Leonardo Castellani
We argue that Wigner's friend thought experiment does not support observer dependence of quantum states. An analysis in terms of history vectors suggests that quantum collapse is to be understood as collapse of histories rather than collapse of states.
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The measurement problem is seen as an ambiguity of quantum mechanics, or, beyond that, as a contradiction within the theory: Quantum mechanics offers two conflicting descriptions of the Wigner's-friend experiment. As we argue in this note there are, however, obstacles from within quantum mechanics and regarding our perspective onto doing physics towards fully describing a measurement. We conclude that the ability to exhaustively describe a measurement is an assumption necessary for the common framing of the measurement problem and ensuing suggested solutions.
Classical information and collapse in Wigner's friend setups
该研究在维格纳朋友实验中引入维格纳与朋友间的经典通信，通过调控准经典信道的信息泄露量，实现了从悖论性、可违反定域友好性不等式的情形到有效坍缩情形的平滑过渡。
What Do Black Holes Teach Us About Wigner's Friend?
本文探讨黑洞悖论与维格纳朋友悖论的联系，指出黑洞悖论支持内在关系性而非有效关系性，并暗示可能的逆行因果性。
Wigner's friend's memory and the no-signaling principle
本文研究了维格纳的朋友实验中记忆变化与无通信原则的冲突，指出朋友对记忆变化的意识可通过额外寄存器建模，但该模型违反了无通信条件。
We present a discussion of the extended Wigner's friend thought experiment proposed by Frauchiger and Renner in [1]. We show by using various arguments, including textbook quantum mechanics and the ontological approach of Contexts, Systems, Modalities (CSM), that no contradiction arises if one admits that agents must agree on what is considered as a system and what is not. In such a contextually objective approach of quantum mechanics, the apparent contradiction is automatically removed. We also discuss why this mutual agreement between agents is already implicit in the standard formulations o…
The original Wigner's-Friend scenarios
本文探讨了韦igner朋友场景的不同解释，通过去相干理论提供了对悖论的统一解决方法。
Proietti et al. (arXiv:1902.05080) reported on an experiment designed to settle, or at least to throw light upon, the paradox of Wigner's friend. Without questioning the rigor or ingenuity of the experimental protocol, I argue that its relevance to the paradox itself is rather limited.
Reassessing the strength of a class of Wigner's friend no-go theorems
本文分析了“观察者独立事实的no-go定理”与“Local Friendliness no-go定理”，指出其存在缺陷，结论是它们未能对物理实在本质施加重大约束。
The firewall paradox is Wigner's friend paradox
本文指出黑洞物理中的防火墙悖论依赖于允许不同观察者结合视角的隐含规则，而无需引力的扩展维格纳朋友悖论已证明该规则与量子理论冲突，表明两者同源。
Foundational puzzles on quantum universality: From Wigner's friend to black holes
本论文探讨量子理论中的测量问题与黑洞信息悖论，提出扩展维格纳的朋友论证及统一悖论，并分析叠加黑洞的探测器响应，深化对量子实在性和引力的理解。
Physicians define death as the "irreversible" breakdown of all brain-functions including brain-stem. By "irreversible" they mean a damage that is beyond the human capacity to restore the patient's healthy state. In the same line I propose to complete the definition of quantum physics in [1] by Principle D (Detection): "Detection outcomes (like death) are ordinarily irreversible and observer-independent". It is then argued that this principle excludes generalization of quantum superposition to visible objects bearing observer-dependent outcomes. However this exclusion is not absolute: It rather…
It is shown that the absence of an objective existence of the results of quantum measurements cannot be proved by known experiments. There are also general arguments confirming this conclusion.
This is a comment on the paper by Hagar and Hemmo (quant-ph/0512095) in which they suggest that information-theoretic approaches to quantum theory are incomplete.
