Electrical currents in a quantum spin Hall insulator are confined to the boundary of the system. The charge carriers can be described as massless relativistic particles, whose spin and momentum are coupled to each other. While the helical character of those states is by now well established experimentally, it is a fundamental open question how those edge states interact with each other when brought in spatial proximity. We employ a topological quantum point contact to guide edge channels from opposite sides into a quasi-onedimensional constriction, based on inverted HgTe quantum wells. Apart from the expected quantization in integer steps of 2e 2 /h, we find a surprising additional plateau at e 2 /h. We explain our observation by combining band structure calculations and repulsive electron-electron interaction effects captured within the Tomonaga-Luttinger liquid model. The present results may have direct implications for the study of one-dimensional helical electron quantum optics, Majorana-and potentially para-fermions. The quantum spin Hall effect has been predicted in several systems [1][2][3][4] and was first realized in HgCdTe/HgTe quantum wells [5]. Later, this phase was observed in other material systems such as InAs/GaSb double quantum wells [6] and in monolayers of WTe 2 and bismuthene [7,8]. The defining properties of this state, related to its helical nature, are well established by numerous experiments such as the observation of conductance quantization of two spin polarized edge channels G 0 = 2e 2 /h with e the electron charge and h the Planck's constant [5]. Additionally, non-local edge transport and spin-polarization of the edge channels were demonstrated by suitable transport experiments [9,10]. We instead target a still open question, namely how helical edge states interact with each other.A quantum point contact (QPC) can be used to guide * All three authors contributed equally to this work, email: Jonas.Strunz@physik.uni-wuerzburg.de edge channels from opposite boundaries of the sample into a constriction. Such a device allows for studies of charge and spin transfer mechanisms by, e.g., adjusting the overlap of the edge states [11][12][13][14][15][16][17][18][19][20]. Besides the general interest in the study of transport processes in such a device, the appropriate model to describe the essential physics and to capture interaction effects of helical edge states is still unclear. The one-dimensionality of the helical edge modes suggests a description in terms of the Tomonaga-Luttinger liquid when electron-electron interactions are taken into account. In this respect, the QPC setup provides an illuminating platform as it may give rise to particular backscattering processes.We present the realization of a QPC based on HgTe quantum wells as evidenced by the observation of the expected conductance steps in integer values of G 0 . The newly developed lithographic process allows the fabrication of sophisticated nanostructures based on topological materials without lowering the material quality. It t...

In the version of this Letter originally published online, in the Methods subsection 'Explicit expressions for correlators' , the citation of equation ( 9a) was incorrect; it should have been a citation of equation (8a). Also, in equation ( 8c), on the right-hand side the final term was given as +2O a O b O c ; it should have been −O a O b O c . In equation (8d), the final seven terms on the right-hand side were given as −6O a O b O cAll versions of the Letter have been amended.