Topological Quantum Computation Research Articles

Pressure-Dependent Superconductivity in Topological Dirac Semimetal SrCuBi.

The discovery of superconducting states in diverse topological materials generated a burgeoning interest to explore a topological superconductor and to realize a fault-tolerant topological quantum computation. A variety of routes to realize topological superconductors have been proposed and many types of topological materials have been developed. However, a pristine topological material with a natural superconducting state is relatively rare as compared to topological materials with artificially induced superconductivity. Here, we report that the planar honeycomb structured three-dimensional (3D) topological Dirac semimetal (TDS) SrCuBi, which is the Zintl phase, shows a natural surface superconductivity at 2.1 K under ambient pressure. It is clearly identified from theoretical calculations that a topologically non-trivial state exists on the (100) surface. Further, its superconducting transition temperature (Tc) increases by applying pressure, exhibiting a maximal Tc of 4.8 K under 6.2GPa. We believe that this discovery opens up a new possibility of exploring exotic Majorana fermions at the surface of 3D TDS superconductors. This article is protected by copyright. All rights reserved.

Resonant exchange of Chiral Majorana Fermions modulated by two parallel quantum dots.

Resonant exchange of the chiral Majorana fermions (MFs) that is coupled to two parallel Majorana zero
modes (MZMs) or two parallel quantum dots (QDs) is investigated. We find that, in the two QDs coupling
case, the resonant exchange for the chiral MFs is analogous to that in the MZM coupling case. We further
propose a circuit based on topological superconductor (TSC), which is formed by the proximity coupling of
a quantum anomalous Hall insulator (QAHI) and a s-wave superconductor, to observe the resonant exchange
of chiral MFs pairs. The numerical calculations show that the resonant transmission of the chiral MFs can be
adjusted by varying the coupling parameters. It is particularly noteworthy that, by only modulating the coupling
strength between the two QDs, the resonant exchange may be switched on or off. By adding another MZM, the
non-Abelian braidinglike operation can be realized. Therefore, our design scheme may provide another way for
non-Abelian braiding operation of MFs and the findings may have potential application value in the realization
of topological quantum computers.

Majorana subsystem qubit codes that also correct odd-weight errors

Abstract A potential platform for topological quantum computation is the Majorana-based tetron architecture. Its building blocks are superconducting islands called tetrons, which host four Majorana zero modes. Existing error correcting codes can correct even-weight errors on tetrons. In a previous proposal by us, we had shown that incorporating tetrons in the stabilizer group allows us to correct a combination of odd-weight errors and even-weight errors on tetrons. In this work, we show that inclusion of tetrons in the gauge group lets us create subsystem codes from conventional Pauli stabilizer codes, which can correct both kinds of errors. Compared to the previous approach, the current approach lets us construct codes with fewer stabilizer generators. This leads to shorter fault-tolerant sequence length, and improves the fault-tolerant pseudothreshold by as much as 84%.

Symmetry fractionalized (irrationalized) fusion rules and two domain-wall Verlinde formulae

We investigate the composite systems consisting of topological orders separated by gapped domain walls. We derive a pair of domain-wall Verlinde formulae, that elucidate the connection between the braiding of interdomain excitations labeled by pairs of anyons in different domains and quasiparticles in the gapped domain wall with their respective fusion rules. Through explicit non-Abelian examples, we showcase the calculation of such braiding and fusion, revealing that the fusion rules for interdomain excitations are generally fractional or irrational. By investigating the correspondence between composite systems and anyon condensation, we unveil the reason for designating these fusion rules as symmetry fractionalized (irrationalized) fusion rules. Our findings hold promise for applications across various fields, such as topological quantum computation, topological field theory, conformal field theory, and parton physics.

