Quantum computers promise to revolutionize technology, but first they must overcome decoherence: the loss of quantum information caused by environmental noise. Topological quantum computers aim to do this by storing information in protected states called Majorana modes, but identifying materials that can support these modes has proved tricky and sometimes controversial.
Researchers in the US and Ireland have now developed a method that could make it easier. Using a modified form of scanning tunnelling microscopy (STM) with a superconducting tip, they built a tool that maps subtle features of a material’s internal quantum state – an achievement that could reveal which materials contain the elusive Majorana modes.
Going on a Majorana hunt
Unlike regular particles, a Majorana particle is its own antiparticle. It is also, strictly speaking, hypothetical – at least in its fundamental form. “So far, no one has definitively found this particle,” says Séamus Davis of University College Cork, who co-led the research with Dung-Hai Lee of the University of California, Berkeley. However, Davis adds, “all serious theorists believe that it should exist in our universe”.
Majorana modes are a slightly different beast. Rather than being fundamental particles, they are quasiparticle excitations that exhibit Majorana-like properties, and theory predicts that they should exist on the edges or surfaces of certain superconducting materials. But not every superconductor can host these states. The material must be topological, meaning its electrons are arranged in a special, symmetry-protected way. And unlike in most conventional superconductors, where electrons pair up with their spins pointing in opposite directions, the paired electrons in these materials have their spins aligned.
To distinguish these characteristics experimentally, Davis, Lee and colleagues invented what Davis calls “a new type of quantum microscope”. This special version of STM uses a superconducting tip to probe the surface of another superconductor. When the tip and sample interact, they produce telltale signals of so-called Andreev bound states (ABSs), which are localized quantum states that arise at boundaries, impurities or interfaces within a material.
The new microscope does more than just detect these states, however. It also lets users tweak the coupling strength between tip and sample to see how the energy of the ABS changes. This is critical, as it helps researchers determine whether the superconductor is chiral, meaning that the movement of its electron pairs has a preferred direction that doesn’t change when time runs backward. This breaking of time-reversal symmetry is characteristic of Majorana surface states. Hence, if a certain material shows both ABSs and chirality, scientists know it’s the material they’re looking for: a so-called topological superconductor.
Gonna catch a big one?
To demonstrate the method, the team applied it to uranium ditelluride (UTe₂), a superconductor with the desired electron pairing that was previously considered a strong candidate for topological superconductivity. Alas, measurements with the new microscope showed that UTe₂ doesn’t fit the bill.
“If UTe2 superconductivity did break time reversal and sustain a chiral state, then we would have imaged Majoranas and proven it is a topological superconductor,” says Davis. “But UTe2 does not break that symmetry.”
Despite this disappointment, Steven Kivelson, a theoretical physicist at Stanford University in the US who was not involved in the research, says that studying UTe₂’s superconducting state could still be useful. “Searching for topological superconductors is interesting in its own right,” he says.
While some physicists are sceptical that topological superconductors will deliver on their quantum computing potential, citing years of ambiguous data and unfulfilled claims, that scepticism doesn’t necessarily translate to disinterest. Even if such materials never lead to a working quantum computer, Kivelson believes understanding them is still essential. “One doesn’t need these sexy buzzwords to justify the importance of this work,” he says.
According to Davis, the value of the team’s work lies in the tool it introduces. The Andreev STM method, especially when combined with tip tuning and quasiparticle interference imaging, allows researchers to identify topological superconductors definitively. The technique also offers something more commonly-used bulk techniques cannot achieve: a real-space, high-resolution view of the superconductor’s pairing symmetry, including node imaging and phase variation across the material’s surface.
The team is now using its method to survey other candidate materials, including UPt₃, which Davis describes as “the most likely one” to show the right properties. “If we find one which has Majoranas on the surface, that will open the door to applications,” he says.
The “strategic objective”, Davis adds, would be to get away from trying to create Majorana modes in engineered systems such as nanowires layered with superconductors, as companies such as Microsoft and Nokia are doing. Finding an intrinsic topological superconductor would, he suggests, be simpler.
The research is published in Science.
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