Abstract: Described is a semiconductor device comprising a substrate having a surface; a mesa arranged on the surface of the substrate, the mesa having a perimeter; and one or more gate electrodes. The mesa is obtainable by selective area growth, and comprises a semiconductor heterostructure for hosting a 2-dimensional electron gas or a 2-dimensional hole gas. The one or more gate electrodes are configured to deplete electrically portions of the semiconductor heterostructure to define a boundary of an active region of the semiconductor heterostructure, the boundary being spaced from the perimeter of the mesa. By using a selective-area-grown mesa and defining the boundary of the active region electrostatically, improved electronic properties may be obtained, for example by avoiding the diffuse scattering of charge carriers. Also provided is a method for fabricating the device, and a use of one or more gate electrodes to define an active region of a semiconductor component.

Abstract: A method to evaluate a semiconductor-superconductor heterojunction for use in a qubit register of a topological quantum computer includes (a) measuring one or both of a radio-frequency (RF) junction admittance of the semiconductor-superconductor heterojunction and a sub-RF conductance including a non-local conductance of the semiconductor-superconductor heterojunction, to obtain mapping data and refinement data; (b) finding by analysis of the mapping data one or more regions of a parameter space consistent with an unbroken topological phase of the semiconductor-superconductor heterojunction; and (c) finding by analysis of the refinement data a boundary of the unbroken topological phase in the parameter space and a topological gap of the semiconductor-superconductor heterojunction for at least one of the one or more regions of the parameter space.

Abstract: A method of fabricating a device, comprising forming portions of electronic circuitry and a shadow wall structure over a substrate, and subsequently depositing a conducting layer over the substrate by angled deposition of a conducting material in at least a first deposition direction at an acute angle relative to the plane of the substrate. The shadow wall structure is arranged to cast a shadow in the deposition, leaving areas where the conducting material is not deposited. The shadow wall structure comprises one or more gaps each shorter than a shadow length of a respective part of the shadow wall structure casting the shadow into the gap, to prevent the conducting material forming in the gaps and to thereby create regions of said upper conducting layer that are electrically isolated from one another. These are arranged to form conducting elements for applying signals to, and/or receiving signals from, the electronic circuitry.

Abstract: A method to evaluate a semiconductor-superconductor heterojunction for use in a qubit register of a topological quantum computer includes (a) measuring one or both of a radio-frequency (RF) junction admittance of the semiconductor-superconductor heterojunction and a sub-RF conductance including a non-local conductance of the semiconductor-superconductor heterojunction, to obtain mapping data and refinement data; (b) finding by analysis of the mapping data one or more regions of a parameter space consistent with an unbroken topological phase of the semiconductor-superconductor heterojunction; and (c) finding by analysis of the refinement data a boundary of the unbroken topological phase in the parameter space and a topological gap of the semiconductor-superconductor heterojunction for at least one of the one or more regions of the parameter space.

Abstract: A method of fabricating a device, comprising forming portions of electronic circuitry and a shadow wall structure over a substrate, and subsequently depositing a conducting layer over the substrate by angled deposition of a conducting material in at least a first deposition direction at an acute angle relative to the plane of the substrate. The shadow wall structure is arranged to cast a shadow in the deposition, leaving areas where the conducting material is not deposited. The shadow wall structure comprises one or more gaps each shorter than a shadow length of a respective part of the shadow wall structure casting the shadow into the gap, to prevent the conducting material forming in the gaps and to thereby create regions of said upper conducting layer that are electrically isolated from one another. These are arranged to form conducting elements for applying signals to, and/or receiving signals from, the electronic circuitry.

Abstract: A method to evaluate a semiconductor-superconductor heterojunction for use in a qubit register of a topological quantum computer includes measuring a radio-frequency (RF) junction admittance of the semiconductor-superconductor heterojunction to obtain mapping data; finding by analysis of the mapping data one or more regions of a parameter space consistent with an unbroken topological phase of the semiconductor-superconductor heterojunction; measuring a sub-RF conductance including a non-local conductance of the semiconductor-superconductor heterojunction in each of the one or more regions of the parameter space, to obtain refinement data; and finding by analysis of the refinement data a boundary of the unbroken topological phase in the parameter space and a topological gap of the semiconductor-superconductor heterojunction for at least one of the one or more regions of the parameter space.

Abstract: A method to evaluate a semiconductor-superconductor heterojunction for use in a qubit register of a topological quantum computer includes measuring a radio-frequency (RF) junction admittance of the semiconductor-superconductor heterojunction to obtain mapping data; finding by analysis of the mapping data one or more regions of a parameter space consistent with an unbroken topological phase of the semiconductor-superconductor heterojunction; measuring a sub-RF conductance including a non-local conductance of the semiconductor-superconductor heterojunction in each of the one or more regions of the parameter space, to obtain refinement data; and finding by analysis of the refinement data a boundary of the unbroken topological phase in the parameter space and a topological gap of the semiconductor-superconductor heterojunction for at least one of the one or more regions of the parameter space.

Abstract: A method for producing a planar free surface comprising embedded, contactable nanostructures includes arranging at least one nanostructure on a surface of an initial substrate; applying a first layer to the surface of the initial substrate, wherein the first layer embeds the at least one nanostructure; applying a target substrate to the first layer; and separating the initial substrate from the first layer such that the at least one nanostructure embedded in the first layer has a planar free surface. An additional layer is applied to the surface of the initial substrate before the at least one nanostructure is applied to the initial substrate, and in that the initial substrate is removed from the first layer using a solvent.

Abstract: A method for producing a planar free surface comprising embedded, contactable nanostructures includes arranging at least one nanostructure on a surface of an initial substrate; applying a first layer to the surface of the initial substrate, wherein the first layer embeds the at least one nanostructure; applying a target substrate to the first layer; and separating the initial substrate from the first layer such that the at least one nanostructure embedded in the first layer has a planar free surface. An additional layer is applied to the surface of the initial substrate before the at least one nanostructure is applied to the initial substrate, and in that the initial substrate is removed from the first layer using a solvent.