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Nonplanar qubit with tunable gauge symmetry
Authors:
Muqing Yu,
Han Bi,
Hengli Lo,
Vishvesha Sridhar,
Guilherme Delfino,
Dmitry Green,
Claudio Chamon,
Nadya Mason,
Andrew P. Higginbotham
Abstract:
Circuit quantum electrodynamics embeds Josephson junction qubits within superconducting cavities, and has emerged as a leading approach to quantum computing and quantum simulation. Despite the many permutations of circuit geometry that have been explored, Josephson connectivities have so far been planar, making them effectively low-dimensional. Here we show that a non-planar qubit -- a $3\times3$…
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Circuit quantum electrodynamics embeds Josephson junction qubits within superconducting cavities, and has emerged as a leading approach to quantum computing and quantum simulation. Despite the many permutations of circuit geometry that have been explored, Josephson connectivities have so far been planar, making them effectively low-dimensional. Here we show that a non-planar qubit -- a $3\times3$ crossbar Josephson array -- gives rise to flux-tunable $\mathbb{Z}_3$ combinatorial gauge symmetry (CGS), potentially enabling spin-liquid behavior when networked into a lattice. The observed excitation spectrum shows excellent agreement with predictions from a neural network trained to generate variational quantum states, demonstrating that we have predictive power over our high-dimensional quantum system. Fine-structure splittings near the CGS point are compatible with weak tunneling or symmetry breaking due to experimental imperfections. We additionally use the superconducting cavity to externally induce symmetry breaking, observing a restoration of symmetry at the CGS point where ground states differ only by a $\mathbb{Z}_3$ phase. This work initiates a general program exploring lattice gauge theories using the toolbox of circuit quantum electrodynamics. More broadly, introducing non-planar Josephson connectivities opens a vast space for experimental and theoretical exploration of structures in almost any imaginable dimensionality and geometry.
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Submitted 15 July, 2026;
originally announced July 2026.
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Single-ion anisotropy-stabilized short-period helimagnetism in frustrated chiral Co$_5$TeO$_8$
Authors:
Priya R. Baral,
Ravi Yadav,
Victor Ukleev,
Thomas LaGrange,
Ivica Živković,
Wen Hua Bi,
Marek Bartkowiak,
Robert Cubitt,
Nina-Juliane Steinke,
Vladimir Pomjakushin,
Yurii Skourski,
Henrik M. Rønnow,
Oleg V. Yazyev,
Arnaud Magrez,
Jonathan S. White
Abstract:
Chiral spin textures in magnetic insulators promise magneto-electric (ME) spintronics with orders-of-magnitude lower power consumption than metallic systems. However, realizing the short magnetic periods required for high-density device integration remains difficult, as conventional Dzyaloshinskii-Moriya interaction (DMI)-based mechanisms typically constrain spiral periods to tens of nanometers. W…
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Chiral spin textures in magnetic insulators promise magneto-electric (ME) spintronics with orders-of-magnitude lower power consumption than metallic systems. However, realizing the short magnetic periods required for high-density device integration remains difficult, as conventional Dzyaloshinskii-Moriya interaction (DMI)-based mechanisms typically constrain spiral periods to tens of nanometers. While theory predicts that strong single-ion anisotropy (SIA) on frustrated lattices can stabilize complex non-coplanar textures, the potential for using this mechanism to engineer such compact textures remains largely unexplored. Here we report that a cubic chiral insulator Co$_5$TeO$_8$ provides an experimental example of this paradigm. Comprehensive neutron scattering and magnetometry reveal helimagnetic spirals with continuously tunable pitch of 5.7-10 nm embedded in a complex phase diagram spanning eight distinct phases. Capacitance anomalies throughout the phase diagram indicate magneto-electric coupling, pointing to the possibility of future $E$-field control of these textures. The temperature- and field-dependence of the helical wavevector strongly support a scenario in which site-dependent SIA provides the leading contribution to the selection of the helical period from a frustration-induced degenerate manifold. Consistent with this interpretation, $ab\,initio$ calculations place SIA approximately an order of magnitude above DMI, distinct from conventional helimagnets. Co$_5$TeO$_8$ thus offers an experimental realization of sub-10 nm helimagnetism and motivates a design principle for anisotropy-engineered correlated insulators.
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Submitted 4 June, 2026; v1 submitted 17 December, 2025;
originally announced December 2025.
