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Physics Today

A last word on quantum Darwinism

Art Hobson

Published Nov 30, 2015• 1 citation
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Abstract

In the opening paragraphs of “Quantum Darwinism, classical reality, and the randomness of quantum jumps” (Physics Today, October 2014, page 44), Wojciech Zurek invokes an analogue to Schrödinger’s cat and describes how the environment-driven process of decoherence removes the interferences that would otherwise exist between the live and dead states. He thus explains how the environment effectively “measures” the state of the cat. The article leaves me uncertain whether Zurek judges those comments to be a resolution of the quantum measurement or “Schrödinger’s cat” problem, but some readers might conclude that they indeed offer such a solution. Those readers would be wrong. As Stephen Adler has shown,11. S. L. Adler, Stud. Hist. Philos. Mod. Phys. 34, 135 (2003). https://doi.org/10.1016/S1355-2198(02)00086-2 and as decoherence expert Maximilian Schlosshauer has stated,22. M. Schlosshauer, Rev. Mod. Phys. 76, 1267 (2005). https://doi.org/10.1103/RevModPhys.76.1267 decoherence does not solve the measurement problem because it does not solve the so-called “problem of outcomes.”Decoherence theory shows how interactions with environmental quanta (photons, air molecules, and so forth) convert a quantum system’s superposition state α|s1⟩ + β|s2⟩ into the entangled “measurement state” α|s1⟩|e1⟩ + β|s2⟩|e2⟩, where |e1⟩ and |e2⟩ are orthogonal many-body states of the environment. Thus the environment acts just like a “which-slit” detector in the double-slit experiment with photons. Such a which-slit measurement transforms each photon from its premeasurement superposition over both slits into a measured photon whose single-slit states |s1⟩ and |s2⟩ are entangled with the two orthogonal states |e1⟩ and |e2⟩ of the detector. That removes the interferences from—that is, it decoheres—the photon.At that point, the theory runs into the problem of outcomes: The measurement state is still a superposition, albeit one in which the measured quantum no longer interferes with itself. By what logic does that imply definite (but indeterminate) individual outcomes? In other words, by what logic is Schrödinger’s cat converted into a cat that is either dead or alive rather than one that is both dead and alive? Zurek’s analysis does nothing to resolve that question.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. S. L. Adler, Stud. Hist. Philos. Mod. Phys. 34, 135 (2003). https://doi.org/10.1016/S1355-2198(02)00086-2, Google ScholarCrossref2. M. Schlosshauer, Rev. Mod. Phys. 76, 1267 (2005). https://doi.org/10.1103/RevModPhys.76.1267, Google ScholarCrossref© 2015 American Institute of Physics.

DOI
10.1063/pt.3.3002

Related papers

Classical selection and quantum Darwinism

Ruth E. Kastner

This short letter in response to Wojciech Zurek’s article “Quantum Darwinism, classical reality, and the randomness of quantum jumps” points out extant rebuttals in the literature to some of the author’s key claims.On the first page of his arXiv.org version of the Physics Today article (http://arxiv.org/abs/1412.5206), Zurek states thatdecoherence selects preferred pointer states that survive interaction with the environment. They are localized and effectively classical. They persist while their superpositions decohere. Decoherence marks the border between quantum and classical, alleviating concern about flagrant … manifestations of quantumness in the macroscopic domain.Here we consider emergence of “the classical” starting at a more fundamental pre-decoherence level, tracing the origin of preferred pointer states.However, the idea that preferred pointer states naturally “emerge” from the quantum level has been refuted in the literature, in particular in a paper11. R. E. Kastner, Stud. Hist. Philos. Mod. Phys. 48, 56 (2014), and references therein. https://doi.org/10.1016/j.shpsb.2014.06.004 showing that “classical” pointer states do not emerge unless a key aspect of classicality has been tacitly assumed from the beginning. In other words, the “quantum Darwinism” program is fatally circular.The assumption generating the circularity usually takes the form of a predesignated system that is considered separable from its environment (the same assumption appears in the discussion about information flow in Zurek’s article). The system acquires decohered observable (or stable) states because it is presumed to be distinguishable from a designated set of environmental subsystems, all assumed as having random phases—with respect to each other and the system. The only correlations between the system and the environmental subsystems are assumed to be established via the designated Hamiltonians.However, in the absence of this sort of predesignated partitioning of all degrees of freedom into the system and its surroundings—measurement apparatus, environment, and so forth—where the initial phase of each subsystem is random, the desired decoherence and emergence of pointer states, or einselection, do not follow. The partitioning is inevitably based on what a human observer would be able to identify and measure, such as distinguishable atoms and molecules, so the account is dependent on assuming the classical realm of the observer.The need for this pre-partitioning fatally conflicts with the claim that classicality emerges naturally from the quantum realm: Absent a pre-partitioning of all degrees of freedom into uncorrelated systems of interest, classical pointer states do not emerge. With unitary-only dynamics (lacking nonunitary collapse), the quantum realm does not have any a priori preference for the assumed uncorrelated degrees of freedom. On the contrary, a unitary-only evolution would typically begin with a maximally entangled universal state.As I noted in reference 11. R. E. Kastner, Stud. Hist. Philos. Mod. Phys. 48, 56 (2014), and references therein. https://doi.org/10.1016/j.shpsb.2014.06.004, one can observe the decoherence process experimentally, but that observation does not demonstrate that einselection occurs in a unitary-only dynamics or that classicality emerges in such a dynamics. A key missing ingredient in quantum Darwinism is some real physical randomization process, such as collapse, that could create an observer-independent, physical partitioning at the quantum level. Without such a process, quantum Darwinism contains the same kind of logical circularity as Boltzmann’s H-theorem, which attempted to derive irreversible thermodynamic laws from reversible laws. Boltzmann inadvertently smuggled in irreversibility by assuming molecular chaos; quantum Darwinists smuggle in classicality via their partitioning of the universe into distinguishable systems of interest that interact with mutually randomized environmental subsystems.In his article, Zurek does not address this refutation of quantum Darwinism and repeats the refuted claims and circular assumptions. Similar criticisms of quantum Darwinism’s presumption of classically distinguishable primordial systems, raised by Jasmina Jeknić-Dugić, Miroljub Dugić, and collaborators,2–42. J. Jeknić-Dugić, M. Dugić, A. Francom, Int. J. Theor. Phys. 53, 169 (2014). https://doi.org/10.1007/s10773-013-1794-x3. J. Jeknić-Dugić, M. Dugić, A. Francom, M. Arsenijević, Open Access Libr. J. 1, e501 (2014). https://doi.org/10.4236/oalib.11005014. M. Dugić, J. Jeknić, Int. J. Theor. Phys. 45, 2215 (2006). https://doi.org/10.1007/s10773-006-9186-0 and Chris Fields’s concern about the division of primordial degrees of freedom into a classically distinguishable system and environment55. C. Fields, Phys. Essays 24, 518 (2011). https://doi.org/10.4006/1.3644391 have also not been countered by proponents of quantum Darwinism.I acknowledge and thank Miroljub Dugić for his helpful comments.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. R. E. Kastner, Stud. Hist. Philos. Mod. Phys. 48, 56 (2014), and references therein. https://doi.org/10.1016/j.shpsb.2014.06.004, Google ScholarCrossref2. J. Jeknić-Dugić, M. Dugić, A. Francom, Int. J. Theor. Phys. 53, 169 (2014). https://doi.org/10.1007/s10773-013-1794-x, Google ScholarCrossref3. J. Jeknić-Dugić, M. Dugić, A. Francom, M. Arsenijević, Open Access Libr. J. 1, e501 (2014). https://doi.org/10.4236/oalib.1100501, Google ScholarCrossref4. M. Dugić, J. Jeknić, Int. J. Theor. Phys. 45, 2215 (2006). https://doi.org/10.1007/s10773-006-9186-0, Google ScholarCrossref5. C. Fields, Phys. Essays 24, 518 (2011). https://doi.org/10.4006/1.3644391, Google ScholarCrossref© 2015 American Institute of Physics.

