Intrinsic Asymmetry Within Quantum Systems Breaks Fundamental Particle Physics Rules

Di Xiao and colleagues at Huazhong University of Science and Technology, in collaboration with University of Hamburg, and Wuhan Institute of Quantum Technology, have identified sublattice kinks in bipartite fermionic lattices as intrinsically exhibiting charge-conjugation violation, a key concept in particle physics. The violation arises from the graph topology of the system’s Hamiltonian, without requiring external symmetry breaking, and results in a measurable population asymmetry between different configurations. This asymmetry leads to an imbalanced production of sublattice kinks during dynamic processes, suggesting a potential pathway for observation using cold-atom quantum simulators and offering new insights into the microscopic origins of charge-conjugation violation.

Topological defects in fermionic lattices induce spontaneous charge-conjugation violation

Charge-conjugation violation can arise intrinsically within specific quantum systems. This discovery centres on sublattice kinks, topological defects within bipartite fermionic lattices, and their unusual behaviour. Whether this phenomenon is a broader characteristic of many-body quantum systems remains an open question. The origin of this violation lies not in external manipulation, but in the graph topology of the system’s Hamiltonian — a departure from the usual assumption that charge-conjugation must be broken by some external field or coupling.

A bipartite lattice is a structure composed of two interweaving subnetworks. Demonstrating this intrinsic charge-conjugation violation is significant because it moves beyond reliance on artificially broken symmetry, offering a new pathway to explore fundamental symmetries in quantum materials. Furthermore, existing cold-atom technology offers a realistic pathway for experimental verification of these predictions.

Bipartite lattice geometry imprints asymmetry on kink quasiparticles

The microscopic origin of intrinsic charge-conjugation violation (CCV) is a graph topology, with population asymmetry as the macroscopic consequence. Kinks, also termed domain walls, are topological defects acting as interfaces between distinct domains in various quantum systems and can be considered a quasiparticle family. They emerge in low-energy quantum systems, such as the φ4 and sine-Gordon models, as well as spin chains, which can be implemented in condensed matter and ultracold atomic settings.

These systems have been explored as a testbed for fundamental phenomena, with magnetic kinks simulating confinement-deconfinement transitions and collision effects between quarks. Related phenomena, like meson formation and thermodynamic properties, are also observed. Kinks possess naturally opposite configurations, kink and antikink, allowing the mimicking and testing of particle-antiparticle related phenomena, such as charge-conjugation violation.

The CCV of fundamental particles is believed to be responsible for the particle-antiparticle imbalance, constituting a key ingredient in the Sakharov conditions explaining the matter-antimatter asymmetry of the universe. In a bipartite fermionic lattice, quasiparticles exist as sublattice kinks, offering intrinsic CCV between the kink and antikink configurations. This CCV manifests as a difference in chemical potential between the kink and antikink configurations, arising directly from the topology of the underlying bipartite fermionic lattice.

This provides a concrete mechanism for emergent quasiparticle CCV, generating a hidden leaf-like structure in the eigenenergy spectrum and leading to imbalanced excitation between the kink and antikink configurations. The bipartite fermionic lattice enriches the kink family with the sublattice kink, offering an experimentally feasible testbed for both the microscopic origin and macroscopic consequence of CCV employing ultracold atomic ensembles.

Researchers modelled this using a one-dimensional bipartite lattice of spin-polarized fermions with open boundaries and one fermion per unit cell. Each cell has two sites — left and right sublattices — and a strong repulsive interaction prevents any cell from holding more than one fermion, confining the dynamics to a restricted set of allowed states. The lattice topology itself, captured in a Fermi-Hubbard Hamiltonian, encodes intra-cell hopping between the two sublattice sites, inter-cell hopping between adjacent cells, and nearest-neighbour interactions. The allowed states naturally partition into regions where all occupied sites sit on the same sublattice — L-domains and R-domains — with the boundary between a pair of opposing domains forming a sublattice kink, treated as a quasiparticle residing on the bond between adjacent cells.

Two kink types exist: the kink, where an L-domain lies to its left and an R-domain to its right, and the antikink with the opposite arrangement. These form a particle–antiparticle pair, and the charge-conjugation operation corresponds to flipping the sublattice occupation of every fermion simultaneously. To capture their full dynamics, the team transformed the Hamiltonian into a kink-picture description, introducing virtual boundary bonds to distinguish the two degenerate no-kink ground states. In this picture the Hamiltonian separates into a real-bond term — encoding the kink and antikink chemical potentials alongside hopping and pair creation and annihilation — and a boundary term coupling the real lattice to the virtual edges. The asymmetry in chemical potential between kink and antikink is a direct consequence of the bipartite lattice geometry, and is the mathematical signature of charge-conjugation violation without any external symmetry breaking.

Researchers demonstrated that sublattice kinks in bipartite fermionic lattices exhibit intrinsic charge-conjugation violation without external symmetry breaking. This violation originates from the graph topology of the system’s Hamiltonian, resulting in an unequal population of different configurations. The population asymmetry creates a distinctive leaf-like structure in the system’s energy spectrum and leads to an imbalanced production of these kinks during dynamic processes. The authors suggest this setup is well-suited for experimental investigation using cold-atom quantum simulators.