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Cu2OSeO3 Turns Trigonal with Structural Transformation and Implications for Skyrmions
Authors:
Alla Arakcheeva,
Priya Ranjan Baral,
Wen Hua Bi,
Christian Jandl,
Oleg Janson,
Arnaud Magrez
Abstract:
The formation and characteristics of magnetic skyrmions are strongly governed by the symmetry of the underlying crystal structure. In this study, we report the discovery of a new trigonal polymorph of Cu2OSeO3, observed exclusively in nanoparticles. Electron diffraction and density functional theory calculations confirm its R3m space group, sharing C3v symmetry with Néel-type skyrmion hosts. This…
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The formation and characteristics of magnetic skyrmions are strongly governed by the symmetry of the underlying crystal structure. In this study, we report the discovery of a new trigonal polymorph of Cu2OSeO3, observed exclusively in nanoparticles. Electron diffraction and density functional theory calculations confirm its R3m space group, sharing C3v symmetry with Néel-type skyrmion hosts. This polymorph is likely stabilized by surface effects, suggesting that size-induced structural changes may drive a transformation from Bloch-type to Neel-type skyrmions in Cu2OSeO3. This hypothesis is consistent with prior unexplained observations of Neel-type skyrmions at the surfaces of bulk crystals, which may result from surface-specific structural distortions. Overall, these findings provide insights into the interplay between size, structure, and magnetism, opening pathways for controlling skyrmionic properties in nanoscale systems.
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Submitted 11 August, 2025;
originally announced August 2025.
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Possible High temperature Superconductivity above 200K mediated by Bose Einstein Condensation of exciton
Authors:
Shisheng Lin,
Shaoqi Huang,
Minhui Yang,
Xin Chen,
Hongjia Bi,
Kangchen Xiong
Abstract:
Exciton mediated superconductor is a fascinating quantum phase of matter that occurs when excitons become the dominant excitation in materials, which is also very promising for high temperature superconductor. However, there is no experimental report of exciton mediated superconductivity. Herein, we realize exciton mediated superconductivity from exciton insulator, where the onset transition tempe…
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Exciton mediated superconductor is a fascinating quantum phase of matter that occurs when excitons become the dominant excitation in materials, which is also very promising for high temperature superconductor. However, there is no experimental report of exciton mediated superconductivity. Herein, we realize exciton mediated superconductivity from exciton insulator, where the onset transition temperature can reach larger than 200 K. More profoundly, Bose-Einstein condensation (BEC) of exciton can facilitate the formation of exciton insulator and a transition happened from extremely high resistivity of 107 Ω at 153.5 K to 100 Ω at 125 K, indicating a superconducting transition. The resistance of exciton mediated superconductivity can not be absolutely reach zero possibly as a result of drag effect between electrons/holes and excitons. We reveal one rule for exciton mediated superconductivity is that the resistivity is an inverse linear function of the current through the BEC superfluid state, which should be ascribed to the Andreev-Bashkin effect, which reflects the coupling between the BEC state of exciton and Cooper pairs mediated by excitons. Furthermore, a record transition temperature above 200 K has been found for one superconducting sample, which shows the Josephson oscillation dominated by a pendulum-like equation caused by the quantum coupling between superconductor and BEC superfluid of exciton.
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Submitted 19 December, 2024;
originally announced December 2024.
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Optical response of edge modes in time-reversal symmetric topological superconductors
Authors:
Hirokazu Kobayashi,
Han Bi,
James Jun He,
Seishiro Ono
Abstract:
Topological superconductors and Majorana edge modes at their boundaries have been theoretically predicted. However, their experimental observation remains controversial. Recent theoretical studies suggest that chiral Majorana edge modes exhibit distinct spatially-resolved optical conductivity compared to chiral Dirac edge modes. In this work, we investigate the optical conductivity and spatially-r…
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Topological superconductors and Majorana edge modes at their boundaries have been theoretically predicted. However, their experimental observation remains controversial. Recent theoretical studies suggest that chiral Majorana edge modes exhibit distinct spatially-resolved optical conductivity compared to chiral Dirac edge modes. In this work, we investigate the optical conductivity and spatially-resolved optical conductivity induced by Majorana edge modes and Dirac edge modes under time-reversal symmetry and crystalline symmetry. We conduct numerical calculations and analytical calculations with edge effective theory for two-dimensional ${\mathbb Z}_2$ topological insulators, strong topological superconductors, and topological crystalline superconductors. Our results show that even under time-reversal symmetry and crystalline symmetry, Majorana edge modes and Dirac edge modes exhibit different optical responses.
