Path-Erasing Basis in Quantum Interference

• Path-Erasing Basis is a measurement framework that erases which-path information via superposition, enabling coherent interference in quantum field setups like WIT.
• It enables controlled constructive or destructive interference by matching gap-to-acceleration ratios and adjusting phases, analogous to Λ-type EIT in quantum optics.
• Finite-time effects introduce tolerance windows in parameter matching, affecting amplitude overlap and the resultant detector response in practical implementations.

A path-erasing basis is a measurement basis that eliminates which-path information in quantum interferometry, enabling coherent addition of amplitude contributions from multiple disjoint physical pathways. In the context of relativistic quantum field theory, it plays a fundamental role in interference experiments involving alternative accelerated worldlines of particle detectors, such as those described in the framework of Worldline-Induced Transparency (WIT). In WIT, post-selecting the detector-ancilla composite system in the path-erasing basis destroys all information about which branch (worldline) the detector traversed, allowing branch amplitudes to combine coherently and giving rise to conditional suppression or restoration of Unruh-type responses depending on the relative phase and parameter matching.

1. Path-Erasing Basis: Definition and Construction

The path-erasing basis is constructed from superpositions of orthogonal ancilla states, which encode the "which-path" information for physically distinct worldline branches. Given a labeling ancilla with basis $\{\ket{a_1}, \ket{a_2}\}$ corresponding to two possible branches traversed by the detector, the path-erasing (or "dark-state") basis is defined as: $$\ket{+} = \frac{1}{\sqrt{2}}(\ket{a_1}+\ket{a_2}), \qquad \ket{-} = \frac{1}{\sqrt{2}}(\ket{a_1}-\ket{a_2})$$ Postselection onto $\ket{+}$ ensures that no information about the branch index persists, erasing all which-path distinguishability.

2. Role in Quantum Interference with Alternative Worldlines

In the WIT setup, a single Unruh-DeWitt detector is initialized in a coherent state with amplitudes $\alpha_k$ along each worldline (branch $k$ with corresponding acceleration $a_k$ and energy gap $\omega_k$). After the interaction with the Minkowski vacuum, the final state is measured in the path-erasing basis, enforcing indistinguishability between excitation events on either branch. The total amplitude in the $\ket{+}$ channel becomes: $$A_\text{total} = \alpha_1 \mathcal{I}_1 + \alpha_2 \mathcal{I}_2$$ where $\mathcal{I}_k$ are the on-shell transition amplitudes for each branch, and the phases $\alpha_k$ can be tuned to enforce destructive or constructive interference.

3. Destructive Interference and Suppression Conditions

Coherent addition in the path-erasing basis enables complete cancellation of the first-order excitation amplitude if two conditions are met:

1. Gap-to-acceleration ratio matching:

$$\frac{\omega_1}{a_1} = \frac{\omega_2}{a_2} \equiv \Lambda$$

2. Phase cancellation:

$\theta = \pi + \Lambda \log(a_1/a_2) \pmod{2\pi}$

With these conditions, amplitudes for excitation on both branches add with opposite phase, leading to $A_\text{total}=0$ and suppressing the Unruh detector response. Reversing the phase restores the response, demonstrating amplitude-level control.

4. Operational Indistinguishability and Orthogonality Structure

When measured in the path-erasing basis, the detector's excitation events become operationally indistinguishable, meaning that the physical processes along different worldlines cannot be discerned from the measurement outcome. In the Unruh-mode formalism, only branches exciting identical mode labels ($\Omega_1 = \Omega_2$) interfere; otherwise, contributions are orthogonal. In the Minkowski plane-wave basis, orthogonality arises from distinct spectral supports generated by mismatched $\omega_k/a_k$ ratios. Both approaches confirm that only amplitude contributions with coincident Unruh-mode resonance are subject to path-erased interference.

5. Finite-Time Effects and Tolerance Windows

Exact suppression is an idealization. With finite interaction duration $T$ (e.g., Gaussian switching), each branch excites a band of Unruh labels of width $\Delta \Omega_k \sim 1/(a_k T)$. The degree of amplitude overlap—and interference—is governed by

$$\exp\left[ -\frac{a_1^2 a_2^2 T^2}{2(a_1^2 + a_2^2)} (\Delta \Lambda)^2 \right]$$

with $\Delta\Lambda = (\omega_1/a_1 - \omega_2/a_2)$. Significant suppression is achieved only when

$$|\Delta\Lambda| \lesssim \frac{\sqrt{a_1^2 + a_2^2}}{a_1 a_2} \frac{1}{T}$$

defining a finite tolerance window and quantifying the residual signal when matching is imperfect.

6. Analogy to Λ-Type Electromagnetically Induced Transparency

WIT and the use of a path-erasing basis is directly analogous to Λ-type EIT in quantum optics. There, destructive interference between two ground-state pathways to a common excited state renders the medium transparent. In WIT, two disjoint worldlines function as the ground-state legs, the common excited state is the detector excitation, the vacuum field is the reservoir, and post-selection onto the path-erasing basis produces the analogue of the dark-state superposition. The matching condition $\omega_1/a_1 = \omega_2/a_2$ corresponds to two-photon resonance.

7. Theoretical Significance and Computational Formalisms

The utility of the path-erasing basis in the analysis of quantum interference between alternative worldlines is manifested in both Unruh-mode expansions and Minkowski plane-wave bases. In both cases, the physical requirement of matching gap-to-acceleration ratios and appropriate phase tuning is necessary for interference, reflecting mode orthogonality and operator-level indistinguishability, respectively. The path-erasing basis thus shifts traditional interpretations of the Unruh effect, demonstrating that detector excitation rates are amplitude-level quantities modifiable by quantum superposition and measurement postselection (Azizi, 23 Jan 2026).