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arXiv:2503.17720 · Physical review. B./Physical review. B

Exchangeless braiding of Majorana zero modes in weakly coupled Kitaev chains

Robin Quade, Michael Potthoff

Published Jul 30, 2025• 2 citations
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Abstract

Exchangeless braiding of Majorana modes is studied in minimal networks of weakly hybridized Kitaev chains of finite length using a rigorous many-body framework. In particular, for two coupled chains it is shown that exchangeless braiding is achieved by $2\ensuremath{\pi}$ rotations of the phase $\ensuremath{\phi}$ of the superconducting order parameter of one of the chains. This braiding protocol is verified by the numerical calculation of the non-Abelian Wilczek-Zee phase of the low-energy many-body subspaces ${\mathcal{H}}_{0}(\ensuremath{\phi})$ based on the Bertsch-Robledo ground-state overlap formula. In the parameter space spanned by the total chain length, the strength of the weak hybridization connecting the chains, and the on-site potential, we identify two regions with different braiding outcomes, i.e., a projective ${\ensuremath{\sigma}}_{x}$ gate and a projective ${\ensuremath{\sigma}}_{z}$-gate phase. The transition between these phases is a continuous crossover, the location of which is reliably given by a simple four-Majorana mode model. This demonstrates the resilience of the anyonic properties of the Majorana modes against finite-size effects and weak links between Kitaev chains.

