Post O: The Quantum Switch Experiment and the Reality of Superposition: A Challenge to QBism
Weekend Bonus Post
Author’s Note: This post is a collaborative work between ChatGPT 4o and the author, emerging from a discussion on the Quantum Switch experiment in which the author disagreed with the model’s conclusions. Through debate, the LLM came to align with the author’s interpretation. The author is not an expert in this subject matter and welcomes challenges to the logic and corrections of any errors found.
Title: The Quantum Switch Experiment and the Reality of Superposition: A Challenge to QBism
Abstract:
The Quantum Switch (QS) experiment demonstrates that two classically exclusive orders of events (A then B vs. B then A) can occur simultaneously in a single quantum system, as evidenced by interference patterns in the final photon state. This challenges interpretations of quantum mechanics that treat superposition as epistemic rather than ontological. QBism, which asserts that quantum states represent an observer's subjective beliefs rather than objective reality, struggles to account for the Quantum Switch results. The persistence of interference after decoherence suggests that both histories physically coexisted before measurement, contradicting the QBist framework. This paper analyzes the implications of the QS experiment for QBism and contrasts it with interpretations of quantum mechanics that more naturally accommodate indefinite process order, such as Many-Worlds, Relational Quantum Mechanics, and Objective Collapse theories. The findings suggest that superposition is a physically real phenomenon and that QBism, by treating quantum states as purely epistemic, fails to account for the empirical data.
1. Introduction
The interpretation of quantum mechanics remains one of the most profound unresolved questions in physics. While many interpretations accept quantum superposition as an ontological reality, QBism treats it as a reflection of an agent's subjective knowledge rather than an objective feature of the world.
This raises the fundamental question: Is superposition just a mathematical tool for tracking our uncertainty, or is it a real physical phenomenon?
The Quantum Switch experiment provides a crucial test of this question. It demonstrates a coherent superposition of process order, posing a direct challenge to QBism’s epistemic stance. This paper argues that the results of the Quantum Switch experiment provide empirical evidence that superposition is physically real, rather than merely an update in an agent's beliefs. If correct, this calls into question QBism’s core premise and strengthens interpretations of quantum mechanics that treat the wavefunction as ontologically real.
2. The Quantum Switch Experiment: Empirical Evidence of Indefinite Process Order
At a high level, the Quantum Switch experiment is designed to test whether a quantum system can exist in a superposition of two different process orders at the same time.
Imagine a photon passing through two operations, A and B. In a classical system, the photon must experience either A then B or B then A—one or the other. However, in the Quantum Switch experiment, the photon is placed in a superposition of both process orders simultaneously.
When the photon’s final state is measured, it exhibits an interference pattern that contains information from both process orders. This means that the photon was not in just one order or the other; it was in a genuine quantum superposition of both. The key finding is that this superposition was maintained before measurement, demonstrating that indefinite process order is a real, physical phenomenon.
This is demonstrated by the violation of a causal witness measurement, ruling out any classical explanation in terms of hidden variables or ignorance. Importantly, when which-order information is revealed, the interference disappears. However, the final state of the photon, after interference, carries traces of both process orders, meaning that before measurement, both histories physically contributed to the outcome.
3. QBism’s Epistemic Interpretation of Quantum States
QBism, developed by Christopher Fuchs and colleagues, asserts that the wavefunction does not describe a physically real entity but instead encodes an agent’s subjective probabilities about measurement outcomes. In this view, quantum states are not objective but represent Bayesian belief updates. Measurement is treated as an active process in which the agent updates their knowledge, rather than an event in which a pre-existing reality is revealed.
A useful analogy for QBism is a weather forecast. If the forecast says there is a 70% chance of rain, that probability does not describe an objective “superposition” of rainy and non-rainy worlds. Instead, it reflects our uncertainty about what will happen. QBists argue that quantum states work the same way—before measurement, they do not describe a real superposition, only our incomplete knowledge of the system.
Applying QBism to the Quantum Switch experiment, a QBist would argue that the photon was never truly in both process orders simultaneously. Instead, the interference observed in the final measurement is interpreted as an agent’s probabilistic update upon measurement, rather than evidence of an ontologically real superposition of histories.