Physics Today•Apr 30, 2015•3 citations

A last word on quantum Darwinism

Wojciech Hubert Zurek

Zurek replies: When quantum objects interact, they can entangle, as does a system with an apparatus or environment in models of measurement or of decoherence. The envariance-based derivation I gave of Born’s rule relies on a quantum indeterminacy that precludes simultaneous complete knowledge of both the entangled whole and its parts. Probabilities that quantify the resulting ignorance about parts follow from symmetries of entanglement.The so-called problem of outcomes is therefore not a bug but a feature of quantum physics: The ignorance it reflects leads to Born’s rule.Ignorance disappears only when an observer correlates with an outcome, precipitating a specific perception. Observers acquire information indirectly, using environments—photons, for example—as communication channels.Quantum Darwinism recognizes that decoherence doesn’t just hide quantum information. Rather, an environment is a repository and a conduit for multiple copies of information about outcomes, classical pointer states that survive decoherence intact. Flagrantly quantum effects are simultaneously hidden out of the observer’s reach.11. M. Zwolak, W. H. Zurek, Sci. Rep. 3, 1729 (2013). https://doi.org/10.1038/srep01729Classical physics dealt with an objective reality that was believed to exist and evolve independently of information. Measurements had predetermined outcomes. Information was an inconsequential illusion, irrelevant to what happened. In the classical universe, observers were like puppets, their strings pulled by Newtonian dynamics.Quantum theory shattered that view. Information is no longer a shadow of the classical states of a clockwork universe; it helps define the underlying quantum states. When a state is entangled, the ignorance of outcomes implied by quantum indeterminacy yields the probabilities that are reflected in the mixed state of the system.This interdependence between what is and what is known means that a quantum state represents partly existence and partly information. In the classical world, one could theorize about the precise state of the system and blame ignorance on observers. When, in our quantum universe, a pure entangled state is known, that fact implies complementary ignorance about parts; information and existence are inextricably intertwined.The decohered cat is neither dead nor alive. However, it is also no longer a superposition of dead and alive. The information amplified by decoherence and disseminated—for example, by photons—contains multiple records of the fate of the cat. Observers will agree about what happened and will take for granted the objective nature of the past.22. C. J. Riedel, W. H. Zurek, M. Zwolak, arXiv:1312.0331REFERENCESSection:ChooseTop of pageREFERENCES <<1. M. Zwolak, W. H. Zurek, Sci. Rep. 3, 1729 (2013). https://doi.org/10.1038/srep01729, Google ScholarCrossref2. C. J. Riedel, W. H. Zurek, M. Zwolak, arXiv:1312.0331, Google Scholar© 2015 American Institute of Physics.