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Submitted 15 November, 2024;
originally announced November 2024.
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DPA-2: a large atomic model as a multi-task learner
Authors:
Duo Zhang,
Xinzijian Liu,
Xiangyu Zhang,
Chengqian Zhang,
Chun Cai,
Hangrui Bi,
Yiming Du,
Xuejian Qin,
Anyang Peng,
Jiameng Huang,
Bowen Li,
Yifan Shan,
Jinzhe Zeng,
Yuzhi Zhang,
Siyuan Liu,
Yifan Li,
Junhan Chang,
Xinyan Wang,
Shuo Zhou,
Jianchuan Liu,
Xiaoshan Luo,
Zhenyu Wang,
Wanrun Jiang,
Jing Wu,
Yudi Yang
, et al. (18 additional authors not shown)
Abstract:
The rapid advancements in artificial intelligence (AI) are catalyzing transformative changes in atomic modeling, simulation, and design. AI-driven potential energy models have demonstrated the capability to conduct large-scale, long-duration simulations with the accuracy of ab initio electronic structure methods. However, the model generation process remains a bottleneck for large-scale applicatio…
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The rapid advancements in artificial intelligence (AI) are catalyzing transformative changes in atomic modeling, simulation, and design. AI-driven potential energy models have demonstrated the capability to conduct large-scale, long-duration simulations with the accuracy of ab initio electronic structure methods. However, the model generation process remains a bottleneck for large-scale applications. We propose a shift towards a model-centric ecosystem, wherein a large atomic model (LAM), pre-trained across multiple disciplines, can be efficiently fine-tuned and distilled for various downstream tasks, thereby establishing a new framework for molecular modeling. In this study, we introduce the DPA-2 architecture as a prototype for LAMs. Pre-trained on a diverse array of chemical and materials systems using a multi-task approach, DPA-2 demonstrates superior generalization capabilities across multiple downstream tasks compared to the traditional single-task pre-training and fine-tuning methodologies. Our approach sets the stage for the development and broad application of LAMs in molecular and materials simulation research.
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Submitted 16 August, 2024; v1 submitted 24 December, 2023;
originally announced December 2023.
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Inter-band optical transitions of helical Majorana edge modes in topological superconductors
Authors:
Han Bi,
James Jun He
Abstract:
The search for evidence of Majorana states on the edges of topological superconductors (TSCs) is challenging due to the difficulty of detecting such charge-neutral electronic quasiparticles. Local microwave spectroscopy has been shown to be a possible method to detect propagating Majorana modes, where a spatially focused light beam must be used. Here, we show that helical Majorana modes in TSCs al…
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The search for evidence of Majorana states on the edges of topological superconductors (TSCs) is challenging due to the difficulty of detecting such charge-neutral electronic quasiparticles. Local microwave spectroscopy has been shown to be a possible method to detect propagating Majorana modes, where a spatially focused light beam must be used. Here, we show that helical Majorana modes in TSCs allow inter-band transitions and thus contribute to optical conductivity under a spatially uniform light. The existence of such a signal requires the system to break certain symmetries so that the projection of the charge current operator onto helical Majorana edge states leads to inter-band hybridization terms. The general form of this contribution under a tunable time-reversal breaking field is derived, which is valid in the sub-gap low-frequency regime where the edge energy spectrum is linear, and numerical results are obtained in three TSC models, showing remarkable consistency with the analytical prediction. In comparison, the current operator for normal helical edge states, such as in quantum spin Hall insulators, does not cause inter-band transitions and the related optical conductivity vanishes unless the time-reversal symmetry is broken. Our results may help guide feasible experiments to provide evidence of Majorana edge modes in TSCs.
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Submitted 26 October, 2023;
originally announced October 2023.