The extended Wigner's friend problem deals with two Observers each measuring a sealed laboratory in which a friend is making a quantum measurement. We investigate this problem by relying on the basic rules of quantum mechanics as exposed by Feynman in the well-known "Feynman Lectures on Physics". Although recent discussions have suggested that the extended Wigner's friend problem cannot consistently be described by quantum theory, we show here that a straightforward application of these standard rules results in a non-ambiguous and consistent account of the measurement outcomes for all agents …
The quantum measurement problem can be regarded as the tension between the two alternative dynamics prescribed by quantum mechanics: the unitary evolution of the wave function and the state-update rule (or "collapse") at the instant a measurement takes place. The notorious Wigner's friend gedankenexperiment constitutes the paradoxical scenario in which different observers (one of whom is observed by the other) describe one and the same interaction differently, one --the Friend-- via state-update and the other --Wigner-- unitarily. This can lead to Wigner and his friend assigning different prob…
In a joint paper Jeff Bub and Itamar Pitowski argued that the quantum state represents `the credence function of a rational agent [...] who is updating probabilities on the basis of events that occur'. In the famous thought experiment designed by Wigner, Wigner's friend performs a measurement in an isolated laboratory which in turn is measured by Wigner. Here we consider Wigner's friend as a rational agent and ask what her `credence function' is. We find experimental situations in which the friend can convince herself that updating the probabilities on the basis of events that happen solely in…
Considering a complicated extension of a Wigner's friend scenario, Frauchiger and Renner (FR) allegedly showed that "quantum theory cannot consistently describe the use of itself". However, such a result has been under severe criticism, as it has been convincingly argued to crucially depend on an implicit, non-trivial assumption regarding details of the collapse mechanism. In consequence, the result is not as robust or general as intended. On top of all this, in this work we show that a much simpler arrangement--basically an EPR setting--is sufficient to derive a result fully analogous to that…
"Wigner's friend" refers to a quantum process of which different observers, following the rules of quantum mechanics, give contradictory descriptions. Lostaglio and Bowles have recently claimed to describe a classical system showing the same effect. It is argued that this claim is not justified. it fails to take into account the different meanings of probability in classical and quantum mechanics.
According to QBism, quantum states, unitary evolutions, and measurement operators are all understood as personal judgments of the agent using the formalism. Meanwhile, quantum measurement outcomes are understood as the personal experiences of the same agent. Wigner's conundrum of the friend, in which two agents ostensibly have different accounts of whether or not there is a measurement outcome, thus poses no paradox for QBism. Indeed the resolution of Wigner's original thought experiment was central to the development of QBist thinking. The focus of this paper concerns two very instructive mod…
Conditional probabilities of measurements, quantum time and the Wigner's friend case
本文在无时间量子形式体系中，利用 Gleason-Bush 定理和 POVM 推导非相对论相继测量的单时条件概率，并据此澄清维格纳朋友情景不存在悖论。
Recent developments in elementary quantum mechanics have seen a number of extraordinary claims regarding quantum behaviour, and even questioning internal consistency of the theory. These are, we argue, different disguises of what Feynman described as quantum theory's "only mystery".
The place and role of an Observer in quantum mechanics has been a subject of an ongoing debate since the theory's inception. Wigner brought this question to the fore in a celebrated scenario in which a super-Observer observes a Friend making a measurement. Here we briefly review why this \e{Wigner Friend scenario} has been taken to require the introduction of the Observer's consciousness, or alternatively to show the inconsistency of quantum measurement theory. We will argue that quantum theory can consistently leave observers outside its narrative, by making only minimal assumptions about how…
A general argument is presented against relativistic, unitary, single-outcome quantum mechanics. This is achieved by combining the Wigner's Friend thought experiment with measurements on a Greenberger-Horne-Zeilinger (GHZ) state, and describing the evolution of the quantum state in various inertial frames. Assuming unitary quantum mechanics and single outcomes, the result is that the Born rule must be violated in some inertial frame: in that frame, outcomes are obtained for which no corresponding term exists in the pre-measurement wavefunction.
