The errors that arise from the volatile nature of quantum technologies are a major roadblock on the path to practical quantum computing. We can imagine getting past this blockade by driving straight through it, using a car built to withstand the impact: this is quantum error correction. Alternatively, we might try to drive around the obstacle, bypassing the original problem entirely. To that end, researchers are investigating Majorana fermions – curious quantum objects that are their own antiparticles and are thought to be naturally resilient to quantum errors. So far, however, these quantum objects have proven difficult to create and control.
Researchers at the University of Maryland, US have now identified a more experimentally feasible way to generate Majorana fermions, potentially paving the way for Majorana-based quantum computation. In a paper published in Physical Review Letters, they show that a simple physical system can serve as a flexible platform for observing and manipulating these particles. The platform’s utility derives from its simplicity, says Ruixing Zhang, a postdoctoral researcher at Maryland and lead author of the study. “We don’t have to create additional structures. Nature gives us everything we need,” he says.
Majorana modes pose major challenges
Majorana fermions are not single particles like the electron or the photon. Instead, they are a template for a certain type of particle. After the Italian physicist Ettore Majorana predicted their existence in 1937, physicists hoped that some elementary particles might fit this mould, but subsequent experiments ruled this out for all known particles except the neutrino.
More recently, the Majorana fermion has taken on new life in the confines of ultracold quantum systems. In this context, Majorana fermions can manifest as collective oscillations of electrons. These electronic undulations are called quasiparticles because they behave in many ways like elementary particles, but emerge from the intricate interplay of many particles. Majorana fermions of this type live on the edges of their host materials and are the starting point for generating so-called Majorana zero modes (MZMs), which have zero energy and are further localized as point objects. The MZMs, in turn, can be used to build naturally error-resistant qubits.
Majorana modes are, however, notoriously elusive. In part, this is because it is hard to create the conditions required to generate them in an experimental setting. Many theoretical proposals have predicted MZMs should be present in quasi-2D materials, which consist of a small number of 2D layers stacked on top of each other. However, all previous proposals required heterostructures – that is, structures where the stacked layers have differing material composition and structure. Practically, these heterostructures are difficult if not downright impossible to grow.
To make matters worse, Majorana modes can only be observed indirectly. Like detectives trying to catch a culprit with only circumstantial evidence, physicists have a hard time ruling out alternative explanations for the phenomena they observe. This has led to high-profile premature claims of Majorana discovery, including Microsoft Quantum Lab’s recent retraction of a Nature paper in which they purported to observe MZMs in nanowires.
Ironing out problems
In their new work, Zhang and his coauthor show that Majorana modes should be present in a much simpler setting: thin films of an iron-based superconducting material. Like previous proposals, the system they study is quasi-2D, but crucially all layers are of the same kind. The iron-based thin films naturally accommodate Majorana fermions that are helical – left or right-handed – and move along the edges of the system in their preferred direction. This is due to a special “time-reversal” symmetry, wherein interchanging the left-moving and right-moving quasiparticles makes it look like time is propagating backwards in the system.
With these thin films, making MZMs from helical Majorana fermions is relatively simple. When a magnetic field is applied to the system, the Majorana modes shift from being spread out around the edges of the system to localizing in its corners. Rotating the magnetic field has the effect of transporting each Majorana mode from one corner to another. This magnetic knob can be used to “braid” the Majoranas, which is the cornerstone for logic gates – controlled operations required to perform computation – in topological quantum computers.
At its core, Zhang’s analysis has real-world applications in mind. The thin films he studies can be grown one layer at a time using a technique known as epitaxy, and all of the essential ingredients that are mixed together to produce helical Majorana modes have been previously realized and observed experimentally. Zhang’s work also shows that an electric field, which is easy to apply experimentally, can serve as a “topological switch” for controlling the emergent quasiparticles.
Retraction of Nature paper puts Majorana research on a new path
What’s more, the researchers also propose a new “smoking gun” for confirming the presence of MZMs based on this corner localization. Traditional techniques, which involve analyzing the material’s transport properties, are experimentally challenging and have trouble disqualifying alternative explanations. Zhang’s new method, which centers around measuring the particle density across the thin film, is easier to implement, and facilitates catching the slippery suspects.
The road ahead
The path to large-scale quantum computing is protracted and precarious, but Zhang believes his work shows that it might be more feasible than previously thought to build a quantum computer out of Majorana modes – something that could help overcome the significant issue of quantum errors. “The first step is establishing the possibilities,” he says. “Next, we need to create a blueprint.”
