---
title: Reeh–Schlieder Approximation for Coherent States
url: https://www.emergentmind.com/topics/reeh-schlieder-approximation-for-coherent-states
type: topic
---

# Reeh–Schlieder Approximation for Coherent States

The Reeh–Schlieder approximation for coherent states formalizes and makes explicit the deep nonlocal properties of relativistic quantum field theory (QFT), specifically the ability to approximate any coherent state of a free scalar field by operators localized in the causal complement of a given region. This construction renders the cyclicity aspect of the Reeh–Schlieder theorem operationally explicit for coherent states and underpins rigorous quantum measurement bounds in local QFT, as realized in recent work by Falcone & Conti and its applications to photodetection scenarios [2509.09021], [2601.10354]. Additionally, related protocols in nonrelativistic and remote state preparation contexts demonstrate the generality of the underlying structures [1501.01407].

## 1. Fundamentals: Coherent States and Weyl Operators

A coherent state $\lvert f\rangle$ in a free scalar field theory is defined as the action of a Weyl (displacement) operator on the vacuum:
\[
\hat W(f) = \exp\left[i\,\hat\phi(f)\right],\quad \hat\phi(f) = \int d^4x\,f(x)\,\hat\phi(x),\quad \lvert f\rangle = \hat W(f)\lvert\Omega\rangle,
\]
where $f\in\mathscr S(\mathbb R^4)$ is a real-valued Schwartz test function. Weyl operators satisfy the algebra
\[
\hat W(f)\,\hat W(g) = \exp\left(-\frac{i}{2}[\hat\phi(f),\hat\phi(g)]\right) \hat W(f+g).
\]
The Minkowski vacuum $\lvert\Omega\rangle$ is cyclic and separating for all local algebras $\mathfrak A(\mathcal U)$ associated to bounded open regions $\mathcal U$ in spacetime [2509.09021], [2601.10354].

## 2. Formal Statement of the Reeh–Schlieder Approximation for Coherent States

Given any bounded open region $\mathcal U \subset \mathbb R^4$, with causal complement $\mathcal U'$, and any test function $f$, there exists a family of bounded operators $\hat A_\zeta(f) \in \mathfrak A(\mathcal U')$ such that
\[
\lim_{\zeta\to 0} \|\hat A_\zeta(f)\lvert\Omega\rangle - \lvert f\rangle\| = 0.
\]
This provides a fully constructive and localized version of the Reeh–Schlieder property for coherent states: the coherent state $\lvert f\rangle$ can be approximated arbitrarily well by applying operators entirely outside the support of $f$ [2509.09021].

## 3. Explicit Local Construction of Approximating Operators

The core construction involves several key steps:
- Choose coordinates so that $\mathcal U$ lies within the left Rindler wedge $\mathcal W_L = \{x^1 < -|x^0|\}$; its causal complement is the right wedge $\mathcal W_R$.
- If $\operatorname{supp} f \subset \mathcal U''$, define the spacetime reflection $f\circ J(x) = f(-x^0, -x^1, x^2, x^3)$; $f\circ J$ is now supported in $\mathcal W_R$.
- Let $\Lambda_1(\eta)$ denote a boost in the $x^1$ direction. Analytic continuation via the Bisognano–Wichmann theorem gives
  \[
  \hat U[\Lambda_1(\eta + i\pi)] \hat W(f\circ J)\lvert\Omega\rangle = \hat W\left[f\circ\Lambda_1(-\eta)\right]\lvert\Omega\rangle,\quad \eta\in\mathbb R,
  \]
  where $\hat U$ is the unitary implementing the Lorentz boost.
- For a real analytic “mollifier” $G_\zeta(\eta)$, typically $G_\zeta(\eta) = (2\pi\zeta)^{-1/2}\exp(-\eta^2/(2\zeta))$, define
  \[
  \hat A_\zeta(f) = \int_\mathbb{R} d\eta\, G_\zeta(\eta - i\pi)\, \hat W(f\circ J \circ \Lambda_1(-\eta)).
  \]
Each $\hat A_\zeta(f)$ is localized in $\mathcal W_R \subset \mathcal U'$ and is bounded [2509.09021], [2601.10354]. When $\operatorname{supp} f$ is not contained in $\mathcal U''$, a time-slice argument and partition of unity allow the same logic to be applied via a decomposition $f_0 = \chi f_0 + (1-\chi)f_0$ with appropriate smooth cutoff $\chi$.

## 4. Convergence, Error Estimates, and Trade-offs

Action on the vacuum gives
\[
\hat A_\zeta(f)\lvert\Omega\rangle = \int_{\mathbb R} d\eta\, G_\zeta(\eta)\, \hat W\left[f\circ\Lambda_1(-\eta)\right] \lvert\Omega\rangle.
\]
The approximation error is
\[
\mathcal E_\zeta(f) = \sqrt{1 - \int d\eta\,[2G_\zeta(\eta) - G_{2\zeta}(\eta)]\, \exp\left(W_2[f, f\circ\Lambda_1(\eta)] - W_2(f, f)\right)},
\]
where $W_2(f_1, f_2)$ is the vacuum two-point Wightman functional. Since $G_\zeta \to \delta$, $\mathcal E_\zeta(f) \to 0$ as $\zeta \to 0$. The operator norm satisfies $\|\hat A_\zeta(f)\| \leq \exp(\pi^2/(2\zeta))$.