Bicolor loop models and their long range entanglement

Quantum loop models are well studied objects in the context of lattice gauge theories and topological quantum computing. They usually carry long range entanglement that is captured by the topological entanglement entropy. I consider generalization of the toric code model to bicolor loop models and show that the long range entanglement can be reflected in three different ways: a topologically invariant constant, a sub-leading logarithmic correction to the area law, or a modified bond dimension for the area-law term. The Hamiltonians are not exactly solvable for the whole spectra, but admit a tower of area-law exact excited states corresponding to the frustration free superposition of loop configurations with arbitrary pairs of localized vertex defects. The continuity of color along loops imposes kinetic constraints on the model and results in Hilbert space fragmentation, unless plaquette operators involving two neighboring faces are introduced to the Hamiltonian.

Majorana fermions and its application on topological quantum computer

The majorana fermion represents a kind of particle which is its own antiparticle. This paper aims to analyze the majorana fermions from theoretical aspect and application aspect, including the derivation of Dirac equation and the practicality of topological quantum computer. This paper explores the idea of Majorana fermions from Dirac equation and the anyons which is an important quasi-particle to build a topological quantum computer. After that, the potentials of quantum computer are emphasized as well. Several current major difficulties faced by building a quantum computer have also been discussed, including the decoherence of qubit and the errors during the operation of qubit. The advantages of topological computer are mentioned as well, especially the high resistance to local perturbation. The important property of two-dimensional non-abelian anyons has been discussed as well. Finally, the important relation of majorana fermions with anyons are introduced and the reason why majorana fermions is important has also been revealed.

Majorana fermions in a two-band two-dimensional Fermi system

Majorana fermions represent potential candidates for realizing fault-tolerant topological quantum computation. We obtain the exact mapping between a two-band hybridized model and a model with a topological superconducting state supporting Majorana fermions. The antisymmetric hybridization employed resembles that of spin–orbit coupling. We consider the situation where the model Hamiltonian comprises only inter-band s-wave pairing, i.e. the one in which the superconducting pairing is formed by one fermion from one of the bands and another fermion of opposite spin and momentum from the other band. From this we obtain an effective Hamiltonian of the px+ipy superconductor and calculate the topological invariant that distinguishes the weak and strong pairing phases. We also obtain the gap parameter and chemical potential of the Fermi system under spin–orbit coupling beyond the mean-field approximation. We show that the spin–orbit coupling substantially enhances the density of states at the Fermi surface.

Dynamics of Fricke–Painlevé VI Surfaces

The symmetries of a Riemann surface Σ∖{ai} with n punctures ai are encoded in its fundamental group π1(Σ). Further structure may be described through representations (homomorphisms) of π1 over a Lie group G as globalized by the character variety C=Hom(π1,G)/G. Guided by our previous work in the context of topological quantum computing (TQC) and genetics, we specialize on the four-punctured Riemann sphere Σ=S2(4) and the ‘space-time-spin’ group G=SL2(C). In such a situation, C possesses remarkable properties: (i) a representation is described by a three-dimensional cubic surface Va,b,c,d(x,y,z) with three variables and four parameters; (ii) the automorphisms of the surface satisfy the dynamical (non-linear and transcendental) Painlevé VI equation (or PVI); and (iii) there exists a finite set of 1 (Cayley–Picard)+3 (continuous platonic)+45 (icosahedral) solutions of PVI. In this paper, we feature the parametric representation of some solutions of PVI: (a) solutions corresponding to algebraic surfaces such as the Klein quartic and (b) icosahedral solutions. Applications to the character variety of finitely generated groups fp encountered in TQC or DNA/RNA sequences are proposed.