Physics Today•Nov 30, 2015•0 citations

Classical selection and quantum Darwinism

Wojciech Hubert Zurek

Zurek replies: Repeatability is a textbook postulate—not mine, as the comment by Alexia Auffèves and Philippe Grangier suggests. However, I like the idea of a “context,” provided its origin is quantum. Using an ab initio classical apparatus (made out of quantum atoms!) to supply a context is inconsistent. The Copenhagen interpretation used it as an ingenious and successful ad hoc fix, but it cannot be a fundamental solution. The inability to delineate a quantum–classical boundary makes matters worse. Textbooks adopt such a “shut up and calculate” ploy to paper over the inconsistency of their axioms.Quantum Darwinism and decoherence favor quasi-classical pointer states, accessible indirectly to many observers via the imprints they deposit in the environment. Pointer states in turn depend on the quantum context provided by the environment. They emerge from a quantum substrate and supply context for the quantum states of microsystems.Quantum theory is universally valid. Quantum Darwinism and decoherence explain why macrosystems behave classically: Decoherence limits the validity of the quantum principle of superposition. Preferred states are a compromise between amplification, which calls for many copies of the original, and the no-cloning theorem. An unknown state can’t be cloned, but decoherence can amplify pointer states and disseminate information about them throughout the environment.Observers access only environment fragments, hence they extract only classical information. Quantum coherence, as explained in my article, is out of reach. Observers will agree about states communicated by environment fragments, but only data about pointer states can be shared.11. M. Zwolak, W. H. Zurek, Sci. Rep. 3, 1729 (2013); https://doi.org/10.1038/srep01729A. Streltsov, W. H. Zurek, Phys. Rev. Lett. 111, 040401 (2013). https://doi.org/10.1103/PhysRevLett.111.040401 Objective existence arises from epiontic—that is, epistemic and ontological—quantum states via the proliferation of information. Robust, quasi-classical branches and elusive information about them both emerge via amplification-like decoherence.Deducing the measurement postulates—Born’s rule and hermiticity of observables—from a simple, self-consistent credo is an advance. The assumptions, that states inhabit Hilbert space and evolve unitarily, should be judged by what follows from them and not by an intuitive appeal to classical prejudices. After all, counterintuitive assumptions—for example, that the speed of light is independent of an observer’s motion—have led to deep and valid consequences.The comment by Ruth Kastner states that phase randomness is key for decoherence and quantum Darwinism and that it is unlikely. Both are incorrect: Random phases between states are typical when phases can be defined. Moreover, random phases aren’t essential for decoherence; an outflow of information suffices. Indeed, standard models—for example, of quantum Brownian motion—often employ environments in equilibrium, for which phases are undefined and the question of their randomness is ill-posed. An initial correlation with the environment complicates calculations,22. L. D. Romero, J. P. Paz, Phys. Rev. A 55, 4070 (1997). https://doi.org/10.1103/PhysRevA.55.4070 but it does not eliminate decoherence. As difficulties encountered by quantum computing research show, decoherence is generic and hard to avoid.Quantum Darwinism requires more than decoherence; it relies on an environment that can store and communicate information. Solar radiation enables that,33. C. J. Riedel, W. H. Zurek, Phys. Rev. Lett. 105, 020404 (2010); https://doi.org/10.1103/PhysRevLett.105.020404New J. Phys. 13, 073038 (2011); https://doi.org/10.1088/1367-2630/13/7/073038M. Zwolak, C. J. Riedel, W. H. Zurek, Phys. Rev. Lett. 112, 140406 (2014). https://doi.org/10.1103/PhysRevLett.112.140406 since the Sun is a localized photon source. Quantum Darwinism is impossible in a completely equilibrated environment, whose subsystems cannot store information.The question of how to define systems is indeed of interest, and I have noted it before.44. W. H. Zurek, Phil. Trans. Roy. Soc. A 356, 1793 (1998); https://doi.org/10.1098/rsta.1998.0250Rev. Mod. Phys. 75, 715 (2003). https://doi.org/10.1103/RevModPhys.75.715 The answer must go beyond decoherence: What constitutes a system depends on interactions that make or break it; molecules, for example, would fall apart if atomic interactions weakened.There is no reason to complicate matters by making the definition of a system part of the measurement problem. Systems are there from the start—Schrödinger’s cat is obviously distinct from the nucleus that determines the cat’s fate—and their presence alone does not solve the problem. Indeed, the measurement problem disappears absent a system distinct from a measuring apparatus. Quantum evolution of a universe without parts is deterministic. There is no apparatus to record a definite outcome and so no need to talk about measurement outcomes: There is literally nothing to explain. Recognizing the role of the environment—as another system—is then natural in seeking answers.I am in good company when accused of the H-theorem “mistake.” Still, Ludwig Boltzmann dealt with the evolution of a many-body system that is classical and isolated. In that classical setting, probability was a fault of an observer ignorant of the system’s precise state. Boltzmann’s Stosszahlansatz postulates uncorrelated trajectories and is a way of representing that ignorance. Although the Boltzmann equation is useful, calling on Stosszahlansatz to prove the H-theorem—that is, the second law of thermodynamics—was suspect. Evolution is deterministic and reversible, so while observers’ ignorance may increase, microscopic entropy of the closed system cannot change. Moreover, the information and hence ignorance of observers is irrelevant for classical dynamics.As I show in my article, in our quantum world probabilities arise via entanglement. A single photon that escapes with information about the fate of Schrödinger’s cat is enough to decohere the cat’s state. An observer can never catch up with the photon, so reversibility of the equations of motion notwithstanding, information loss can be irreversible for prosaic yet compelling reasons. The resulting entropy production can be accounted for by decoherence.55. W. H. Zurek, J. P. Paz, Phys. Rev. Lett. 72, 2508 (1994). https://doi.org/10.1103/PhysRevLett.72.2508The whole measurement (or Schrödinger’s cat) setup starts far from equilibrium. Quantum Darwinism and decoherence show why, in such nonequilibrium multipartite settings, pointer states—and not their superpositions—survive and information about them is amplified and recorded in many copies by the environment. Both the nonequilibrium setting and the division of the universe into systems are needed to state the problem. When certain assumptions are needed to state the problem, surely they can be used to solve it.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. M. Zwolak, W. H. Zurek, Sci. Rep. 3, 1729 (2013); https://doi.org/10.1038/srep01729, Google ScholarCrossrefA. Streltsov, W. H. Zurek, Phys. Rev. Lett. 111, 040401 (2013). https://doi.org/10.1103/PhysRevLett.111.040401, , Google ScholarCrossref2. L. D. Romero, J. P. Paz, Phys. Rev. A 55, 4070 (1997). https://doi.org/10.1103/PhysRevA.55.4070, Google ScholarCrossref3. C. J. Riedel, W. H. Zurek, Phys. Rev. Lett. 105, 020404 (2010); https://doi.org/10.1103/PhysRevLett.105.020404, Google ScholarCrossrefNew J. Phys. 13, 073038 (2011); https://doi.org/10.1088/1367-2630/13/7/073038, , Google ScholarCrossrefM. Zwolak, C. J. Riedel, W. H. Zurek, Phys. Rev. Lett. 112, 140406 (2014). https://doi.org/10.1103/PhysRevLett.112.140406, , Google ScholarCrossref4. W. H. Zurek, Phil. Trans. Roy. Soc. A 356, 1793 (1998); https://doi.org/10.1098/rsta.1998.0250, Google ScholarCrossrefRev. Mod. Phys. 75, 715 (2003). https://doi.org/10.1103/RevModPhys.75.715, , Google ScholarCrossref5. W. H. Zurek, J. P. Paz, Phys. Rev. Lett. 72, 2508 (1994). https://doi.org/10.1103/PhysRevLett.72.2508, Google ScholarCrossref© 2015 American Institute of Physics.