In quantum measurement applications, this leads to a trade-off bound for any local POVM element $\hat E_{\mathrm{click}}$ with support in $\mathcal O_{\mathrm{det}}$:
\[
P_{\mathrm{click}}(f) \leq \min_{\zeta>0} \left[\mathcal E_\zeta(f) + e^{\pi^2/(2\zeta)}\sqrt{P_{\mathrm{dark}}}\right]^2,
\]
where $P_{\mathrm{click}}$ and $P_{\mathrm{dark}}$ are the probabilities for click and vacuum-induced dark count, respectively. Lowering $P_{\mathrm{dark}}$ necessarily shrinks the maximal attainable $P_{\mathrm{click}}(f)$ [2601.10354].

## 5. Modeling Detector Response and Quantum Measurement Bounds

To interface with practical setups, especially in quantum optics, the region $\mathcal O_{\mathrm{det}}$ is modeled as a spacetime right-square prism, operated over a time window $\tau$, thickness $l$, and base $L\times L$. The support function $\chi$ encodes the actual detectable spacetime region, smoothed near the boundaries.

An explicit single-mode, normally incident coherent state is parameterized by $\alpha(\mathbf k) = \alpha_0\, \delta^3(\mathbf k - \mathbf k_0)/\sqrt{V_{\mathrm{coh}}}$ in the narrow-band limit, where $N = |\alpha_0|^2 (l+\tau)(L+\tau)^2 / V_{\mathrm{coh}}$ denotes effective photon number, $\Delta\varphi = k_0 (l+\tau)$ counts optical wavelengths across the detector, and $a = (l+\tau)/(L+\tau)$ is the aspect ratio. Numerically minimizing the measurement bound with respect to $\zeta$ illustrates that
- $P_{\mathrm{click},\mathrm{max}}$ is suppressed as $P_{\mathrm{dark}}$ decreases;
- Increasing $N$ raises $P_{\mathrm{click},\mathrm{max}}$;
- Small $\Delta\varphi \ll 1$ or small $a \ll 1$ degrade $P_{\mathrm{click},\mathrm{max}}$ due to undersampling or thin geometry;
- The phase dependence becomes negligible for $\Delta\varphi \gtrsim 10$ but matters for small optical thickness [2601.10354].

## 6. Extensions to Remote State Preparation and Generalizations

Remote state preparation in QFT leverages the Reeh–Schlieder property to create a desired state (e.g., a coherent state) in a target region $A$ by performing suitable operations (via detectors or sources) in the complement $B$. In relativistic QFT, this requires using superoscillatory functions in time to match the frequency requirements of the desired coherent state profile $\alpha(\mathbf k)$, as conventional Fourier uncertainty does not permit exact matching in finite time for all $k$. The design involves synthesizing window functions $\epsilon_j(t)$ such that, after postselecting detector outcomes, the resulting operator matches $D_A[\alpha]$ up to an arbitrarily small error, at the expense of exponentially small success probability as a function of fidelity, bandwidth, and spatial separation [1501.01407]. The techniques extensively use the algebraic structure of field operators and superoscillatory window synthesis.

A plausible implication is that the Reeh–Schlieder approximation for coherent states, with its explicit localization and error control, constitutes a fundamental tool for both foundational analysis of locality in QFT and for setting rigorous bounds in realistic quantum measurement theory in relativistic settings.

## 7. Significance, Limitations, and Physical Insights

Explicit Reeh–Schlieder approximation schemes reveal the operational power and limitations inherent to local quantum field measurements. They show that the vacuum is cyclic not just abstractly but with fully controllable, explicit constructions for a wide class of states (here, coherent states). For experiment, the fundamental bound on distinguishing the vacuum from excitations within any finite region—not just in principle, but quantitatively and constructively—is now accessible.

However, the approximation scheme is rooted in free (Gaussian) field theory with wedge-modular structure matching Lorentz boosts. For interacting fields or those with more complex modular localization, extensions require further analysis. The resource overhead for remote state preparation grows rapidly with fidelity and separation, due to superoscillatory amplification cost, limiting practical applicability though not in principle [1501.01407].

Overall, the Reeh–Schlieder approximation for coherent states bridges constructive algebraic QFT, quantum measurement theory, and operational scenarios in quantum information, with broad implications for fundamental limits of locality, measurement, and remote state control in quantum fields [2509.09021], [2601.10354], [1501.01407].

Source: https://www.emergentmind.com/topics/reeh-schlieder-approximation-for-coherent-states