Majorana zero mode and its lattice construction in iron-based superconductors

Majorana zero modes (MZMs) obey non-Abelian braiding statistics. The braiding of MZMs can be used to construct the basic unit − topological qubit − of the topological quantum computation, which is immune to environmental noise and can achieve fault-tolerant quantum computation. The existing MZM platforms include hybrid structures such as topological insulator/superconductor, semiconducting nanowire/superconductor and 1d magnetic atomic chain/superconductor, and single materials such as 2M-WS<sub>2</sub>, 4Hb-TaS<sub>2</sub>, and iron-based superconductors (IBSs). The IBSs have advantages such as easy to fabricate, pure MZMs and high surviving temperatures of MZMs. Recently, a large-scale, ordered and tunable MZM lattice has been observed in LiFeAs, which provides a promising platform to future topological quantum computation. In this paper, first, we review the experimental observations of MZMs in IBSs, focusing on FeTe<sub>0.55</sub>Se<sub>0.45</sub>, (Li<sub>0.84</sub>Fe<sub>0.16</sub>)OHFeSe, CaKFe<sub>4</sub>As<sub>4</sub> and LiFeAs. Next, we introduce the critical experimental evidences of the MZMs. We also review the recent research work on the ordered and tunable MZM lattice in LiFeAs. Finally, we give conclusion and perspective on future Majorana research.

Fractional Quantum Hall State at Filling Factor ν=1/4 in Ultra-High-Quality GaAs Two-Dimensional Hole Systems.

Single-component fractional quantum Hall states (FQHSs) at even-denominator filling factors may host non-Abelian quasiparticles that are considered to be building blocks of topological quantum computers. Such states, however, are rarely observed in the lowest-energy Landau level, namely at filling factors ν<1. Here, we report evidence for an even-denominator FQHS at ν=1/4 in ultra-high-quality two-dimensional hole systems confined to modulation-doped GaAs quantum wells. We observe a deep minimum in the longitudinal resistance at ν=1/4, superimposed on a highly insulating background, suggesting a close competition between the ν=1/4 FQHS and the magnetic-field-induced, pinned Wigner solid states. Our experimental observations are consistent with the very recent theoretical calculations that predict that substantial Landau level mixing, caused by the large hole effective mass, can induce composite fermion pairing and lead to a non-Abelian FQHS at ν=1/4. Our results demonstrate that Landau level mixing can provide a very potent means for tuning the interaction between composite fermions and creating new non-Abelian FQHSs.

Next-generation even-denominator fractional quantum Hall states of interacting composite fermions.

The discovery of the fractional quantum Hall state (FQHS) in 1982 ushered a new era of research in many-body condensed matter physics. Among the numerous FQHSs, those observed at even-denominator Landau level filling factors are of particular interest as they may host quasiparticles obeying non-Abelian statistics and be of potential use in topological quantum computing. The even-denominator FQHSs, however, are scarce and have been observed predominantly in low-disorder two-dimensional (2D) systems when an excited electron Landau level is half filled. An example is the well-studied FQHS at filling factor [Formula: see text] 5/2 which is believed to be a Bardeen-Cooper-Schrieffer-type, paired state of flux-particle composite fermions (CFs). Here, we report the observation of even-denominator FQHSs at [Formula: see text] 3/10, 3/8, and 3/4 in the lowest Landau level of an ultrahigh-quality GaAs 2D hole system, evinced by deep minima in longitudinal resistance and developing quantized Hall plateaus. Quite remarkably, these states can be interpreted as even-denominator FQHSs of CFs, emerging from pairing of higher-order CFs when a CF Landau level, rather than an electron or a hole Landau level, is half-filled. Our results affirm enhanced interaction between CFs in a hole system with significant Landau level mixing and, more generally, the pairing of CFs as a valid mechanism for even-denominator FQHSs, and suggest the realization of FQHSs with non-Abelian anyons.

Topological magnetic line defects in Fe(Te, Se) high-temperature superconductors

The realization of Majorana zero modes in condensed matter have been attracting enormous interests from fundamental science such as topological quantum computation. Recently iron based superconductors were identified as a high-temperature platform for realizing topological superconductivity and Majorana modes. As unconventional superconductors, one of the most important characteristics of them is that they are in the vicinity of magnetic states due to the strong Hund’s coupling in iron atoms. Here we propose that the line defects with missing Te/Se anions in Fe(Se, Te) superconductors provide the realization of intrinsic antiferromagnetic (AFM) chains with Rashba spin-orbit coupling. Against conventional wisdom, Majorana zero modes (MZMs) can be robustly generated at these AFM chain ends. These results can consistently explain the recent experimental observation of zero-energy end states in line defects of monolayer Fe(Te, Se)/SrTiO3 by scanning tunneling microscopy (STM) measurements. Our research not only demonstrates an unprecedented interplay among native line defect, emergent magnetism and topological superconductivity but also explores a high-temperature platform for Majorana fermions.

