Suppression of Decoherence at Exceptional Transitions

TL;DR

The paper shows that decoherence under non-Hermitian environments can be controlled by exceptional points, in contrast to the standard Hermitian case where criticality enhances decoherence. By analyzing spin-chain and ultracold Fermi gas environments, it demonstrates that decoherence can be either enhanced or strongly suppressed near EPs depending on the balance between Hermitian and non-Hermitian system–environment couplings, with suppression occurring when $\delta_x=\delta_y$ near $h_y=h_x$. The mechanism is tied to the environmental ground-state susceptibility, which is minimized near EPs for balanced couplings, leading to reduced environmental fluctuations. The authors validate the effect across models and show that the decoherence suppression is experimentally accessible on digital quantum simulators using a novel ancilla-assisted non-unitary evolution protocol and adaptive trotterization, highlighting non-Hermitian criticality as a practical resource for coherence protection in quantum technologies.

Abstract

Decoherence is strongly influenced by environmental criticality, with conventional Hermitian critical points universally 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 the 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.

Figures (7)

• Figure 1: (a) Schematic illustration of a central qubit (system) coupled to its non-Hermitian environment formed by interacting spins in a complex transverse field. (b1, b2) Dynamics of qubit coherence $C[\rho(t)]$ as the non-Hermitian environment approaches its EPs, $h_{y}\rightarrow h_{x}=1$. The environment is formed by an Ising chain with a complex transverse field [see Eq. (\ref{['eq:Ising_model']})]. (b1) Coherence dynamics for balanced Hermitian and non-Hermitian system-environment couplings ($\delta_{x}=\delta_{y}$), showing pronounced suppression of decoherence near the EP. (b2) Coherence dynamics in the absence of non-Hermitian coupling ($\delta_{y}=0$), showing the conventional enhancement of decoherence near the EP. (c1, c2) Same as (b1, b2), but for a non-Hermitian environment formed by Heisenberg spin chain in a complex transverse field. All simulations use $N=25$, $J=1/2$, $h_{x}=1$, and $\sqrt{\delta_{x}^{2}+\delta_{y}^{2}}=0.1$. See text for more details.
• Figure 2: Dynamics of qubit coherence $C[\rho(t)]$ as a non-Hermitian environment formed by non-Hermitian Fermi gases in optical lattices approaches its EPs, $h_{y}\rightarrow h_{x}=1$. (a) Coherence dynamics for balanced Hermitian and non-Hermitian system-environment couplings ($\delta_{x}=\delta_{y}$), showing pronounced suppression of decoherence near the EP. (b) Coherence dynamics in the absence of non-Hermitian coupling ($\delta_{y}=0$), showing the conventional enhancement of decoherence near the EP. All simulations use $N=10$, $J=0.1$, $U=0.4$, $h_{x}=1$, $\sqrt{\delta_{x}^{2}+\delta_{y}^{2}}=0.1$. See text for more details.
• Figure 3: Simulation of qubit coherence dynamics on a quantum circuit, with the non-Hermitian environment formed by an Ising chain in a complex transverse field. (a) Quantum circuit protocol for simulating the first time step of the dynamics. (b) Coherence dynamics obtained from the quantum circuit. Hollow (solid) markers denote results from $2\times10^{6}$ trajectories on the qasm_simulator with (without) noise from ibm_brisbane, provided by IBM Qiskit cross_2018_ibm. Solid curves show exact results from numerical calculations. Simulations use $J=1/100$, $h_{x}=1$, $\delta_{x}=\delta_{y}=0.5$, and an optimized time step $dt$. See text for more details.
• Figure S1: Dynamics of the qubit coherence $C[\rho(t)]$ as a non-Hermitian environment formed by non-Hermitian Ising model under different coupling strengths $\delta_{x}=|\boldsymbol{\delta}|\sin\theta$ and $\delta_{y}=|\boldsymbol{\delta}|\cos\theta$ approaches its EPs, $h_{y}\rightarrow h_{x}=1$. (a-d) The $C[\rho(t)]$ during time $t\in[0,3]$ with $\theta=45\pi/200$ (a), $\theta=48\pi/200$ (b), $\theta=52\pi/200$ (c), $\theta=55\pi/200$ (d). All simulations use $N=25$ and $J=1/2$, $h_{x}=1$, $|\boldsymbol{\delta}|=\sqrt{\delta_{x}^{2}+\delta_{y}^{2}}=0.1$. See text for more details.
• Figure S2: Dynamics of the qubit coherence $C[\rho(t)]$ as a non-Hermitian environment formed by non-Hermitian Ising model with fixed complex field $h_{y}$ under different $\theta$. All simulations use $N=25$ and $h_{y}=0.9$, $J=1/2$, $h_{x}=1$ under different couplings $\delta_{x}=|\boldsymbol{\delta}|\sin\theta$, $\delta_{y}=|\boldsymbol{\delta}|\cos\theta$, $|\boldsymbol{\delta}|=\sqrt{\delta_{x}^{2}+\delta_{y}^{2}}=0.1$. See text for more details.