DOI
10.1103/n7bl-slgm

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Exchangeless braiding of Majorana modes is studied in minimal networks of weakly hybridized Kitaev chains of finite length using a rigorous many-body framework. In particular, for two coupled chains it is shown that exchangeless braiding is achieved by 2\ensuremathπ2\ensuremath{\pi}2\ensuremathπ rotations of the phase \ensuremathϕ\ensuremath{\phi}\ensuremathϕ of the superconducting order parameter of one of the chains. This braiding protocol is verified by the numerical calculation of the non-Abelian Wilczek-Zee phase of the low-energy many-body subspaces H0(\ensuremathϕ){\mathcal{H}}_{0}(\ensuremath{\phi})H0​(\ensuremathϕ) based on the Bertsch-Robledo ground-state overlap formula. In the parameter space spanned by the total chain length, the strength of the weak hybridization connecting the chains, and the on-site potential, we identify two regions with different braiding outcomes, i.e., a projective \ensuremathσx{\ensuremath{\sigma}}_{x}\ensuremathσx​ gate and a projective \ensuremathσz{\ensuremath{\sigma}}_{z}\ensuremathσz​-gate phase. The transition between these phases is a continuous crossover, the location of which is reliably given by a simple four-Majorana mode model. This demonstrates the resilience of the anyonic properties of the Majorana modes against finite-size effects and weak links between Kitaev chains.
We simulate the non-Abelian exchange of Majorana zero modes (MZMs) on a quantum computer. Rather than utilizing MZMs at the boundaries of quantum Ising chains, which are typically represented as nonlocal operators on a quantum computer, using a Kitaev lattice allows us to exploit a local representation of MZMs. We detail the protocol for braiding two and four MZMs in terms of a spin Hamiltonian, i.e. physical qubit Hamiltonian. Projecting this onto a subspace of states, we extract an effective Hamiltonian which drives a non-Abelian Berry phase. Using several approximations, we construct a set of gates which mimics this accumulation of non-Abelian phase and process this construction on a quantum computer. For two and four MZMs, we realize braiding fidelities of approximately 85 and 47%, respectively.
The realization and manipulation of Majorana zero modes have drawn significant attention for their crucial role in enabling topological quantum computation. Conventional approaches to the braiding of Majorana zero modes rely on adiabatic processes. In this work, using a composite 2-Kitaev-chain system accommodating Majorana zero modes as a working example, we propose a nonadiabatic and non-Abelian geometry phase-based protocol to execute operations on these Majorana zero modes. This is possible by locally coupling the edge sites of both quantum chains with an embedded lattice defect, successfully simulating the braiding operation of two Majorana modes in a highly nonadiabatic fashion. To further enhance the robustness against control imperfections, we apply a multiple-pulse composite strategy to our quantum chain setting for second-order protection of the braiding operations. Our proposal can also support the fast and robust realization of the $\ensuremath{\pi}/8$ gate, an essential ingredient for universal quantum computation. This work hence offers a potential pathway toward the nonadiabatic and fault-tolerant control of Majorana zero modes.
In a recent breakthrough experiment [Nature (London) 614, 445 (2023)], signatures of Majorana zero modes have been observed in tunnel spectroscopy for a minimal Kitaev chain constructed from coupled quantum dots. However, as Ising anyons, Majoranas' most fundamental property of non-Abelian statistics is yet to be detected. Moreover, the minimal Kitaev chain is qualitatively different from topological superconductors in that it supports Majoranas only at a sweet spot. Therefore, it is not obvious whether non-Abelian characteristics such as braiding and fusion can be demonstrated in this platform with a reasonable level of robustness. In this work, we theoretically propose a protocol for detecting the Majorana fusion rules in an artificial Kitaev chain consisting of four quantum dots. In contrast with the previous proposals for semiconductor-superconductor hybrid nanowire platforms, here we do not rely on mesoscopic superconducting islands, which are difficult to implement in quantum dot chains. To show the robustness of the fusion protocol, we discuss the effects of three types of realistic imperfections on the fusion outcomes, e.g., diabatic errors, dephasing errors, and calibration errors. We also propose a fermion parity readout scheme using quantum capacitance. Our work will shed light on future experiments on detecting the non-Abelian properties of Majorana modes in a quantum dot chain.
A quantum computer based on Majorana qubits would contain a large number of zero-energy Majorana states. This system can be modelled as a connected network of the Ising-Kitaev chains alternating the "trivial" and "topological" regions, with the zero-energy Majorana fermions localized at their interfaces. The low-energy sector of the theory describing such a network can be formulated in terms of leading-order couplings between the Majorana zero modes. I consider a minimal model exhibiting effective couplings between four Majorana zero modes - the nonuniform Ising-Kitaev chain, containing two "topological" regions separated by a "trivial" region. Solving the model exactly, I show that for generic values of the model parameters the four zero modes are localized at the four interface points of the chain. In the special case where additional inversion symmetry is present, the Majorana zero modes are "delocalized" between two interface points. In both cases, the low-energy sector of the theory can be formulated in terms of the localized Majorana fermions, but the couplings between some of them are independent of their respective separations: the exact solution does not support the "nearest-neighbor" form of the effective low-energy Hamiltonian.
A quantum computer based on Majorana qubits would contain a large number of zero-energy Majorana states. This system can be modelled as a connected network of the Ising-Kitaev chains alternating the “trivial” and “topological” regions, with the zero-energy Majorana fermions localized at their interfaces. The low-energy sector of the theory describing such a network can be formulated in terms of leading-order couplings between the Majorana zero modes. I consider a minimal model exhibiting effective couplings between four Majorana zero modes – the nonuniform Ising-Kitaev chain, containing two “topological” regions separated by a “trivial” region. Solving the model exactly, I show that for generic values of the model parameters the four zero modes are localized at the four interface points of the chain. In the special case where additional inversion symmetry is present, the Majorana zero modes are “delocalized” between two interface points. In both cases, the low-energy sector of the theory can be formulated in terms of the localized Majorana fermions, but the couplings between some of them are independent of their respective separations: the exact solution does not support the “nearest-neighbor” form of the effective low-energy Hamiltonian.
The topic of Majorana zero modes has been a focal point of the condensed matter physicscommunity in recent times due in part to their potential applications in quantum computation.Most notably, the exotic exchange statistics of Majorana zero modes form the basis oftopologically protected quantum gates. The physical exchange, or braiding, of Majorana zeromodes is often modeled in two-dimensional systems or by using networks of wires. Recentwork has established that Majorana zero modes can be braided in a purely one-dimensionalsetting with a periodic, or Floquet, drive. Specifically, the work of Bauer et al. in Ref.[1] demonstrates that a working braiding protocol can be simulated on a Kitaev chain withperiodically modulated parameters. This Floquet system gives rise to both Majorana zeromodes and Majorana pi modes, the latter of which serves as an additional degree of freedomin the braiding protocol.We reproduce the results of Ref. [1] through numerical simulation and extend theirbraiding protocol such that it includes both disorder and interactions. We demonstrate thatthe protocol remains successful when the disorder is weak compared to the parameters of thesystem. In the case of strong disorder, we demonstrate that the protocol fails. In addition,we show that the protocol is quite sensitive to the presence of interactions. We illustratethat interactions cause a severe delocalization of the Majorana zero modes leading to thefailure of the protocol