Analysis of Majorana-based quantum computation

In recent years, the realization of quantum computation is a compelling research direction. To realize an error-tolerate quantum computer, topological quantum computation is considered as the most promising direction. Plenty of scholars have proved that non-abelian particles may play an important role on it. As a matter of fact, among particles obey non-abelian statistics, Majorana fermions is a typical example. On this basis, this study aims at introducing the development of Majorana-based quantum computation. To be specific, it will contain the conceptual framework of topological computation and introduce some recent research about Majorana-based quantum computation including the principles, configurations as well as the state-of-art analysis results. In addition, the current limitations for these studies will be demonstrated accordingly. Moreover, based on the summary the development of this field in recent years, this research will provide reference direction for future researchers. Overall, these results shed light on guiding further exploration of quantum computers.

The ZX-calculus as a language for topological quantum computation

Unitary fusion categories formalise the algebraic theory of topological quantum computation. These categories come naturally enriched in a subcategory of the category of Hilbert spaces, and by looking at this subcategory, one can identify a collection of generators for implementing quantum computation. We represent such generators for the Fibonacci and Ising models, namely the encoding of qubits and the associated braid group representations, with the ZX-calculus and show that in both cases, the Yang–Baxter equation is directly connected to an important rule in the complete ZX-calculus known as the P-rule, which enables one to interchange the phase gates defined with respect to complementary bases. In the Ising case, this reduces to a familiar rule relating two distinct Euler decompositions of the Hadamard gate as Z- and X-phase gates, whereas in the Fibonacci case, we give a previously unconsidered exact solution of the P-rule involving the Golden ratio. We demonstrate the utility of these representations by giving graphical derivations of the single-qubit braid equations for Fibonacci anyons and the single- and two-qubit braid equations for Ising anyons. We furthermore present a fully graphical procedure for simulating and simplifying braids with the ZX-representation of Fibonacci anyons.

Quantum computation at the edge of a disordered Kitaev honeycomb lattice

We analyze propagation of quantum information along chiral Majorana edge states in two-dimensional topological materials. The use of edge states may facilitate the braiding operation, an important ingredient in topological quantum computations. For the edge of the Kitaev honeycomb model in a topological phase, we discuss how the edge states can participate in quantum-information processing, and consider a two-qubit logic gate between distant external qubits coupled to the edge. Here we analyze the influence of disorder and noise on properties of the edge states and quantum-gate fidelity. We find that realistically weak disorder does not prevent one from implementation of a high-fidelity operation via the edge.

Experimental Simulation of Topological Quantum Computing with Classical Circuits

The key obstacle to the realization of a scalable quantum computer is overcoming environmental and control errors. Topological quantum computation attracts great attention because it emerges as one of the most promising approaches to solving these problems. Various theoretical schemes for building topological quantum computation have been proposed. However, experimental implementation has always been a great challenge because it has proved to be extremely difficult to create and manipulate topological qubits in real systems. Therefore, topological quantum computation has not been realized in experiments yet. Herein, the first experimental simulation of topological quantum computation with classical circuits is reported. Based on the proposed new scheme with circuits, not only Majorana‐like edge states are simulated experimentally, but also T junctions are constructed for simulating the braiding process. Furthermore, the feasibility of simulated topological quantum computing through a set of one‐ and two‐qubit unitary operations is demonstrated. Finally, the simulation of Grover's search algorithm demonstrates that simulated topological quantum computation is ideally suited for such tasks. The developed circuit‐based topological quantum‐computing simulator can provide important references for developing future topological quantum circuits.