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Structure, heat capacity and Raman spectra of mm-sized Ba$_{2}$MgWO$_{6}$ single crystals synthesized by BaCl$_{2}$-MgCl$_{2}$ flux method
Authors:
Jana Pásztorová,
Wen Hua Bi,
Richard Gaal,
Karl Krämer,
Ivica Živković,
Henrik M. Rønnow
Abstract:
We present a new method of Ba$_{2}$MgWO$_{6}$ single crystal synthesis that allows to grow larger crystals using BaCl$_{2}$ and MgCl$_{2}$ flux. Difficulties to grow single crystal of a size suitable for macroscopic material property measurements caused the majority of characterisation being published on polycrystalline samples. Single crystal diffraction and energy dispersive X-ray analysis confi…
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We present a new method of Ba$_{2}$MgWO$_{6}$ single crystal synthesis that allows to grow larger crystals using BaCl$_{2}$ and MgCl$_{2}$ flux. Difficulties to grow single crystal of a size suitable for macroscopic material property measurements caused the majority of characterisation being published on polycrystalline samples. Single crystal diffraction and energy dispersive X-ray analysis confirmed high quality of synthesised samples. Heat capacity measurements from 300~K to 2~K do not show any transitions. However, Raman spectra measured down to 77~K contain additional peaks at all temperatures probed, which is in a contrast with only 4 Raman active modes expected from the reducible representation. This calls for a more detailed study of potential symmetry breaking that could also influence the electronic properties of the material.
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Submitted 12 December, 2022;
originally announced December 2022.
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Asynchronous and coherent dynamics in balanced excitatory-inhibitory spiking networks
Authors:
Hongjie Bi,
Matteo Di Volo,
Alessandro Torcini
Abstract:
Dynamic excitatory-inhibitory (E-I) balance is a paradigmatic mechanism invoked to explain the irregular low firing activity observed in the cortex. However, we will show that the E-I balance can be at the origin of other regimes observable in the brain. The analysis is performed by combining simulations of sparse E-I networks composed of N spiking neurons with analytical investigations of low dim…
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Dynamic excitatory-inhibitory (E-I) balance is a paradigmatic mechanism invoked to explain the irregular low firing activity observed in the cortex. However, we will show that the E-I balance can be at the origin of other regimes observable in the brain. The analysis is performed by combining simulations of sparse E-I networks composed of N spiking neurons with analytical investigations of low dimensional neural mass models. The bifurcation diagrams, derived for the neural mass model, allow to classify the asynchronous and coherent behaviours emerging any finite in-degree K. In the limit N >> K >> 1 both supra and sub-threshold balanced asynchronous regimes can be observed. Due to structural heterogeneity the asynchronous states are characterized by the splitting of the neurons in three groups: silent, fluctuation and mean driven. The coherent rhythms are characterized by regular or irregular temporal fluctuations joined to spatial coherence similar to coherent fluctuations observed in the cortex over multiple spatial scales. Collective Oscillations (COs) can emerge due to two different mechanisms. A first mechanism similar to the pyramidal-interneuron gamma (PING) one. The second mechanism is intimately related to the presence of current fluctuations, which sustain COs characterized by an essentially simultaneous bursting of the two populations. We observe period-doubling cascades involving the PING-like COs finally leading to the appearance of coherent chaos. For sufficiently strong current fluctuations we report a novel mechanism of frequency locking among collective rhythms promoted by these intrinsic fluctuations. Our analysis suggest that despite PING-like or fluctuation driven COS are observable for any finite in-degree K, in the limit N >> K >> 1 these solutions result in two coexisting balanced regimes: an asynchronous and a fully synchronized one.
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Submitted 19 October, 2021; v1 submitted 31 August, 2021;
originally announced August 2021.
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A topological attractor of vortices as a clock generator based on polariton superfluids
Authors:
Xuemei Sun,
Gang Wang,
Kailin Hou,
Huarong Bi,
Yan Xue,
Alexey Kavokin
Abstract:
We reveal a topologically protected persistent oscillatory dynamics of a polariton superfluid, which is driven non-resonantly by a super-Gaussian laser beam in a planar semiconductor microcavity subjected to an external C-shape potential. We find persistent oscillations, characterized by a topological attractor, that are based on the dynamical behavior of small Josephson vortices rotating around t…
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We reveal a topologically protected persistent oscillatory dynamics of a polariton superfluid, which is driven non-resonantly by a super-Gaussian laser beam in a planar semiconductor microcavity subjected to an external C-shape potential. We find persistent oscillations, characterized by a topological attractor, that are based on the dynamical behavior of small Josephson vortices rotating around the outside edge of the central vortex. The attractor is being formed due to the inverse energy cascade accompanied by the growth of the incompressible kinetic energy. The attractor displays a remarkable stability towards perturbations and it may be tuned by the pump laser intensity to two distinct frequency ranges: 20.16$\pm$0.14 GHz and 48.4$\pm$1.2 GHz. This attractor is bistable due to the chirality of the vortex. The switching between two stable states is achieved by altering the pump power or by adding an extra incoherent Gaussian pump beam.