Physics Today•Apr 30, 2015•9 citations

On measurement and quantum nondemolition

H. D. Zeh

In his letter, Christopher Monroe recommends that the term “quantum nondemolition” be abandoned (PHYSICS TODAY, January 2011, page 8). I agree, but for somewhat different reasons. Quantum measurement theory has always used the concept of ideal measurements, or “measurements of the first kind,” in which the state of the measured system does not change—that is, it is not “demolished”—if it is already in an eigenstate of the observable. Although an ideal measurement is often a good approximation when dealing with conserved particles, its realization requires special effort in the case of photons, which are usually absorbed while being measured. The concept of ideal measurements is certainly meaningful, but I do not agree with the suggestion that the repeated preparation of the same photon state should be regarded as being equivalent to its nondemolition during a measurement. The term nondemolition may be quite useful in photon experiments or in the construction of gravitational-wave antennae, where it was first used, as far as I know. But in a general context, the traditional terminology would be more to the point, since nonabsorption of the measured photon does not necessarily define an ideal measurement.Much of the confusion originates in the abridged description of a measurement as a stochastic jump from a superposition a∣0〉 + b∣1〉 of a microscopic system into ∣0〉 or ∣1〉 with probabilities ∣a∣2 or ∣b∣2, respectively. That description is a relic from the times when quantum theory was expected to apply only to isolated atoms, electrons, and so forth. (It does not seem to be a genuine part of the Copenhagen interpretation, which describes measurement outcomes in classical terms and often assumes that the wavefunction loses its meaning thereafter.) In most situations, though, one has to include other interacting systems in the unitary description. In fact, all of the much-discussed weirdness of quantum theory has been derived by using entangled quantum states for all involved systems. Therefore, most of the inflationary confusing vocabulary of quantum weirdness—including spooky action, quantum teleportation, quantum eraser, quantum Darwinism, sudden death, quantum information, and even quantum jumps—could be abandoned if one used instead a consistent description in terms of entangled wavefunctions, at least as far as this entanglement remains relevant.11. See, for example, H. D. Zeh, Foundations of Physics 40 9-10 1476 (2010). https://doi.org/10.1007/s10701-009-9383-9 Usually this is so until irreversible decoherence, which was itself derived from unitary interaction with the environment, has occurred in the chain of interactions leading to observation. However, I think it is definitely wrong to say that decoherence theory allows us to “think of any measurement as a [quantum nondemolition] measurement plus a possible interaction with a reservoir.” Although I personally prefer the assumption that unitary dynamics is universal, I agree that some stochastic dynamics has to be used somewhere along the chain of interactions that ends at the observer. It does not matter whether you describe this observed indeterminism in terms of a collapse or a branching of a universal wavefunction into dynamically autonomous components (unlike the collapse, the branching does not require any new dynamical postulate; it is a consequence of the Schrödinger equation with local interactions). Such a real or apparent collapse is an important part of the effective dynamics of “our quantum world”—our “branch,” if you like—that should be carefully analyzed rather than being used ad hoc in a pragmatic and confusing manner. In neither case would it describe a mere increase of information about an initially incompletely known quantum state of the involved systems; the indeterminism is described by a change of a state, which is known to be different from an ensemble of potential states.REFERENCESSection:ChooseTop of pageREFERENCES <<1. See, for example, H. D. Zeh, Foundations of Physics 40 9-10 1476 (2010). https://doi.org/10.1007/s10701-009-9383-9 , Google ScholarCrossref, ISI© 2011 American Institute of Physics.

