&Bullet; physics 14, 39
Control over eddies that arise in magnet-superconductor heterostructures could result in qubits that are immune to the effects of their surroundings.
Quantum computers can solve difficult numerical problems much faster than conventional computers, but their implementation is made more difficult by errors that result from the interaction of the quantum system with its environment. A promising approach to developing fault-tolerant quantum computers that are resistant to errors due to interactions with the environment is the concept of topological quantum computing. This is where operations are carried out on anyons – quasi-particles whose collective states are largely immune to environmental influences. However, the creation of these quasiparticles in materials has proven difficult. Now, Alexander Petrović from Nanyang Technical University, Singapore, and colleagues have made a 2D structure that, according to models, could accommodate anyone  .
Anyons occur in two dimensions and are different from other particles such as electrons and photons. In particular, anyons can obey non-Abelian statistics, which means that if you take a few anyons and swap their positions, the final state of the system depends on the order in which each swap is made  . This path dependency could be used to encode information in a form that is robust to localized interference, which is why non-Abelian anyons are an attractive choice for qubits.
Quasi-particles that obey non-Abelian statistics were first observed in materials with the fractional quantum Hall effect, which occurs in 2D electron gases exposed to strong magnetic fields. Unfortunately, these materials are impractical for generic quantum computations  . Another system that could accommodate non-Abelian quasiparticles like anyons is an unconventional 2D systemp-Wave superconductor  . These superconductors differ from conventional onesso-Wave superconductors in such a way that the spins are aligned in the Cooper pairs that carry the current. Physicists expect that the components of an anyon – known as Majorana null modes – should appear in p-Wave superconductors in the center of eddies that can form in the material. But these superconductors have proven difficult to implement.
Theoretically is a so-Wave superconductors can be used in a. being transformed p-Wave superconductor by coupling a conventional one so-Wave superconductor with a semiconducting nanowire that exhibits significant spin-orbit interaction in the presence of a moderate magnetic field. Some experimental evidence suggests that this leads to Majorana null modes at both ends of the nanowire  . The coupling of theso-Wave superconductors to a spatially rotating magnetic field could be a p-Wave superconductors and Majorana null modes without the need for strong spin-orbit interaction  .
Building on the concept of a rotating magnetic field, one fascinating idea in creating a Majorana null mode hosting platform is to couple a vortex in a traditional superconductor with a magnetic vortex called a skyrmion  . In this magnetic structure (Fig. 1) the spins have an inclination that depends on their distance from the skyrmion center and the “number of turns” that describes the magnetic topology  . Most materials that have skyrmions contain magnetic layers of iron (Fe) or cobalt (Co) stacked with layers of heavy transition metals like platinum (Pt) or iridium (Ir) that provide the strong spin-orbit coupling, those responsible for the formation of this magnetic structure.
Much theoretical work discusses the possibility of generating the desired skyrmion-vortex coupling in heterostructures that combine magnetic and superconducting layers. However, experimental studies of these systems are still rare. Both parts of the heterostructure must remain within certain temperature and magnetic field strength ranges in order to realize the desired topological phase, and the length scales of skyrmions and eddies must be similar in order to study their coupling. In recent years, advances have been made in controlling the temperature and radius of skyrmions with moderate magnetic fields  .
Now Petrović and his colleagues have developed a heterostructure that achieves stable skyrmion-vortex coexistence with control over the coupling between these two topological stimuli. The researchers are experimentally studying the interactions between a magnetic [Ir/FeCo/Pt] Sandwich structure that can accommodate skyrmions and a superconducting niobium (Nb) layer that is either in direct contact with the magnet or separated by a thin insulating MgO layer  . A small applied magnetic field germinates in the sandwich-structure skyrmions with spins rotating in the radial plane from the core to the periphery. Scattering magnetic fields of the skyrmions induce eddies in the Nb film, which were observed with scanning tunneling spectroscopy.
The researchers explain the mechanism of skyrmion vortex induction using an effect observed when a two-dimensional electron gas is spin polarized at an interface. This spin polarization leads to a spin accumulation and, due to the spin-orbit coupling, to a charge current at the interface. Whether the spin polarization is induced by the exchange coupling in direct contact between the magnetic and superconducting layer or only by the stray magnetic field can be regulated by the presence or absence of the intervening insulator layer. Since the temperature changes the ratio of the superconductor vortex radius to the Skyrmion radius (in each case by 50 nm), the coupling strength can be adjusted.
Magnet-superconductor heterostructures provide a fascinating playground for the generation of topological phases that house exotic quasiparticles in a thin-film structure, and this demonstration of skyrmion-vortex coupling is an important step in the generation of Majorana null modes. However, it is still a challenge to separate them spatially and energetically from other states so that they can be observed directly. The next step to do this is to create skyrmions of various shapes  or with a magnetic order, which is described by different numbers of windings  . This step requires fine-tuning of the magnetic interactions and careful selection of magnetic materials, but should, in conjunction with conventional superconductors, lead to increased stability of the Majorana null modes. We can expect more exciting discoveries on the way to topological quantum computing.
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