It is demonstrated that ‘quantum eraser’ (QE) experiments do not erase any information. Nor do they demonstrate retrocausation or ‘temporal nonlocality’ in their ‘delayed choice’ form, beyond standard EPR correlations. It is shown that the erroneous erasure claims arise from assuming that the improper mixed state of the signal photon physically prefers either the ‘which way’ or ‘both ways’ basis, when no such preference is warranted. The latter point is illustrated through comparison of the QE spatial state space with the spin-1/2 space of particles in the EPR-spin experiment.
It is demonstrated that ‘quantum eraser’ (QE) experiments do not erase any information. Nor do they demonstrate retrocausation or ‘temporal nonlocality’ in their ‘delayed choice’ form, beyond standard EPR correlations. It is shown that the erroneous erasure claims arise from assuming that the improper mixed state of the signal photon physically prefers either the ‘which way’ or ‘both ways’ basis, when no such preference is warranted. The latter point is illustrated through comparison of the QE spatial state space with the spin-1/2 space of particles in the EPR-spin experiment.
Ever since the proposal of delayed choice quantum erasure and subsequent realization in the experiment by Kim et al., the interpretation and implications of delayed-choice experiments have remained a subject of intense foundational debate. This paper resolves the apparent paradox attached to the experiment using standard quantum mechanics. Using an extended Mach-Zehnder interferometer which captures every operational feature of the original experiment, we show that choosing between which-path and erasure detectors is simply a choice of measurement bases, which does not rewrite a photon's past. Furthermore, by mapping the experiment to a two-way Stern-Gerlach framework, we prove that quantum erasure is an expected result of measuring entangled states, not a physical anomaly. Ultimately, through a pedagogical game, we illustrate that the illusion of retrocausality arises from asking illegitimate questions, and that a forward-in-time description is entirely sufficient to explain the logic.
Abstract The delayed-choice quantum eraser has long been a subject of controversy, and has been looked at as being incomprehensible to having retro-causal effect in time. Here the delayed-choice quantum eraser is theoretically analyzed using standard quantum mechanics. Employing a Mach–Zehnder interferometer, instead of a conventional two-slit interference, brings in surprising clarity. Some common mistakes in interpreting the experiment are pointed out. It is demonstrated that in the delayed mode there is no which-way information present after the particle is registered on the screen or the final detectors, contrary to popular belief. However, it is shown that another kind of path information is present even after the particle is registered in the final detectors. The registered particle can be used to predict the results of certain yet to be made measurements on the which-way detector. This novel correlation can be tested in a careful experiment. It is consequently argued that there is no big mystery in the experiment, and no retro-causal effect whatsoever.
According to Quantum Mechanics, the particles can exhibit either particle properties or wave properties depending on the experimental set up (wave-particle dualism). A special behavior occurs for a system of two entangled particles 1 and 2 that propagate along two different directions in the space. In such a case, the entangled particle 2 should exhibit either the particle behavior or the wave behavior depending on the kind of measurement that is performed on particle 1 whatever is the actual distance between the two particles. The apparently surprising fact is that the "choice" of what kind of measurement is performed on particle 1 can be also made when particle 2 has been already detected (delayed-choice quantum eraser).These theoretical predictions have been confirmed by the experiments and seem to suggest that a future measurement can affect a past event. Recently, both the concepts of "delayed choice" and of "quantum erasure" have been criticized by Ellerman and by Kastner. In this paper we propose an alternative analysis of the delayed-choice quantum eraser with two entangled particles and we show that it does not imply an inversion of the natural cause-effect temporal order of the physical events.
Introduction In recent years, numerous quantum optics experiments have demonstrated novel quantum effects based on quantum amplitude superposition. Some of these empirical observations raise disturbing questions about the nature of reality, particularly concerning quantum nonlocality. In this chapter we discuss some experiments that suggest (to us, at least) that the information void, that uncharted regime between signal preparation and outcome detection, is not well modeled by classical spacetime. Something strange seems to be lurking there. For the purposes of exposition, the sequence of experiments we discuss in this chapter does not follow the historical sequence of those experiments, We discuss first the delayed-choice quantum eraser experiment of Kim et al. (Kim et al., 2000), referred to here as KIM. This leads us to define a heuristic measure of which-path information that is central to the theme of this chapter. This leads to a discussion of Wheeler's thought experiments on the conundrum of delayed choice in physics, particularly evident in the phenomenon of galactic lensing (Wheeler, 1983), and its empirical implementation, the delayed-choice interferometer experiment of Jacques et al., referred to as JACQUES. Our final experiment in this sequence is the double-slit quantum eraser of Walborn et al. (Walborn et al., 2002), referred to as WALBORN. That experiment gives a significant challenge for quantized detector networks (QDN) to reproduce its empirical results. Such experiments have led to suggestions that interference patterns formed by particles impacting on a screen may be influenced in some way by decisions made long after those particles had landed on that screen. Our objective in this chapter is to show by a detailed QDN analysis of those experiments that quantum principles do not support these suggestions. Nevertheless, something bizarre seems to be there. There are two complementary aspects in these experiments that have deeply worried physicists. The first involves spatial nonlocality and is the point of Wheeler's concerns. Historically, it has been a great source of debate among physicists.
The quantum eraser variant of the double-slit experiment, and its 'delayed choice' sub-variant, are considered from the perspective of weak value and weak measurement theory (which is briefly reintroduced here). The interference fringes that appear when measuring certain spin states, which can then be 'erased' when measuring other spin states, are shown to be anomalous weak values that depend on particular post-selection choices. By framing the choice of spin measurement as a weak value of a certain weak measurement, it is then made clear what physical claims can and cannot be made about what occurs in the quantum eraser experiment. Specifically, claims about the choice of spin-state `retrocausally' influencing the choice of slit(s) for the particles to travel through are discredited, and a simple framework is presented for understanding how the fringes arise and why they can be 'erased'.
The delayed-choice quantum eraser represents an interesting experiment that exemplifies Bohr's principle of complementarity in a beautiful way. According to the complementarity principle, in a two-path interference experiment, the knowledge of which path was taken by the particle and the appearance of interference are mutually exclusive. Even when the which-path information is merely retained in specific quantum path-markers, without being actually read, it suffices to eliminate interference. Nevertheless, if this path information is erased in some manner, the interference re-emerges, a phenomenon referred to as the quantum eraser. An intriguing aspect of this experiment is that if the path information is erased after the particle has been detected on the screen, the interference still reappears, a phenomenon known as the delayed-choice quantum eraser. This observation has led to the interpretation that the particle can be influenced to exhibit characteristics of either a particle or a wave based on a decision made long after it has been registered on the screen. This idea has sparked considerable debate and discussions surrounding retrocausality. This controversy is reviewed here, and a detailed resolution provided. Quanta 2025; 14: 66–74.