall states (in the second Landau level). Here we present measurements of charge and neutral modes in these states. For both ν=7/3 and ν=8/3 states, we found a quasiparticle charge e=1/3 and an upstream neutral mode only in ν=8/3-excluding the possibility of non-abelian Read-Rezayi states and supporting Laughlin-like states. The absence of an upstream neutral mode in the ν=7/3 state also proves that edge reconstruction was not present in the ν=7/3 state, suggesting its absence also in ν=5/2 state, and thus may provide further support for the non-abelian anti-pfaffian nature of the ν=5/2 state.

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Upstream neutral modes, counter propagating to charge modes and carrying energy without net charge, had been predicted to exist in some of the fractional quantum Hall states and were recently observed via noise measurements. Understanding such modes will assist in identifying the wavefunction of these states, as well as shedding light on the role of Coulomb interactions within edge modes. Here, operating mainly in the n ¼ 2/3 state, we place a quantum dot a few micrometres upstream of an ohmic contact, which serves as a 'neutral modes source'. We show that the neutral modes heat the input of the dot, causing a net thermo-electric current to flow through it. Heating of the electrons leads to a decay of the neutral mode, manifested in the vanishing of the thermo-electric current at T4110 mK. This set-up provides a straightforward method to investigate upstream neutral modes without turning to the more cumbersome noise measurements.

This work was motivated by the quest for observing interference of fractionally charged quasi particles. Here, we study the behavior of an electronic Mach-Zehnder interferometer (MZI) at the integer quantum Hall effect (IQHE) regime at filling factors greater than one. Both the visibility and the drift velocity were measured, and found to be highly correlated as function of filling factor. As the filling factor approached unity, the visibility quenched, not to recover for filling factors smaller than unity. Alternatively, the velocity saturated around a minimal value at unity filling factor. We highlight the significant role interactions between the interfering edge and the bulk play, as well as that of the defining potential at the edge. Shot noise measurements suggest that phaseaveraging (due to phase randomization), rather than single particle decoherence, is likely to be the cause of the dephasing in the fractional regime.Interference of multiple electron trajectories is generally used to better understand coherent electron phenomena and their dephasing processes. Yet, the implementation of a high visibility electronic interferometer faces challenges, such as restricting the number of trajectories and achieving sufficiently long coherence and thermal lengths. Therefore, chiral edge channels in the quantum Hall effect (QHE) regime [1], being 1D-like channels running along the edge of a two dimensional electron gas (2DEG), are ideal as distinct trajectories in electron interferometers [2,3,4]. Based on these edge channels, the electronic Mach-Zehnder interferometer (MZI) is a true two-path interferometer, with the enclosed magnetic flux between the two paths controlling the Aharonov-Bohm (AB) phase [5,6]. Indeed, its practical success allowed innovative studies of coherence and dephasing [7,8,9], as well as studies of the nature of electron-electron interactions in the integer QHE regime [10,11].

Controlling the transmission of electrical current using a quantum point contact constriction paved a way to a large variety of experiments in mesoscopic physics. The increasing interest in heat transfer in such systems fosters questions about possible manipulations of quantum heat modes that do not carry net charge (neutral modes). Here we study the transmission of upstream neutral modes through a quantum point contact in fractional hole-conjugate quantum Hall states. Employing two different measurement techniques, we were able to render the relative spatial distribution of these chargeless modes with their charged counterparts. In these states, which were found to harbor more than one downstream charge mode, the upstream neutral modes are found to flow with the inner charge mode—as theoretically predicted. These results unveil a universal upstream heat current structure and open the path for more complex engineering of heat flows and cooling mechanisms in quantum nano-electronic devices.