4. Where QBism Fails: The Challenge from QS
While QBism allows for a pragmatic interpretation of quantum mechanics, it faces severe difficulties in accounting for the Quantum Switch experiment’s findings.
Persistence of Interference Post-Decoherence: If the photon had only epistemic uncertainty about which process order it took, the final state should not retain interference effects from both histories. The fact that it does suggests that both orders were real prior to collapse.
Violation of the Causal Witness Measurement: The Quantum Switch experiment rules out hidden-variable explanations, indicating that the system was genuinely in an indefinite process order. If superposition were merely a state of belief rather than reality, this violation should not occur.
Dependence of Interference on Prior Superposition: If superposition were purely epistemic, revealing which-order information should not physically erase the interference pattern. The fact that it does strongly suggests that superposition had an objective physical existence before measurement.
Imagine a meteorologist predicts a 70% chance of rain, but it never actually rains. Yet, when you step outside, imagine you see fresh puddles and wet footprints—clear signs that the rain had real physical effects. That is what happens in the QS experiment. If the rain were just a belief update, these traces wouldn’t exist. Similarly, in the QS experiment, the photon’s final state retains evidence of both process orders, proving that superposition was physically real before measurement, rather than just an agent’s probability update.
5. Alternative Interpretations That Better Accommodate QS
Several interpretations of quantum mechanics provide more natural explanations for the Quantum Switch results:
Many-Worlds Interpretation (MWI): The superposition of process orders is real because all histories exist within the universal wavefunction. The Quantum Switch experiment is fully consistent with MWI’s framework, where both process orders exist in parallel branches until decoherence occurs.
*Footnote:* An interesting subtlety arises in the MWI perspective: if all histories exist in separate branches of the universal wavefunction, why does the Quantum Switch experiment leave measurable evidence of both process orders within a single outcome? If strict branching had already occurred, the photon should only reflect a single history in any given measurement. One possible resolution is that before full decoherence, interference effects still allow branches to influence each other—a form of ‘pre-decoherence mixing’ rather than absolute separation. If true, this suggests that Many-Worlds might need to refine how it treats intermediate stages of branching in cases of indefinite process order.*
Relational Quantum Mechanics (RQM): The order of events is relative to the observer, meaning that process order is not an absolute property of reality. This aligns with the QS experiment’s demonstration that process order is indeterminate until measurement.
*Footnote:* A potential challenge for RQM arises from the Quantum Switch experiment. If quantum states are purely relational—existing only in relation to an observer or system—then why does the final photon state retain evidence of both process orders even before an observer extracts information? The persistence of interference effects suggests that superposition had a reality beyond any specific observer’s measurement. This raises the question of whether RQM can fully account for cases where quantum effects leave measurable traces independent of a defined observer-system interaction. If process order is truly observer-relative, why does the final state of the photon appear to 'remember' both interactions? Further exploration is needed to clarify whether RQM can reconcile this result with its core framework.*
Objective Collapse Theories (GRW, Penrose, etc.): These interpretations accept that superposition is real but unstable. If Quantum Switch experiments were performed on larger systems, they could test whether superpositions of process order collapse under specific conditions, providing insights into the boundary between quantum and classical physics.
6. Conclusion: The Reality of Superposition and the Demise of QBism?
The Quantum Switch experiment provides strong empirical evidence that quantum superposition is an ontological feature of reality, not merely an agent’s epistemic uncertainty. The presence of interference effects from both process orders, even after decoherence, is inconsistent with QBism’s claim that quantum states merely encode subjective probabilities. While interpretations such as MWI, RQM, and Objective Collapse theories provide viable explanations for the Quantum Switch results, QBism appears increasingly untenable in light of this evidence.
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The below discussion is extremely lengthy but is the record of the long and winding road to how the ideas for the above paper came about. It is not necessary to read it to grasp the ideas in the paper, but it might help and there are some other interesting points raised. (I think seeing the whole process is also interesting in and of itself).
If you have anything you would like to see discussed in this Substack please don’t hesitate to reach out at editor@unconsciousconstructs.com! And, If this resonated with you, consider sharing it with someone else who might enjoy it!
Note: The indented text highlighted in grey are my prompts and the un-highlighted text is that of the LLM.