From Unitarity to Irreversibility: The Role of Infinite Tensor Products and Nested Wigner's Friends
本文探讨了从可逆量子过程到不可逆过程的转变，通过无限张量积与嵌套维格纳朋友场景的关系，揭示了幺正等价性的破坏机制。
On the significance of Wigner's Friend in contexts beyond quantum foundations
本文论证维格纳朋友悖论的核心不依赖于量子物理，可通过经典思想实验重现，并指出其本质是“限制A”：物理理论无法对所有观测者给出概率描述，该限制也出现在宇宙学玻尔兹曼大脑问题中。
Wigner Friend scenarios -- in which an external agent describes quantum mechanically a laboratory in which a Friend is making a measurement -- give rise to possible inconsistencies due to the ambiguous character of quantum measurements. In this work, we investigate Wigner Friend scenarios in which the external agents can probe in a non-invasive manner the dynamics inside the laboratories. We examine probes that can be very weakly coupled to the systems measured by the Friends, or to the pointers or environments inside the laboratories. These couplings, known as Weak Measurements, are asymptoti…
Questioning the absoluteness of free choices when internalized in Wigner's friend scenarios
本文探讨了在威格纳朋友实验框架下自由选择的绝对性问题，基于Pusey-Barrett-Rudolph定理提出论证，挑战局部性假设下的绝对性结论。
A scalable tripartite Wigner's friend scenario
本文构建了一个包含三个实验室的可扩展维格纳朋友场景，通过Q#量子模拟和IBM量子计算机验证了布鲁克纳的不可能定理，模拟结果与理论一致，而量子计算机因CNOT门等导致误差略高。
In 1962, Eugene P. Wigner introduced a thought experiment that highlighted the incompatibility in quantum theory between unitary evolution and wave function reduction in a measurement. This work resulted in a class of thought experiments often called Wigner's Friend Scenarios, which have been providing insights over many frameworks and interpretations of quantum theory. Recently, a no-go theorem obtained by Daniela Frauchiger and Renato Renner brought attention back to the Wigner's Friend and its potential of putting theories to test. Many answers to this result pointed out how timing in the t…
Agreement and Compatibility in Wigner's Friend Paradox
本文通过贝叶斯视角重新审视维格纳朋友悖论，论证维格纳与朋友的描述在经典和量子层面均无矛盾，并揭示兼容性与一致性是理解该悖论的关键。
Extended Wigner's friend paradoxes do not require nonlocal correlations
该研究表明扩展的维格纳朋友悖论无需非局域关联，通过利用非语境性失效证明构建不可能定理，依赖新的交换无关性假设，为测量问题研究提供新视角。
The purpose of this comment is to show that a reinterpretation of the results from the Letter: "Physics and Metaphysics of Wigner's Friends: Even performed pre-measurements have no results" allows for reaching the conclusion "pre-measurements have no resul" [arXiv:2003.07464] based only on postulates of quantum mechanics without additional assumptions on irreversibility. Additionally, with supplementary reasoning based on Bell's theorem, one can show that unitary decoherence cannot be solely responsible for the quantum-to-classical transition, and an additional irreversibility model is require…
Extended Wigner's Friend Scenarios with Agent-like Observers on Quantum Computers
本研究在量子计算机上实现具有类智能体功能的维格纳朋友场景，验证局部友好性不等式的量子违背，为未来使用人工智能体进行LF测试奠定基础。
"The unambiguous account of proper quantum phenomena must, in principle, include a description of all relevant features of experimental arrangement" (Bohr). The measurement process is composed of pre-measurement (quantum correlation of the system with the pointer variable), and an irreversible decoherence via interaction with an environment. The system ends up in a probabilistic mixture of the eigenstates of the measured observable. For the pre-measurement stage, any attempt to introduce an 'outcome' leads, as we show, to a logical contradiction, $1=i$. This nullifies claims that a modified co…
The Wigner's friend paradox concerns one of the most puzzling problems of quantum mechanics: the consistent description of multiple nested observers. Recently, a variation of Wigner's gedankenexperiment, introduced by Frauchiger and Renner, has lead to new debates about the self-consistency of quantum mechanics. At the core of the paradox lies the description of an observer and the object it measures as a closed system obeying the Schrödinger equation. We revisit this assumption to derive a necessary condition on a quantum system to behave as an observer. We then propose a simple single-photon…