Physics Today•Jul 1, 2011•2 citations

More on the quantum measurement problem

Art Hobson

In his June 2022 Quick Study (page 62), David Mermin argues that “the quantum state of a system expresses only the belief of the particular physicist who assigns it to the system.” Applying that to quantum measurements, he finds that “the acquisition of further information by that physicist … can lead to an abrupt change in those probabilities and thus to an updating of the quantum state that the physicist uses to represent them. There is no quantum measurement problem.”But wavefunctions have been collapsing ever since the Big Bang, with no assistance from physicists. An apparatus’s display of a measurement outcome occurs even if the experimenters happen to be out of the room. When a cosmic-ray proton strikes a sand grain on Mars and moves the grain, a quantum measurement occurs and the proton’s wavefunction collapses regardless of the absence of humans.Roger Carpenter and Andrew Anderson of the University of Cambridge performed a “Schrödinger’s cat” experiment that demolishes Mermin’s interpretation. Instead of connecting a Geiger counter to a cat-killing device, they mercifully connected it to a hammer that would fall without harm. Their strategy was to split information about the experimental result between two observers in such a way that neither observer can know the outcome. The observers learn the outcome later by sharing their information. The question is then, Did the hammer fall at the time of the experiment or later, when the observers became conscious of the outcome? The result: The hammer fell when the nucleus decayed, not later when the observers became conscious of the outcome. I think nearly all physicists would have predicted that. My hat is off to Carpenter and Anderson, who reported it with a straight face.11. R. H. S. Carpenter, A. J. Anderson, Ann. Fond. Louis de Broglie 31, 45 (2006).Humans and their consciousness have nothing to do with quantum physics. Photons, electrons, and the like, as well as their states, are real configurations of fields that have existed throughout the universe since the Big Bang.22. A. Hobson, Am. J. Phys. 81, 211 (2013). https://doi.org/10.1119/1.4789885Nevertheless, I agree with Mermin’s title: There is no quantum measurement problem, because quantum physics, with no special interpretation and without a collapse postulate, logically implies that superpositions collapse nonlocally to a single definite outcome.33. A. Hobson, Quantum Eng. 2022, 1 (2022). https://doi.org/10.1155/2022/5889159ReferencesSection:ChooseTop of pageReferences <<CITING ARTICLES1. R. H. S. Carpenter, A. J. Anderson, Ann. Fond. Louis de Broglie 31, 45 (2006). Google Scholar2. A. Hobson, Am. J. Phys. 81, 211 (2013). https://doi.org/10.1119/1.4789885, Google ScholarCrossref, ISI3. A. Hobson, Quantum Eng. 2022, 1 (2022). https://doi.org/10.1155/2022/5889159, Google ScholarCrossref© 2022 American Institute of Physics.

Physics Today•Nov 1, 2022•1 citation

Resolution of the problem of definite outcomes

Art Hobson

This paper resolves the problem of definite outcomes, also known as "Schrodinger's cat" and the "problem of wavefunction collapse." It's the problem of understanding how nature goes from a coherent superposition of possible outcomes to a single definite outcome. This "measurement problem" has not to my knowledge been solved. The analysis stems from an insight into entanglement demonstrated in two nonlocality experiments involving pairs of momentum-entangled photons. These show that an entangled state of two subsystems is not a superposition of products of paired states, but instead a superposition of correlations between paired states. The "premeasurement state" is just such a superposition with the proviso that one subsystem is the detector of the other subsystem. This state is not a paradoxical superposition of macroscopic detector states; it is instead a superposition of correlations between detector states and microscopic quantum states. Thus, Schrodinger's cat isn't "smeared" between dead and alive but instead represents a nonlocal superposition of correlations between states of the cat and states of the nucleus: This is what we want. We also critique one previously claimed resolution of the measurement problem and seven previously claimed proofs that the measurement problem is unsolvable.

