Quantum simulation approach to ultra-weak magnetic anisotropy in a frustrated spin-1/2 antiferromagnet

Abstract

The intrinsic equivalence between electron spin and qubit offers a natural foundation for quantum simulations of magnetic materials. However, incorporating magnetocrystalline anisotropy (MCA), a key feature of real magnets, remains a major challenge. Here, we develop a quantum simulation framework for MCA in CuSb2O6, a spin-1/2 antiferromagnet with alternating ferromagnetic chains arising from frustrated, anisotropic exchange interactions in a nearly square lattice. The $\mathrm{Cu}^{2+}$ spin network is modeled as a four-qubit square lattice, with four paired ancilla qubits introduced to encode angle-dependent MCA. This two-qubit representation per spin site resolves the limitation that squared Pauli operators yield only the identity, enabling MCA terms to be faithfully embedded into quantum circuits. Using the variational quantum eigensolver, we determine an exceptionally small easy-axis MCA constant, just 0.00022% of the nearest-neighbor exchange interaction, yet sufficient to drive a spin-flop transition with $90^{\circ}$ spin reorientation and strong angular variation in magnetic torque. Beyond this regime, the simulations uncover a half-saturated magnetic phase at ultra-high fields, stabilized by anisotropic next-nearest-neighbor interactions. Our findings demonstrate the feasibility of resource-efficient quantum simulations of complex magnetic phenomena in real materials.