Since the breakthrough discovery of the Majorana particle in 2012 in Delft, researchers faced great challenges. The group of professor Leo Kouwenhoven at QuTech and Microsoft collaborated with theorists and material scientists of various institutes to understand the next steps required to improve the experiments. Now, the scientists provide a definite proof for Majorana existence paving the way towards Majorana quantum bits.
Majorana quasiparticles appear in materials in extremely restricted conditions. When a nanowire made from a semiconductor is connected to a superconductive material, researchers see a so-called zero-bias peak in the case of certain electric and magnetic fields. This signal is the main characteristic of the presence of Majoranas.
In the first experiment of 2012, the zero-bias peak was noisy, and difficult to see. This made the Majorana appearance debatable. In the years that followed, researchers worked very hard on improving the theory, materials and the experimental fabrications. The past months multiple breakthroughs followed each other. Transport in the required materials is improved in two steps: high-quality interfaces and superclean Majorana transport. Furthermore, the design of nano-hashtags allowed for future exchange of Majorana particles, the final step required for topological quantum computing.
Now the researchers in Delft combine all improvements in an experiment to show the quantized conductance of the zero-bias peak. This perfect quantization of the Majorana conductance is the final proof of the existence of the Majorana’s. Zhang: ‘it is a direct consequence of the particle-antiparticle property.’
They have observed perfectly quantized 2e2/h conductance at zero energy, the long sought signature of the Majorana states. This quantization implies that in this latest generation of semiconducting-superconducting devices zero-energy states exhibit perfect electron-hole symmetry and thus allow for perfect Andreev reflection. These remarkable results may finally end the debate and convince most of the skeptics out there.
Perfect quantum computer
This experiment closes a chapter in the quest for Majorana particles, and opens a new chapter to work towards quantum information processing based on their properties. Their unique physical characteristics make the Majorana particles much more stable than the majority of other qubits. Making and regulating these Majoranas on the way to creating this topological quantum computer is still challenging. The level of control and understanding that is reached now, allows for the exploration of Majorana quantum computing. The researchers now aim to combine the previous breakthroughs in one experiment to realize a qubit based on four Majorana particles.
‘For that, we need to scale up to more complicated networks such as the nano-hashtags, ‘Hao Zhang explains, ‘and then we finally have a qubit that is protected by its own topology.’
Majorana zero-modes—a type of localized quasiparticle—hold great promise for topological quantum computing. Tunnelling spectroscopy in electrical transport is the primary tool for identifying the presence of Majorana zero-modes, for instance as a zero-bias peak in differential conductance. The height of the Majorana zero-bias peak is predicted to be quantized at the universal conductance value of 2e2/h at zero temperature (where e is the charge of an electron and h is the Planck constant), as a direct consequence of the famous Majorana symmetry in which a particle is its own antiparticle. The Majorana symmetry protects the quantization against disorder, interactions and variations in the tunnel coupling. Previous experiments, however, have mostly shown zero-bias peaks much smaller than 2e2/h, with a recent observation of a peak height close to 2e2/h. Here we report a quantized conductance plateau at 2e2/h in the zero-bias conductance measured in indium antimonide semiconductor nanowires covered with an aluminium superconducting shell. The height of our zero-bias peak remains constant despite changing parameters such as the magnetic field and tunnel coupling, indicating that it is a quantized conductance plateau. We distinguish this quantized Majorana peak from possible non-Majorana origins by investigating its robustness to electric and magnetic fields as well as its temperature dependence. The observation of a quantized conductance plateau strongly supports the existence of Majorana zero-modes in the system, consequently paving the way for future braiding experiments that could lead to topological quantum computing.