Kragujevac Journal of Science•Jan 1, 2025•0 citations
In the opening paragraphs of “Quantum Darwinism, classical reality, and the randomness of quantum jumps” (Physics Today, October 2014, page 44), Wojciech Zurek invokes an analogue to Schrödinger’s cat and describes how the environment-driven process of decoherence removes the interferences that would otherwise exist between the live and dead states. He thus explains how the environment effectively “measures” the state of the cat. The article leaves me uncertain whether Zurek judges those comments to be a resolution of the quantum measurement or “Schrödinger’s cat” problem, but some readers might conclude that they indeed offer such a solution. Those readers would be wrong. As Stephen Adler has shown,11. S. L. Adler, Stud. Hist. Philos. Mod. Phys. 34, 135 (2003). https://doi.org/10.1016/S1355-2198(02)00086-2 and as decoherence expert Maximilian Schlosshauer has stated,22. M. Schlosshauer, Rev. Mod. Phys. 76, 1267 (2005). https://doi.org/10.1103/RevModPhys.76.1267 decoherence does not solve the measurement problem because it does not solve the so-called “problem of outcomes.”Decoherence theory shows how interactions with environmental quanta (photons, air molecules, and so forth) convert a quantum system’s superposition state α|s1⟩ + β|s2⟩ into the entangled “measurement state” α|s1⟩|e1⟩ + β|s2⟩|e2⟩, where |e1⟩ and |e2⟩ are orthogonal many-body states of the environment. Thus the environment acts just like a “which-slit” detector in the double-slit experiment with photons. Such a which-slit measurement transforms each photon from its premeasurement superposition over both slits into a measured photon whose single-slit states |s1⟩ and |s2⟩ are entangled with the two orthogonal states |e1⟩ and |e2⟩ of the detector. That removes the interferences from—that is, it decoheres—the photon.At that point, the theory runs into the problem of outcomes: The measurement state is still a superposition, albeit one in which the measured quantum no longer interferes with itself. By what logic does that imply definite (but indeterminate) individual outcomes? In other words, by what logic is Schrödinger’s cat converted into a cat that is either dead or alive rather than one that is both dead and alive? Zurek’s analysis does nothing to resolve that question.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. S. L. Adler, Stud. Hist. Philos. Mod. Phys. 34, 135 (2003). https://doi.org/10.1016/S1355-2198(02)00086-2, Google ScholarCrossref2. M. Schlosshauer, Rev. Mod. Phys. 76, 1267 (2005). https://doi.org/10.1103/RevModPhys.76.1267, Google ScholarCrossref© 2015 American Institute of Physics.
This short letter in response to Wojciech Zurek’s article “Quantum Darwinism, classical reality, and the randomness of quantum jumps” points out extant rebuttals in the literature to some of the author’s key claims.On the first page of his arXiv.org version of the Physics Today article (http://arxiv.org/abs/1412.5206), Zurek states thatdecoherence selects preferred pointer states that survive interaction with the environment. They are localized and effectively classical. They persist while their superpositions decohere. Decoherence marks the border between quantum and classical, alleviating concern about flagrant … manifestations of quantumness in the macroscopic domain.Here we consider emergence of “the classical” starting at a more fundamental pre-decoherence level, tracing the origin of preferred pointer states.However, the idea that preferred pointer states naturally “emerge” from the quantum level has been refuted in the literature, in particular in a paper11. R. E. Kastner, Stud. Hist. Philos. Mod. Phys. 48, 56 (2014), and references therein. https://doi.org/10.1016/j.shpsb.2014.06.004 showing that “classical” pointer states do not emerge unless a key aspect of classicality has been tacitly assumed from the beginning. In other words, the “quantum Darwinism” program is fatally circular.The assumption generating the circularity usually takes the form of a predesignated system that is considered separable from its environment (the same assumption appears in the discussion about information flow in Zurek’s article). The system acquires decohered observable (or stable) states because it is presumed to be distinguishable from a designated set of environmental subsystems, all assumed as having random phases—with respect to each other and the system. The only correlations between the system and the environmental subsystems are assumed to be established via the designated Hamiltonians.However, in the absence of this sort of predesignated partitioning of all degrees of freedom into the system and its surroundings—measurement apparatus, environment, and so forth—where the initial phase of each subsystem is random, the desired decoherence and emergence of pointer states, or einselection, do not follow. The partitioning is inevitably based on what a human observer would be able to identify and measure, such as distinguishable atoms and molecules, so the account is dependent on assuming the classical realm of the observer.The need for this pre-partitioning fatally conflicts with the claim that classicality emerges naturally from the quantum realm: Absent a pre-partitioning of all degrees of freedom into uncorrelated systems of interest, classical pointer states do not emerge. With unitary-only dynamics (lacking nonunitary collapse), the quantum realm does not have any a priori preference for the assumed uncorrelated degrees of freedom. On the contrary, a unitary-only evolution would typically begin with a maximally entangled universal state.As I noted in reference 11. R. E. Kastner, Stud. Hist. Philos. Mod. Phys. 48, 56 (2014), and references therein. https://doi.org/10.1016/j.shpsb.2014.06.004, one can observe the decoherence process experimentally, but that observation does not demonstrate that einselection occurs in a unitary-only dynamics or that classicality emerges in such a dynamics. A key missing ingredient in quantum Darwinism is some real physical randomization process, such as collapse, that could create an observer-independent, physical partitioning at the quantum level. Without such a process, quantum Darwinism contains the same kind of logical circularity as Boltzmann’s H-theorem, which attempted to derive irreversible thermodynamic laws from reversible laws. Boltzmann inadvertently smuggled in irreversibility by assuming molecular chaos; quantum Darwinists smuggle in classicality via their partitioning of the universe into distinguishable systems of interest that interact with mutually randomized environmental subsystems.In his article, Zurek does not address this refutation of quantum Darwinism and repeats the refuted claims and circular assumptions. Similar criticisms of quantum Darwinism’s presumption of classically distinguishable primordial systems, raised by Jasmina Jeknić-Dugić, Miroljub Dugić, and collaborators,2–42. J. Jeknić-Dugić, M. Dugić, A. Francom, Int. J. Theor. Phys. 53, 169 (2014). https://doi.org/10.1007/s10773-013-1794-x3. J. Jeknić-Dugić, M. Dugić, A. Francom, M. Arsenijević, Open Access Libr. J. 1, e501 (2014). https://doi.org/10.4236/oalib.11005014. M. Dugić, J. Jeknić, Int. J. Theor. Phys. 45, 2215 (2006). https://doi.org/10.1007/s10773-006-9186-0 and Chris Fields’s concern about the division of primordial degrees of freedom into a classically distinguishable system and environment55. C. Fields, Phys. Essays 24, 518 (2011). https://doi.org/10.4006/1.3644391 have also not been countered by proponents of quantum Darwinism.I acknowledge and thank Miroljub Dugić for his helpful comments.