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Submitted 25 September, 2023; v1 submitted 16 March, 2021;
originally announced March 2021.
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Theta-nested gamma oscillations in next generation neural mass models
Authors:
Marco Segneri,
Hongjie Bi,
Simona Olmi,
Alessandro Torcini
Abstract:
Theta-nested gamma oscillations have been reported in many areas of the brain and are believed to represent a fundamental mechanism to transfer information across spatial and temporal scales. In a series of recent experiments in vitro it has been possible to replicate with an optogenetic theta frequency stimulation several features of cross-frequency coupling among theta and gamma rhythms observed…
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Theta-nested gamma oscillations have been reported in many areas of the brain and are believed to represent a fundamental mechanism to transfer information across spatial and temporal scales. In a series of recent experiments in vitro it has been possible to replicate with an optogenetic theta frequency stimulation several features of cross-frequency coupling among theta and gamma rhythms observed in behaving animals. In order to reproduce the main findings of these experiments we have considered a new class of neural mass models able to reproduce exactly the macroscopic dynamics of spiking neural networks. In this framework, we have examined two set-ups able to support collective gamma oscillations: the pyramidal interneuronal network gamma (PING) and the interneuronal network gamma (ING). In both set-ups we observe the emergence of theta-nested gamma oscillations by driving the system with a sinusoidal theta-forcing in proximity of a Hopf bifurcation. These mixed rhythms display always phase amplitude coupling. However 2 different types of nested oscillations can be identified: one characterized by a perfect phase locking between theta and gamma rhythms, corresponding to an overall periodic behaviour; another one where the locking is imperfect and the dynamics is quasi-periodic or even chaotic. From our analysis it emerges that the locked states are more frequent in the ING set-up. In agreement with the experiments, we find theta-nested gamma oscillations for forcing frequencies in the range [1:10] Hz, whose amplitudes grow proportionally to the forcing one and which are clearly modulated by the theta phase. At variance with experimental findings, the gamma-power peak does not shift to higher frequencies by increasing the theta frequency. This effect can be obtained, in or model, only by incrementing, at the same time, also the noise or the forcing amplitude.
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Submitted 9 March, 2020;
originally announced March 2020.
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Coexistence of fast and slow gamma oscillations in one population of inhibitory spiking neurons
Authors:
Hongjie Bi,
Marco Segneri,
Matteo di Volo,
Alessandro Torcini
Abstract:
Oscillations are a hallmark of neural population activity in various brain regions with a spectrum covering a wide range of frequencies. Within this spectrum gamma oscillations have received particular attention due to their ubiquitous nature and to their correlation with higher brain functions. Recently, it has been reported that gamma oscillations in the hippocampus of behaving rodents are segre…
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Oscillations are a hallmark of neural population activity in various brain regions with a spectrum covering a wide range of frequencies. Within this spectrum gamma oscillations have received particular attention due to their ubiquitous nature and to their correlation with higher brain functions. Recently, it has been reported that gamma oscillations in the hippocampus of behaving rodents are segregated in two distinct frequency bands: slow and fast. These two gamma rhythms correspond to dfferent states of the network, but their origin has been not yet clarified. Here, we show theoretically and numerically that a single inhibitory population can give rise to coexisting slow and fast gamma rhythms corresponding to collective oscillations of a balanced spiking network. The slow and fast gamma rhythms are generated via two different mechanisms: the fast one being driven by the coordinated tonic neural firing and the slow one by endogenous fluctuations due to irregular neural activity. We show that almost instantaneous stimulations can switch the collective gamma oscillations from slow to fast and vice versa. Furthermore, to make a closer contact with the experimental observations, we consider the modulation of the gamma rhythms induced by a slower (theta) rhythm driving the network dynamics. In this context, depending on the strength of the forcing, we observe phase-amplitude and phase-phase coupling between the fast and slow gamma oscillations and the theta forcing. Phase-phase coupling reveals different theta-phases preferences for the two coexisting gamma rhythms.
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Submitted 29 June, 2019;
originally announced July 2019.