Energy spectroscopy of controlled coupled quantum-wire states

Quantum ballistic transport in electron waveguides (EWGs)(1,2) is based on coherent quantum states arising from the one-dimensional (I D) confinement in nanometre-scale constrictions. Semiconductor EWGs have received considerable renewed interest for quantum logic devices(3-7) and theoretical concepts(8-11) in the context of solid-state quantum information processing(12). Implementation in real-world quantum circuits requires the unambiguous experimental distinction between all involved energy levels. However, such knowledge of EWGs investigated for wavefunction hybridization(13/15) is solely based on estimates. Here, we present coupled EWGs that allow single-mode control and manipulation of mode coupling at temperatures as high as that of liquid-helium (4.2 K) and above. We demonstrate high-resolution energy spectroscopy of each EWG subband ladder and the ID coupled states involved. The results verify the power of advanced nanolithography and its ability to open the door to the scalable semiconductor quantum circuits envisaged today.