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. R. E. Kastner, Stud. Hist. Philos. Mod. Phys. 48, 56 (2014), and references therein. https://doi.org/10.1016/j.shpsb.2014.06.004, Google ScholarCrossref2. J. Jeknić-Dugić, M. Dugić, A. Francom, Int. J. Theor. Phys. 53, 169 (2014). https://doi.org/10.1007/s10773-013-1794-x, Google ScholarCrossref3. J. Jeknić-Dugić, M. Dugić, A. Francom, M. Arsenijević, Open Access Libr. J. 1, e501 (2014). https://doi.org/10.4236/oalib.1100501, Google ScholarCrossref4. M. Dugić, J. Jeknić, Int. J. Theor. Phys. 45, 2215 (2006). https://doi.org/10.1007/s10773-006-9186-0, Google ScholarCrossref5. C. Fields, Phys. Essays 24, 518 (2011). https://doi.org/10.4006/1.3644391, Google ScholarCrossref© 2015 American Institute of Physics.
Zurek replies: When quantum objects interact, they can entangle, as does a system with an apparatus or environment in models of measurement or of decoherence. The envariance-based derivation I gave of Born’s rule relies on a quantum indeterminacy that precludes simultaneous complete knowledge of both the entangled whole and its parts. Probabilities that quantify the resulting ignorance about parts follow from symmetries of entanglement.The so-called problem of outcomes is therefore not a bug but a feature of quantum physics: The ignorance it reflects leads to Born’s rule.Ignorance disappears only when an observer correlates with an outcome, precipitating a specific perception. Observers acquire information indirectly, using environments—photons, for example—as communication channels.Quantum Darwinism recognizes that decoherence doesn’t just hide quantum information. Rather, an environment is a repository and a conduit for multiple copies of information about outcomes, classical pointer states that survive decoherence intact. Flagrantly quantum effects are simultaneously hidden out of the observer’s reach.11. M. Zwolak, W. H. Zurek, Sci. Rep. 3, 1729 (2013). https://doi.org/10.1038/srep01729Classical physics dealt with an objective reality that was believed to exist and evolve independently of information. Measurements had predetermined outcomes. Information was an inconsequential illusion, irrelevant to what happened. In the classical universe, observers were like puppets, their strings pulled by Newtonian dynamics.Quantum theory shattered that view. Information is no longer a shadow of the classical states of a clockwork universe; it helps define the underlying quantum states. When a state is entangled, the ignorance of outcomes implied by quantum indeterminacy yields the probabilities that are reflected in the mixed state of the system.This interdependence between what is and what is known means that a quantum state represents partly existence and partly information. In the classical world, one could theorize about the precise state of the system and blame ignorance on observers. When, in our quantum universe, a pure entangled state is known, that fact implies complementary ignorance about parts; information and existence are inextricably intertwined.The decohered cat is neither dead nor alive. However, it is also no longer a superposition of dead and alive. The information amplified by decoherence and disseminated—for example, by photons—contains multiple records of the fate of the cat. Observers will agree about what happened and will take for granted the objective nature of the past.22. C. J. Riedel, W. H. Zurek, M. Zwolak, arXiv:1312.0331REFERENCESSection:ChooseTop of pageREFERENCES <<1. M. Zwolak, W. H. Zurek, Sci. Rep. 3, 1729 (2013). https://doi.org/10.1038/srep01729, Google ScholarCrossref2. C. J. Riedel, W. H. Zurek, M. Zwolak, arXiv:1312.0331, Google Scholar© 2015 American Institute of Physics.
Zurek replies: Repeatability is a textbook postulate—not mine, as the comment by Alexia Auffèves and Philippe Grangier suggests. However, I like the idea of a “context,” provided its origin is quantum. Using an ab initio classical apparatus (made out of quantum atoms!) to supply a context is inconsistent. The Copenhagen interpretation used it as an ingenious and successful ad hoc fix, but it cannot be a fundamental solution. The inability to delineate a quantum–classical boundary makes matters worse. Textbooks adopt such a “shut up and calculate” ploy to paper over the inconsistency of their axioms.Quantum Darwinism and decoherence favor quasi-classical pointer states, accessible indirectly to many observers via the imprints they deposit in the environment. Pointer states in turn depend on the quantum context provided by the environment. They emerge from a quantum substrate and supply context for the quantum states of microsystems.Quantum theory is universally valid. Quantum Darwinism and decoherence explain why macrosystems behave classically: Decoherence limits the validity of the quantum principle of superposition. Preferred states are a compromise between amplification, which calls for many copies of the original, and the no-cloning theorem. An unknown state can’t be cloned, but decoherence can amplify pointer states and disseminate information about them throughout the environment.Observers access only environment fragments, hence they extract only classical information. Quantum coherence, as explained in my article, is out of reach. Observers will agree about states communicated by environment fragments, but only data about pointer states can be shared.11. M. Zwolak, W. H. Zurek, Sci. Rep. 3, 1729 (2013); https://doi.org/10.1038/srep01729A. Streltsov, W. H. Zurek, Phys. Rev. Lett. 111, 040401 (2013). https://doi.org/10.1103/PhysRevLett.111.040401 Objective existence arises from epiontic—that is, epistemic and ontological—quantum states via the proliferation of information. Robust, quasi-classical branches and elusive information about them both emerge via amplification-like decoherence.Deducing the measurement postulates—Born’s rule and hermiticity of observables—from a simple, self-consistent credo is an advance. The assumptions, that states inhabit Hilbert space and evolve unitarily, should be judged by what follows from them and not by an intuitive appeal to classical prejudices. After all, counterintuitive assumptions—for example, that the speed of light is independent of an observer’s motion—have led to deep and valid consequences.The comment by Ruth