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arXiv:2607.26919

Quantum Fisher information of the Klein--Gordon, $φ^4$, and Dirac vacua

Preslav Asenov, Alessio Serafini

hep-thquant-phSubmitted Jul 29, 2026• Announced Jul 30, 2026
v1 · Jul 29, 2026v2 · Sep 8, 2026
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Abstract

The quantum Fisher information (QFI) of the vacuum of three quantum field theories is evaluated with respect to the mass parameter of each theory. All field theories are considered on a $(d+1)$-dimensional Euclidean spacetime. We consider the Klein-Gordon field, a quartic interaction ($φ^4$) field theory, and the free Dirac field. In the case of the quartic interaction theory, the QFI is given perturbatively to first order in the interaction constant. For the free KG case, we find a $m^{d-2}$ dependence of the QFI, and thus no dependence for $d=2$, in agreement with the holographic duality characterizing the theory. The introduction of a quartic interaction is shown to lead to a QFI divergence in $d=3$ and to reduce the available information in the $d=0$ system. The vacuum QFI with respect to the free Dirac field mass is shown to be UV-divergent for $d=2$ and $d=3$, mass-dependent for $d=1$, and zero for $d=0$.

Comments
15 pages, 1 figure

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Quantum Fisher information of the Klein--Gordon, φ4φ^4φ4, and Dirac vacua
The quantum Fisher information (QFI) of the vacuum of three quantum field theories is evaluated with respect to the mass parameter of each theory. All field theories are considered on a (d+1)(d+1)(d+1)-dimensional Euclidean spacetime. We consider the Klein-Gordon field, a quartic interaction (φ4φ^4φ4) field theory, and the free Dirac field. In the case of the quartic interaction theory, the QFI is given perturbatively to first order in the interaction constant. For the free KG case, we find a md−2m^{d-2}md−2 dependence of the QFI, and thus no dependence for d=2d=2d=2, in agreement with the holographic duality characterizing the theory. The introduction of a quartic interaction is shown to lead to a QFI divergence in d=3d=3d=3 and to reduce the available information in the d=0d=0d=0 system. The vacuum QFI with respect to the free Dirac field mass is shown to be UV-divergent for d=2d=2d=2 and d=3d=3d=3, mass-dependent for d=1d=1d=1, and zero for d=0d=0d=0.
We investigate the performance of quantum Fisher information (QFI) under the Unruh-Hawking effect, where one of the observers (e.g., Rob) is uniformly accelerated with respect to other partners. In the context of relativistic quantum information theory, we demonstrate that quantum Fisher information, as an important measure of the information content of quantum states, has a rich and subtle physical structure compared with entanglement or Bell nonlocality. In this work, we mainly focus on the parametrized (and arbitrary) pure two-qubit states, where the weight parameter $\ensuremath{\theta}$ and phase parameter $\ensuremath{\phi}$ are naturally introduced. Intriguingly, we prove that QFI with respect to $\ensuremath{\theta}$ (${\mathcal{F}}_{\ensuremath{\theta}}$) remains unchanged for both scalar and Dirac fields. Meanwhile, we observe that QFI with respect to $\ensuremath{\phi}$ (${\mathcal{F}}_{\ensuremath{\phi}}$) decreases with the increase of acceleration rrr but remains finite in the limit of infinite acceleration. More importantly, our results show that the symmetry of ${\mathcal{F}}_{\ensuremath{\phi}}$ (with respect to $\ensuremath{\theta}=\ensuremath{\pi}/4$) has been broken by the influence of the Unruh effect for both cases.
The Klein–Gordon equation for spinless particles, as well as the Weyl–Pauli and Dirac equations for particles with spin, may be derived from one information-theoretic principle. Consider a gedanken experiment whereby the mean position of a particle in a central force field is to be estimated by means of one position measurement. An efficient (optimum) estimate obeys a condition of minimum Fisher information, or minimum precision, according to the second law of thermodynamics. When the Fisher information is minimized subject to a constraint on the mean-square kinetic energy for the particle, the solution is a probability law on position whose corresponding wave amplitude obeys the Klein-Gordon equation, the Weyl–Pauli equation, or the Dirac equation, depending on whether scalar (without spin) or vector (with spin) solutions are sought. The minimized Fisher information is proportional to the mean-square kinetic energy, minus the rest energy of the particle.
