Suppression of Decoherence at Exceptional Transitions

Abstract

Decoherence is strongly influenced by environmental criticality, with conventional Hermitian critical points typically enhancing the loss of quantum coherence. Here, we show that this paradigm is fundamentally altered in non-Hermitian environments. Focusing on qubits coupled to non-Hermitian spin chains and interacting ultracold Fermi gases, we find that approaching exceptional points can either enhance or strongly suppress decoherence, depending on the balance between Hermitian and non-Hermitian system-environment couplings. In particular, when these couplings are comparable, decoherence is dramatically suppressed at exceptional transitions. We trace this behavior to the distinct response of the environmental ground state near non-Hermitian degeneracies and demonstrate the robustness of this effect across multiple models. Finally, we show that the predicted suppression of decoherence is directly observable on current digital quantum simulation platforms. Our results establish exceptional points as a concrete mechanism for suppressing decoherence and identify non-Hermitian criticality as a new avenue for coherence control in open quantum systems and quantum technologies.

The paper asks whether exceptional points (EPs) in non-Hermitian environments enhance qubit decoherence, as environmental quantum criticality often does in Hermitian settings, or can instead suppress it. It studies a central qubit coupled to complex-field Ising and Heisenberg chains, and to an interacting one-dimensional Fermi gas with spin-flip driving and spin-dependent gain–loss terms. Coherence is measured by the reduced qubit’s $\ell_1$ coherence after normalization of the nonunitarily evolved joint state. Numerical calculations show contrasting behavior as $h_y\to h_x$: purely Hermitian system–environment coupling enhances decoherence, whereas balanced Hermitian and non-Hermitian couplings, $\delta_x=\delta_y$, strongly suppress it. Representative calculations use $N=25$ spins or $N=10$ lattice sites, with $\sqrt{\delta_x^2+\delta_y^2}=0.1$. Supplementary parameter scans show suppression also occurs over a finite neighborhood of balanced coupling. The proposed mechanism is that the EP selects a polarized environmental ground state that the balanced coupling cannot drive away from; ground-state-overlap calculations support this interpretation. A digital-simulation protocol uses ancilla measurements and postselection to implement nonunitary dynamics. Two-site-environment Qiskit simulations, including an IBM Brisbane noise model and $2\times10^6$ trajectories, reproduce the predicted suppression; these are simulations rather than hardware measurements. Adaptive Trotter steps are chosen to keep deviations from exact coherence below $10^{-2}$. The paper states that analytical treatment away from the EP is intractable, so robustness nearby is established numerically. The coherence-protection setting assumes an initially prepared environmental ground state and conditional, postselected non-Hermitian evolution.