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ALT The Jaynes-Cummings (JC) Hamiltonian is a paradigmatic model of light-matter interaction and, more generally, qubit-boson interactions, widely used across atomic, optical, and superconducting qubit platforms. In the multi-qubit setting, where n qubits are identically coupled to a single boson mode, this interaction is known as the Tavis-Cummings (TC) Hamiltonian. The structure of the TC model is usually understood in terms of two standard symmetries: permutation invariance of the qubits and a U(1) symmetry associated with conservation of the total excitation number. Here we identify an additional, independent "accidental" symmetry of the TC Hamiltonian and construct the corresponding conserved observable. We show that, for n>2 qubits, this symmetry imposes strong constraints on the realizable unitary transformations. These constraints persist in the presence of the global Jz Hamiltonian, but are removed by adding JzΒ², even though JzΒ² preserves both permutation invariance and the U(1) s
ALT Characterization of time-frequency (TF) quantum states requires reliable reconstruction of their TF distributions. However, imperfect transmission or measurement channels can distort reconstructed joint spectral intensities (JSIs), especially when the underlying perturbation mechanism is unknown. Here, we experimentally demonstrate a reconstruction and correction framework that uses a TF grid state as an intrinsic frequency-domain reference. By analyzing the displacement of the grid points, a Gaussian process regression model is employed to reconstruct a correction mapping for the nonlinear coordinate deformation without assuming a prior physical model of the distortion. The learned mapping reduces the residual coordinate deviation of the TF grid state by approximately a factor of 11 and, when applied to an independent frequency-entangled test state, improves the Gaussian-shape fidelity from 76.2% to 90.0%. These results establish TF grid states as practical metrological resources for
ALT EDITORS' SUGGESTION Polar and quadratic magneto-optical Kerr effects in nonmagnetic/ferromagnet bilayers for spin-orbit torque measurements 5 June, 2026 Spin orbit torque is one of the key technologies in developing next-generation spintronic devices. Various approaches have been developed to characterize the efficiency of spin orbit torque. Recent studies suggest that the optical approach provides a better means to study the efficiency compared to the conventional electrical approach. Here, the authors clarify the reason behind this. It is the small quadratic component of the magneto-optical Kerr effect compared to its polar counterpart that defines the high accuracy. Yukihiro Marui, Masashi Kawaguchi, Kohji Nakamura, and Masamitsu Hayashi Phys. Rev. B 113, 214419 (2026)