Abstract We analyze the quantum Fisher information (QFI) for a particle detector coupled to a thermal electromagnetic field in the cosmic string spacetime. It is found that vacuum fluctuation, temperature, detector polarization and nontrivial spacetime topology affect the QFI. Vacuum fluctuation and temperature degrade QFI. When deficit angle parameter ν = 1 and detector is far away from the string, the results in free Minkowski spacetime are restored. When detector is close to the string, QFI can be effectively enhanced; specially when detector lies in the string and is not polarized along axial direction, the detector seems like a closed system, and QFI completely is not affected by the thermal electromagnetic fluctuation. For nonzero detector–string distance, QFI appears oscillatory behaviors as detector–string distance varies. In addition, this detector eventually evolves to a thermal state, which is not related to the initial state and other various parameters, which implies QFI cannot persist in for long evolution time in the cosmic string spacetime. In principle, the detector polarization, detector–string distance, temperature and deficit angle parameter provide us with more parameters to steer the QFI behaviors, which might be helpful to enhance the precision of sensing the characters of cosmic string spacetime.
Abstract In this paper, we analytically explore the effect of Hawking radiation on quantum Fisher information (QFI) for Dirac particles in the background of Schwarzschild black hole. What surprise us is that the Hawking radiation could not disturb QFI with respect to the state parameter ( ). Additionally, we also find in the case of an almost extreme black hole, or when the black hole is approximate to evaporating completely, QFI with respect to the Hawking temperature T ( ) always being zero indicates that the parameter estimation precision about the Hawking temperature is minimum in the two situations above. In particular, the maximal is obtained when the Hawking temperature is a fixed value for every state parameter . Furthermore, we investigate how the Hawking effect influences the distribution of the QFI over subsystems in physical accessible regions and find that QFI can be either subadditive or superadditive, depending on the specific parameter under the Hawking effect. At last, we also study the QFI of other possible bipartite divisions, which are physically inaccessible, and the nontrivial results reveal that QFI is averagely distributed in different physical regions.
Quantum Fisher information (QFI) associated with local metrology has been used to parameter estimation in open quantum systems. In this work, we calculated the QFI for a moving Unruh–DeWitt detector coupled with massless scalar fields in n-dimensional spacetime, and analyzed the behavior of QFI with various parameters, such as the dimension of spacetime, evolution time, and Unruh temperature. We discovered that the QFI of state parameter decreases monotonically from 1 to 0 over time. Additionally, we noted that the QFI for small evolution times is several orders of magnitude higher than the QFI for long evolution times. We also found that the value of QFI decreases at first and then stabilizes as the Unruh temperature increases. It was observed that the QFI depends on initial state parameter θ, and Fθ is the maximum for θ = 0 or θ = π, Fϕ is the maximum for θ = π/2. We also obtain that the maximum value of QFI for state parameters varies for different spacetime dimensions with the same evolution time.
In estimating an unknown parameter of a quantum state the quantum Fisher information (QFI) is a pivotal quantity, which depends on the state and its derivate with respect to the unknown parameter. We prove the continuity property for the QFI in the sense that two close states with close first derivatives have close QFIs. This property is completely general and irrespective of dynamics or how states acquire their parameter dependence and also the form of parameter dependence---indeed this continuity is basically a feature of the classical Fisher information that in the case of the QFI naturally carries over from the manifold of probability distributions onto the manifold of density matrices. We demonstrate that, in the special case where the dependence of the states on the unknown parameter comes from one dynamical map (quantum channel), the continuity holds in its reduced form with respect to the initial states. In addition, we show that, when one initial state evolves through two different quantum channels, the continuity relation applies in its general form. A situation in which such a scenario can occur is an open-system metrology where one of the maps represents the ideal dynamics, whereas the other map represents the real (noisy) dynamics. In the making of our main result, we also introduce a regularized representation for the symmetric logarithmic derivative which works for general states even with incomplete rank, and it features continuity similar to the QFI.