Simulating the Hadamard gate in the Fibonacci disk code for universal topological quantum computation

Simulating the Hadamard gate in the Fibonacci disk code for universal topological quantum computation

Quantum double structure in cold atom superfluids

The theory of topological quantum computation is underpinned by two important classes of models. One is based on non-abelian Chern–Simons theory, which yields the so-called SU(2)k anyon models that often appear in the context of electrically charged quantum fluids. The physics of the other is captured by symmetry broken Yang–Mills theory in the absence of a Chern–Simons term and results in the so-called quantum double models. Extensive resources have been invested into the search for SU(2)k anyon quasi-particles, in particular, the so-called Ising anyons (k = 2) of which Majorana zero modes are believed to be an incarnation. In contrast to the SU(2)k models, quantum doubles have attracted little attention in experiments despite their pivotal role in the theory of error correction. Beyond topological error correcting codes, the appearance of quantum doubles has been limited to contexts primarily within mathematical physics, and as such, they are of seemingly little relevance for the study of experimentally tangible systems. However, recent works suggest that quantum double anyons may be found in spinor Bose–Einstein condensates. In light of this, the core purpose of this article is to provide a self-contained exposition of the quantum double structure, framed in the context of spinor condensates, by constructing explicitly the quantum doubles for various ground state symmetry groups and discuss their experimental realisability. We also derive analytically an equation for the quantum double Clebsch–Gordan coefficients from which the relevant braid matrices can be worked out. Finally, the existence of a particle-vortex duality is exposed and illuminated upon in this context.

Wiener–Hopf factorization approach to a bulk-boundary correspondence and stability conditions for topological zero-energy modes

Both the physics and applications of fermionic symmetry-protected topological phases rely heavily on a principle known as bulk-boundary correspondence, which predicts the emergence of protected boundary-localized energy excitations (boundary states) if the bulk is topologically non-trivial. Current theoretical approaches formulate a bulk-boundary correspondence as an equality between a bulk and a boundary topological invariant, where the latter is a property of boundary states. However, such an equality does not offer insight into the stability or the sensitivity of the boundary states to external perturbations. To solve this problem, we adopt a technique known as the Wiener–Hopf factorization of matrix functions. Using this technique, we first provide an elementary proof of the equality of the bulk and the boundary invariants for one-dimensional systems with arbitrary boundary conditions in all Altland–Zirnbauer symmetry classes. This equality also applies to quasi-one-dimensional systems (e.g., junctions) formed by bulks belonging to the same symmetry class. We then show that only topologically non-trivial Hamiltonians can host stable zero-energy edge modes, where stability refers to continuous deformation of zero-energy excitations with external perturbations that preserve the symmetries of the class. By leveraging the Wiener–Hopf factorization, we establish bounds on the sensitivity of such stable zero-energy modes to external perturbations. Our results show that the Wiener–Hopf factorization is a natural tool to investigate bulk-boundary correspondence in quasi-one-dimensional fermionic symmetry-protected topological phases. Our results on the stability and sensitivity of zero-energy modes are especially valuable for applications, including Majorana-based topological quantum computing.

Stencil Lithography‐Assisted Fabrication of Triangular Grid Pb Architecture

AbstractManipulating Majorana zero modes (MZMs) holds significant promise in propelling the field of topological quantum computation. While scanning tunneling microscopy (STM) exhibits remarkable capabilities in characterizing MZMs, additional geometrical constraints are imperative for achieving precise and controllable manipulation of these zero modes. To this end, it is of great help to integrate a micro mask within a molecular beam epitaxy (MBE) system to grow specific patterns containing MZMs. Here, a scenario for manipulating the MZMs in the vortex cores based on a triangular grid architecture is proposed. Additionally, a stencil lithography setup that is compatible with a combined MBE and STM system is introduced and experimentally demonstrated. A fine‐tuning of the Pb patterns, between a hexagonal antidot lattice to a triangular grid array, is achieved by adjusting the spatial gap between the stencil mask and the underlying substrate. This study provides new possibilities for the fabrication and investigation of Majorana devices.