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arXiv:2511.22827

Delayed Choice Quantum Eraser Experiment Revisited: Causality and Informational Coherence

Taku Ohwada

quant-phSubmitted Nov 28, 2025• Announced Dec 1, 2025• 0 citations• Revised Dec 22, 2025 (v3)
v1 · Nov 28, 2025v2 · Dec 15, 2025v3 · Dec 22, 2025
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Abstract

An operationally well-defined delayed-choice quantum-eraser experiment is proposed, realizing a genuine delayed choice within presently available quantum-optical technology. A multimode quantum memory supplies a controlled and verifiable delay, ensuring that the choice operation is applied strictly after the observation event. Electronic single-photon interference detection measurements furnish a direct statistical discriminator between the causal and informational coherence hypotheses, based solely on marginal detection statistics and without any post-selection.

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An operationally well-defined delayed-choice quantum-eraser experiment is proposed, realizing a genuine delayed choice within presently available quantum-optical technology. A multimode quantum memory supplies a controlled and verifiable delay, ensuring that the choice operation is applied strictly after the observation event. Electronic single-photon interference detection measurements furnish a direct statistical discriminator between the causal and informational coherence hypotheses, based solely on marginal detection statistics and without any post-selection.
Quantum superposition is the cornerstone of quantum mechanics, where interference fringes originate in the self-interference of a single photon via indistinguishable photon characteristics. Wheeler's delayed-choice experiments have been extensively studied for the wave-particle duality over the last several decades to understand the complementarity theory of quantum mechanics. The heart of the delayed-choice quantum eraser is in the mutually exclusive quantum feature violating the cause-effect relation. Here, we experimentally demonstrate the quantum eraser using coherent photon pairs by the delayed choice of a polarizer placed out of the interferometer. Coherence solutions of the observed quantum eraser are derived from a typical Mach-Zehnder interferometer, where the violation of the cause-effect relation is due to selective measurements of basis choice.
Quantum superposition lies at the core of quantum mechanics and has been applied to various quantum technologies. Over the past few decades, Wheeler’s delayed-choice thought experiment has been extensively examined to explore the wave-particle duality of the complementary nature of quantum mechanics. The essence of the delayed-choice quantum eraser lies in the contradictory quantum property that violates the cause-effect relationship in classical physics. Our experiments present empirical evidence of the quantum eraser using coherent photon pairs provided in a noninteracting Mach-Zehnder interferometer for the polarization-basis control of output photons. The observed interference fringe is the direct proof of the quantum eraser, where the violation of the cause-effect relationship is due to the selective choices of the measurement events.
In a delayed-choice quantum eraser, interference fringes are obtained by erasing which-way information after the interfering particle has already been irreversibly detected. Following an introductory review of delayed-choice experiments and quantum erasure, we describe the experimental realization of an optical delayed-choice quantum eraser, suitable for advanced undergraduates, based on polarization-entangled pairs of single photons. In our experiment, the delay of the erasure is implemented using two different setups. The first setup employs an arrangement of mirrors to increase the optical path length of the photons carrying which-way information. In the second setup, we use fiber-optic cables to elongate the path of these photons after their passage through the polarization analyzer but prior to their arrival at the detector. We compare our results to data obtained in the absence of a delay and find excellent agreement. This shows that the timing of the erasure is irrelevant, as also predicted by quantum mechanics. The experiment can serve as a valuable pedagogical tool for conveying the fundamentals of quantum mechanics.
Although it may seem The Delayed Choice experiments contradict causality and one could construct an experiment which could possibly affect the past, using Many World interpretation we prove it is not possible. We also find a mathematical background to Which-path information and show why its obtainability prevents system from interfering. We find a system which exhibit both interference and correlation and show why one-particle interference and correlations are complementary. Better visible interference pattern leads to worse correlations and vice versa. Then, using knowledge gained from Quantum Eraser and Delayed Choice experiments we prove there is not an objective reality in a sense of Einstein, Podolsky and Rosen. Furthermore, we discuss the difference between ``outer'' (non-interacting) and ``inner'' (interacting) observer. We find the mathematical relationship between the ``universal'' wave function used by ``outer'' observer and processes the ``inner'' observer sees, which is our small contribution to the measurement problem.
The counterintuitive features of quantum physics challenge many common-sense assumptions. In an interferometric quantum eraser experiment, one can actively choose whether or not to erase which-path information (a particle feature) of one quantum system and thus observe its wave feature via interference or not by performing a suitable measurement on a distant quantum system entangled with it. In all experiments performed to date, this choice took place either in the past or, in some delayed-choice arrangements, in the future of the interference. Thus, in principle, physical communications between choice and interference were not excluded. Here, we report a quantum eraser experiment in which, by enforcing Einstein locality, no such communication is possible. This is achieved by independent active choices, which are space-like separated from the interference. Our setup employs hybrid path-polarization entangled photon pairs, which are distributed over an optical fiber link of 55 m in one experiment, or over a free-space link of 144 km in another. No naive realistic picture is compatible with our results because whether a quantum could be seen as showing particle- or wave-like behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether.