Interferometric and Noise Signatures of Majorana Fermion Edge States in Transport Experiments

Domain walls between superconducting and magnetic regions placed on top of a topological insulator support transport channels for Majorana fermions. We propose to study noise correlations in a Hanbury Brown–Twiss type interferometer and find three signatures of the Majorana nature of the channels. First, the average charge current in the outgoing leads vanishes. Furthermore, we predict an anomalously large shot noise in the output ports for a vanishing average current signal. Adding a quantum point contact to the setup, we find a surprising absence of partition noise which can be traced back to the Majorana nature of the carriers.

Figure 1

Hanbury Brown–Twiss type interferometer. A grounded superconductor surrounded by four magnetic domains is placed on the surface of a three-dimensional topological insulator. The terminals 1, 2, 3, and 4 are connected to the outside circuit and biased at potentials $V_1,\dots ,V_4$ respectively. The magnetizations are chosen such that Dirac electron states at leads 1, 2, 3, and 4 exist and propagate in the direction of the double arrows. Electrons and holes can enter the interferometer at leads 1 and 3, Majorana fermions propagate along the arms $A$, $B$, $C$, and $D$ in the direction of the single arrows and electrons and holes leave through leads 2 and 4. A magnetic flux in the form of $n_v$ vortices threading the superconductor will control the phase difference between the arms of the interferometer.

Figure 2

(a) Modified Hanbury Brown–Twiss interferometer. Majorana excitations will propagate along the boundaries of the two triangular superconducting structures with phases ${\phi }_1$, ${\phi }_2$. An additional short gapped channel appears at the domain wall between the two superconducting regions, forming a quantum point contact characterized by reflection and transmission amplitudes $r$, $t$. The setup is similar to the one proposed in Ref. 7. (b) In the fully transmissive case, $t=1$, $r=0$, the device splits into two separate Mach-Zehnder interferometers, one of which is shown here.