Strain Reshapes Materials to Reliably Find Elusive Quantum States

Scientists at the Indian Institute of Technology Mandi, collaborating with the Indian Institute of Technology Delhi, have developed a new method for manipulating and distinguishing between Majorana-bound states and partially separated Andreev bound states within superconductor-semiconductor heterostructures. The team utilise computational modelling to reveal how even slight application of strain can reshape the low-energy spectrum of nanowires and graphene nanoribbons, effectively tuning the system between trivial states, Andreev bound states, and topological Majorana-bound states. The findings offer a pathway to overcome a key obstacle in topological quantum computation, the difficulty in unambiguously identifying Majorana-bound states amidst mimicking trivial excitations, and provide an experimentally viable means of enhancing their stability in disordered systems.

Strain-induced control of Majorana bound state emergence in nanowires and graphene nanoribbons

A route to transform disorder-induced partially separated Andreev bound states (psABSs) into robust Majorana-bound states (MBSs) has been demonstrated, improving nonlocality by up to a factor of two in strained systems. Distinguishing between trivial and topological states previously presented a significant challenge due to their similar spectral signatures, hindering progress in utilising MBSs for quantum information processing. Now, spatially nonuniform strain provides systematic control over these low-energy excitations in both nanowires and graphene nanoribbons. This control offers a means to stabilise Majorana modes, which are crucial for realising topologically protected qubits, the fundamental building blocks of topological quantum computation. The inherent robustness of these qubits stems from the non-local nature of MBSs, making them less susceptible to decoherence caused by local environmental noise.

Analysis revealed that strain-enhanced nonlocality converts disorder-induced psABSs into well-defined MBSs, clearly demonstrating a pathway for improving the durability of these quantum states. Nonlocality, in this context, refers to the spatial separation of the Majorana modes, with greater separation leading to increased resilience against local perturbations. An analytical framework was also developed, linking strain to a position-dependent topological mass and strain-driven domain-wall motion, offering a real-space criterion for identifying stable Majorana modes. This analytical connection provides valuable insight into the underlying physics governing the strain-induced transformation and allows for predictive design of future devices. While simulations show a factor of two enhancement in nonlocality, they currently lack detailed modelling of realistic device fabrication imperfections, such as variations in nanowire width or interface roughness, potentially impacting the achievable coherence times required for scalable quantum computation. Achieving sufficiently long coherence times remains a major hurdle in the field.

In graphene nanoribbons, strain suppresses subband mixing and lifts degeneracies, stabilising boundary-localized modes. Subband mixing, a phenomenon where different energy bands within the graphene sheet interact, can lead to unwanted hybridisation and degradation of the Majorana modes. Reshaping the electronic spectrum and enhancing state spreading converts weaker signals into strong Majorana modes through this control. Symmetric strain, however, does not consistently yield the strong, isolated Majorana states needed for practical devices, as energy levels accumulate near zero, failing to establish a definitive gap protecting them from disruption and edge localisation proves unreliable. This accumulation of states near zero energy creates a pathway for unwanted transitions and reduces the topological protection afforded by the Majorana modes. Despite this, these findings reinforce the value of exploring strain as a control mechanism for quantum materials, offering a potentially versatile route to engineer desired quantum properties.

Strain-induced modification of electronic structure in superconducting hybrids

Tight-binding Bogoliubov, de Gennes simulations, a computational technique resembling construction with LEGO bricks, underpinned this work. This method allows researchers to discretise the electronic structure of a material, representing it as a lattice of interacting atoms, and then solve the Bogoliubov-de Gennes equations to determine the quasiparticle excitations. This approach is particularly well-suited for modelling complex materials where analytical solutions are intractable. Specifically, these simulations allowed detailed exploration of how strain alters the energy levels of electrons, particularly those forming low-energy states within superconducting hybrid systems. The simulations account for the interplay between the kinetic energy of the electrons, the superconducting pairing potential, the Rashba spin-orbit coupling, and the applied Zeeman field, all crucial factors in the emergence of Majorana-bound states.

Through virtual construction and manipulation of these materials, the subtle interplay between strain, superconductivity, and the emergence of Majorana-bound states was observed, bypassing the limitations of physical fabrication. The method was employed to investigate one-dimensional semiconductor nanowires and graphene nanoribbons exhibiting superconductivity, Rashba spin-orbit coupling, Zeeman fields, and disorder. The Rashba effect, arising from spin-orbit coupling, is essential for creating the necessary conditions for topological superconductivity. Simulations focused on manipulating strain to control low-energy states, without specifying sample sizes or temperatures. The absence of specific temperature parameters highlights the focus on understanding the fundamental physics rather than predicting behaviour at a particular operating point. Further research will need to incorporate temperature effects to assess the stability of the Majorana modes at realistic operating conditions.

Strain’s influence on Majorana-bound states reveals limitations for quantum computation

Controlling quantum states remains a central challenge in developing future technologies. Carefully applied strain offers a pathway to reshape the electronic properties of materials used to create Majorana-bound states, elusive particles considered vital for stable quantum computers. However, simulations reveal a key limitation: symmetric strain alone does not consistently deliver the strong, isolated Majorana states needed for practical devices. Spatially varying strain provides a new method for managing low-energy states within superconducting materials, specifically nanowires and graphene nanoribbons, systematically controlling and interconverting trivial states, psABSs, and topological MBSs, which are essential for building stable quantum computers. The ability to interconvert these states is particularly significant, as it allows for the suppression of unwanted psABSs and the enhancement of the desired MBSs.

The research highlights the importance of precise strain engineering to achieve robust Majorana modes. While the simulations demonstrate the potential of strain as a control parameter, realising this potential in actual devices will require overcoming significant technological challenges, including the development of methods for applying and maintaining spatially nonuniform strain with high precision. Furthermore, the impact of material imperfections and environmental noise on the coherence of the Majorana modes needs to be carefully considered. Despite these challenges, the findings represent a significant step forward in the pursuit of topological quantum computation, offering a promising avenue for creating more stable and reliable quantum computers.

Researchers demonstrated that spatially nonuniform strain can systematically control low-energy states in semiconductor nanowires and graphene nanoribbons with superconductivity. This control is important because it allows for the conversion of unwanted, trivial states into well-separated and robust Majorana-bound states, which are considered essential for building stable quantum computers. The simulations identified regimes where strain enhanced the nonlocality of these states, effectively suppressing problematic excitations. The authors developed an analytical framework to explain these crossovers and further understand the underlying physical mechanisms.