The notorious Wigner's friend thought experiment (and modifications thereof) has in recent years received renewed interest especially due to new arguments that force us to question some of the fundamental assumptions of quantum theory. In this paper, we formulate a no-go theorem for the persistent reality of Wigner's friend's perception, which allows us to conclude that the perceptions that the friend has of her own measurement outcomes at different times cannot "share the same reality", if seemingly natural quantum mechanical assumptions are met. More formally, this means that, in a Wigner's …
Logic meets Wigner's Friend (and their Friends)
本文从认知逻辑与量子力学相容性角度重审 Wigner 的朋友及 FR 悖论，分析观察者纳入、状态指派和知识转移条件，并提出形式化方案以原则性解决此类悖论。
Connecting extended Wigner's friend arguments and noncontextuality
本文建立了扩展维格纳朋友论证与科亨-施佩克非上下文性之间的联系，利用后者推导新的本地友好性不等式，并证明在广泛场景中本地友好性多面体等于贝尔多面体，同时将可能性非上下文性论证转化为本地友好性不可行性证明。
Allowing Wigner's friend to sequentially measure incompatible observables
本文提出一种新的维格纳的朋友场景，其中超级观察者可重复与朋友交互，并证明在此类场景下局部友好性不等式与贝尔不等式等价。
An experiment by Proietti {\it et al} purporting to instantiate the `Wigner's Friend' thought experiment is discussed. It is pointed out that the stated implications of the experiment regarding the alleged irreconcilability of facts attributed to different observers warrant critical review. In particular, violation of a Clauser-Horne-Shimony inequality by the experimental data actually shows that the attribution of measurement outcomes to the ``Friends'' (modeled by internal photons undergoing unitary interactions) is erroneous. An elementary but often overlooked result regarding improper mixt…
The original Wigner's friend paradox is a gedankenexperiment involving an observer described by an external agent. The paradox highlights the tension between unitary evolution and collapse in quantum theory, and is sometimes taken as requiring a reassessment of the notion of objective reality. In this note however we present a classical toy model in which (i) The contradicting predictions at the heart of the thought experiment are reproduced (ii) Every system is in a well-defined state at all times. The toy model shows how puzzles such as Wigner's friend's experience of being in a superpositio…
From observer-dependent facts to frame-dependent measurement records in Wigner friend scenarios
该研究在相对论背景下提出模型，表明维格纳友人场景中支持观察者依赖事实的假设可导致测量记录依赖于惯性参考系，且不同参考系观察者的记录并非洛伦兹变换。
Wigner's Friend Paradox Revisited
本文提出修正的哥本哈根诠释，结合普适量子态、含波函数坍缩的测量过程及系统状态与观察者认知的区分，解决维格纳的朋友悖论，其概率差异可用于证伪相关分析或修正。
Wigner's friend scenarios: on what to condition and how to verify the predictions
本文分析维格纳朋友实验中不同观察者的态指派差异，提出以“泡泡”刻画不可共享信息，并通过可切换泡泡的博弈说明：基于全部原则可得信息，观察者可采纳并验证其他泡泡的预测。
The notion of measurements is central for many debates in quantum mechanics. One critical point is whether a measurement can be regarded as an absolute event, giving the same result for any observer in an irreversible manner. Using ideas from the gedankenexperiment of Wigner's friend it has been argued that, when combined with the assumptions of locality and no-superdeterminism, regarding a measurement as an absolute event is incompatible with the universal validity of quantum mechanics. We consider a weaker assumption: is the measurement event realised relatively to the observer when he only …
Wigner's friend's black hole adventure: an argument for complementarity?
本文通过结合黑洞物理与威金的朋友场景，提出一个统一的论点，证明不存在能够一致描述黑洞物理的后量子理论。
Making sense of relativistic Wigner friend scenarios: a problem for unitary accounts of quantum measurements ?
该研究探讨相对论性Wigner朋友场景，指出测量态更新的参考系依赖性会导致不同参考系的结果解释不一致，表明将复杂智能体行动建模为简单波函数幺正操作存在根本缺陷。
Any theory that admits a Wigner's Friend type multi-agent paradox is logically contextual
本研究证明任何允许维格纳朋友型多智能体悖论的理论必然具有逻辑情境性，并揭示量子弗劳希格-伦纳悖论为逻辑情境性证明，同时构建了无需贝尔非局域性的新悖论。
The extended Wigner's friend, many- and single-worlds and reasoning from observation
本文探讨了孤立系统的概念及弗拉乌奇格与雷纳的扩展威格纳朋友情景，指出测量过程可能使孤立系统近似失效，且任何涉及主观坍缩的量子理论解释均无法成立，同时指出多重世界与单一世界解释无差异，并强调观察推理需考虑量子擦除的可能性。
This is an analysis of the recently published article `Quantum theory cannot consistently describe the use of itself' by D. Frauchiger and R. Renner~\cite{1}. Here I decipher the paradox and analyze it from the point of view of de Broglie-Bohm hidden variable theory (i.e., Bohmian mechanics). I also analyze the problem from the perspective obtained by the Copenhagen interpretation (i.e., the Bohrian interpretation) and show that both views are self consistent and do not lead to any contradiction with a `single-world' description of quantum theory.
