Bounds on relative modular Hamiltonians in general QFT
一般QFT中相对模哈密顿量的界
Adriano Chialastri, Christoph Minz, Ko Sanders
AI总结 利用代数QFT的局域性,通过参考态的模哈密顿量估计两个态之间的相对模哈密顿量及其相对熵,并应用于相干态和自由标量场。
Comments 25 pages, 3 figures
详情
两个态之间的相对熵是量子信息论和量子场论中的一个关键概念。在量子场论中,其计算需要处理相对模哈密顿量,而后者通常难以显式计算。本文利用一般代数QFT的局域性,通过参考态$\hatω$的模哈密顿量来估计两个态$ω$和$\tildeω$之间的相对模哈密顿量,进而估计它们的相对熵,其中$\hatω$可能更易理解。对于合适的态对,我们可以从上方(或下方)用更大区域$V_3$(或更小区域$V_1$)上$\hatω$的模哈密顿量来估计区域$V_2$代数的相对模哈密顿量。适合我们方案的态对和区域选择与Sorkin悖论意义上的超光速信号传递有关。如果$ω=\hatω$,则存在一个幺正算符将$V_3$上的$ω$映射到$\tildeω$,且当我们的上界(或下界)适用时,该算符不允许从$V_3$的类空补集$V_3'$到$V_2$(或从$V_1$到$V_2'$)的超光速信号传递。为了研究我们估计的强度,我们考虑CCR系统的相干态,特别关注自由标量场。即使相对模哈密顿量无法精确计算,我们的估计也适用。对于足够规则的激发,我们通过压缩恢复精确结果。因此,我们的方法在相对模哈密顿量无法精确计算的情况下,为相对熵公式提供了独立证明。对于无质量场,我们还在双锥区域建立了类似结果。这些结果表明我们的估计不会丢失太多信息。
The relative entropy between two states is a key concept in quantum information theory and quantum field theory. In the setting of quantum field theory, its computation requires the handling of relative modular Hamiltonians, which are typically very difficult to compute explicitly. In this paper, we exploit locality properties of general algebraic QFTs to estimate relative modular Hamiltonians between two states, $ω$ and $\tildeω$, and hence also their relative entropy, in terms of the modular Hamiltonian of a reference state $\hatω$, which might be better understood. For suitable pairs of states we can estimate the relative modular Hamiltonian for the algebra of a region $V_2$ from above, resp. from below, in terms of the modular Hamiltonian of $\hatω$ on a larger region $V_3$, resp. a smaller region $V_1$. Pairs of states and choices of regions which are susceptible to our scheme are related to the presence of superluminal signalling in the sense of Sorkin's paradox. If $ω=\hatω$, then there exists a unitary that maps $ω$ to $\tildeω$ on $V_3$ and that does not allow superluminal signalling from the spacelike complement $V_3'$ to $V_2$, resp. from $V_1$ to $V_2'$, if our upper, resp. lower, bound applies. To investigate the strength of our estimates we consider coherent states for CCR systems, focussing particularly on free scalar fields. Our estimates apply even if the relative modular Hamiltonian cannot be computed exactly. For sufficiently regular excitations we recover an exact result by squeezing. Our method thus yields an independent proof for the relative entropy formula in cases where the relative modular Hamiltonian cannot be computed exactly. For massless fields we establish the analogous result also for double cone regions. These results indicate that our estimates do not lose too much information.