It is well-known that the law of total probability does not hold in general in quantum theory. However, the recent arguments on some of the fundamental assumptions in quantum theory based on the extended Wigner's Friend scenario show a need to clarify how the law of total probability should be formulated in quantum theory and under what conditions it still holds. In this work, the definition of conditional probability in quantum theory is extended to POVM measurements. Rule to assign two-time conditional probability is proposed for incompatible POVM operators, which leads to a more general and…
A finite-resources description of a measurement process and its implications for the "Wigner's Friend" scenario
引入有限资源下的量子测量过程模型，利用封闭系统平衡和典型性工具将测量坍缩视为封闭动力学的有效描述，并指出在“维格纳的朋友”场景中，双方承认有限资源视角即可达成一致。
The decoherence interpretation of quantum measurements is applied to Wigner's friend experiments. A framework in which all the experimental outcomes arise from unitary evolutions is proposed. Within it, a measurement is not completed until an uncontrolled environment monitorizes the state composed by the system, the apparatus and the observer. The (apparent) wave-function collapse and the corresponding randomness result from tracing out this environment; it is thus the ultimate responsible for the emergence of definite outcomes. Two main effects arise from this fact. First, external interferen…
A single space-time is too small for all of Wigner's friends
本文证明拒绝“观测事件的绝对性”假设的量子理论解释无法将所有观测事件嵌入单一背景时空，从而与“块宇宙”时空观不相容。
In April 2016, Daniela Frauchiger and Renato Renner published an article online in which they introduce a Gedankenexperiment that led them to conclude that single-world interpretation of quantum theory cannot be self-consistent. In a new version of the paper, published in September 2018, the authors moderate their original claim by concluding that quantum theory cannot be extrapolated to complex systems, at least not in a straightforward manner. The purpose of this article is to offer a careful reconstruction of the F-R argument, which allows us to show that: (i) the argument can be more clear…
Wavefunctions localization, and the Wigner's Friend Paradox in a Framework of Discrete-Space Hypothesis
在混合空间RxQ_p的量子力学框架下，通过非局域哈密顿量的薛定谔方程实现波函数坍缩，解决维格纳朋友悖论，无需观察者干预。
A review and analysis of six extended Wigner's friend arguments
本文综述并统一简化了六种扩展的Wigner's friend论证，指出它们均依赖不可观测的测量关联假设，批判性分析了假设的合理性，同时肯定了其对量子理论本质研究的价值。
In a recent paper, Żukowski and Markiewicz showed that Wigner's Friend (and, by extension, Schrödinger's Cat) can be eliminated as physical possibilities on purely logical grounds. I validate this result and demonstrate the source of the contradiction in a simple experiment in which a scientist S attempts to measure the position of object $|O\rangle = |A\rangle_S + |B\rangle_S$ by using measuring device M chosen so that $|A\rangle_M \approx |A\rangle_S$ and $|B\rangle_M \approx |B\rangle_S$. I assume that the measurement occurs by quantum amplification without collapse, in which M can entangle…
An extended Wigner's friend no-go theorem inspired by generalized contextuality
本文基于非局部性证明与广义情境性证明的对应关系，提出非情境性朋友无矛盾定理，证明量子理论与绝对观测事件和非情境性代理联合假设不一致，扩展了局部朋友无矛盾定理。
Wigner's Friend paradoxes: consistency with weak-contextual and weak-macroscopic realism models
本文从宏观实在论一致性视角审视维格纳的朋友悖论，提出对应宏观不同状态的悖论版本，证明其与弱宏观实在论一致，揭示悖论源于确定性宏观局域实在论失效及贝尔局域性部分假设不成立。
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