Kastner states that phase randomness is key for decoherence and quantum Darwinism and that it is unlikely. Both are incorrect: Random phases between states are typical when phases can be defined. Moreover, random phases aren’t essential for decoherence; an outflow of information suffices. Indeed, standard models—for example, of quantum Brownian motion—often employ environments in equilibrium, for which phases are undefined and the question of their randomness is ill-posed. An initial correlation with the environment complicates calculations,22. L. D. Romero, J. P. Paz, Phys. Rev. A 55, 4070 (1997). https://doi.org/10.1103/PhysRevA.55.4070 but it does not eliminate decoherence. As difficulties encountered by quantum computing research show, decoherence is generic and hard to avoid.Quantum Darwinism requires more than decoherence; it relies on an environment that can store and communicate information. Solar radiation enables that,33. C. J. Riedel, W. H. Zurek, Phys. Rev. Lett. 105, 020404 (2010); https://doi.org/10.1103/PhysRevLett.105.020404New J. Phys. 13, 073038 (2011); https://doi.org/10.1088/1367-2630/13/7/073038M. Zwolak, C. J. Riedel, W. H. Zurek, Phys. Rev. Lett. 112, 140406 (2014). https://doi.org/10.1103/PhysRevLett.112.140406 since the Sun is a localized photon source. Quantum Darwinism is impossible in a completely equilibrated environment, whose subsystems cannot store information.The question of how to define systems is indeed of interest, and I have noted it before.44. W. H. Zurek, Phil. Trans. Roy. Soc. A 356, 1793 (1998); https://doi.org/10.1098/rsta.1998.0250Rev. Mod. Phys. 75, 715 (2003). https://doi.org/10.1103/RevModPhys.75.715 The answer must go beyond decoherence: What constitutes a system depends on interactions that make or break it; molecules, for example, would fall apart if atomic interactions weakened.There is no reason to complicate matters by making the definition of a system part of the measurement problem. Systems are there from the start—Schrödinger’s cat is obviously distinct from the nucleus that determines the cat’s fate—and their presence alone does not solve the problem. Indeed, the measurement problem disappears absent a system distinct from a measuring apparatus. Quantum evolution of a universe without parts is deterministic. There is no apparatus to record a definite outcome and so no need to talk about measurement outcomes: There is literally nothing to explain. Recognizing the role of the environment—as another system—is then natural in seeking answers.I am in good company when accused of the H-theorem “mistake.” Still, Ludwig Boltzmann dealt with the evolution of a many-body system that is classical and isolated. In that classical setting, probability was a fault of an observer ignorant of the system’s precise state. Boltzmann’s Stosszahlansatz postulates uncorrelated trajectories and is a way of representing that ignorance. Although the Boltzmann equation is useful, calling on Stosszahlansatz to prove the H-theorem—that is, the second law of thermodynamics—was suspect. Evolution is deterministic and reversible, so while observers’ ignorance may increase, microscopic entropy of the closed system cannot change. Moreover, the information and hence ignorance of observers is irrelevant for classical dynamics.As I show in my article, in our quantum world probabilities arise via entanglement. A single photon that escapes with information about the fate of Schrödinger’s cat is enough to decohere the cat’s state. An observer can never catch up with the photon, so reversibility of the equations of motion notwithstanding, information loss can be irreversible for prosaic yet compelling reasons. The resulting entropy production can be accounted for by decoherence.55. W. H. Zurek, J. P. Paz, Phys. Rev. Lett. 72, 2508 (1994). https://doi.org/10.1103/PhysRevLett.72.2508The whole measurement (or Schrödinger’s cat) setup starts far from equilibrium. Quantum Darwinism and decoherence show why, in such nonequilibrium multipartite settings, pointer states—and not their superpositions—survive and information about them is amplified and recorded in many copies by the environment. Both the nonequilibrium setting and the division of the universe into systems are needed to state the problem. When certain assumptions are needed to state the problem, surely they can be used to solve it.REFERENCESSection:ChooseTop of pageREFERENCES <<CITING ARTICLES1. M. Zwolak, W. H. Zurek, Sci. Rep. 3, 1729 (2013); https://doi.org/10.1038/srep01729, Google ScholarCrossrefA. Streltsov, W. H. Zurek, Phys. Rev. Lett. 111, 040401 (2013). https://doi.org/10.1103/PhysRevLett.111.040401, , Google ScholarCrossref2. L. D. Romero, J. P. Paz, Phys. Rev. A 55, 4070 (1997). https://doi.org/10.1103/PhysRevA.55.4070, Google ScholarCrossref3. C. J. Riedel, W. H. Zurek, Phys. Rev. Lett. 105, 020404 (2010); https://doi.org/10.1103/PhysRevLett.105.020404, Google ScholarCrossrefNew J. Phys. 13, 073038 (2011); https://doi.org/10.1088/1367-2630/13/7/073038, , Google ScholarCrossrefM. Zwolak, C. J. Riedel, W. H. Zurek, Phys. Rev. Lett. 112, 140406 (2014). https://doi.org/10.1103/PhysRevLett.112.140406, , Google ScholarCrossref4. W. H. Zurek, Phil. Trans. Roy. Soc. A 356, 1793 (1998); https://doi.org/10.1098/rsta.1998.0250, Google ScholarCrossrefRev. Mod. Phys. 75, 715 (2003). https://doi.org/10.1103/RevModPhys.75.715, , Google ScholarCrossref5. W. H. Zurek, J. P. Paz, Phys. Rev. Lett. 72, 2508 (1994). https://doi.org/10.1103/PhysRevLett.72.2508, Google ScholarCrossref© 2015 American Institute of Physics.
In his letter, Christopher Monroe recommends that the term “quantum nondemolition” be abandoned (PHYSICS TODAY, January 2011, page 8). I agree, but for somewhat different reasons. Quantum measurement theory has always used the concept of ideal measurements, or “measurements of the first kind,” in which the state of the measured system does not change—that is, it is not “demolished”—if it is already in an eigenstate of the observable. Although an ideal measurement is often a good approximation when dealing with conserved particles, its realization requires special effort in the case of photons, which are usually absorbed while being measured. The concept of ideal measurements is certainly meaningful, but I do not agree with the suggestion that the repeated preparation of the same photon state should be regarded as being equivalent to its nondemolition during a measurement. The term nondemolition may be quite useful in photon experiments or in the construction of gravitational-wave antennae, where it was first used, as far as I know. But in a general context, the traditional terminology would be more to the point, since nonabsorption of the measured photon does not necessarily define an ideal measurement.Much of the confusion originates in the abridged description of a measurement as a stochastic jump from a superposition a∣0〉 + b∣1〉 of a microscopic system into ∣0〉 or ∣1〉 with probabilities ∣a∣2 or ∣b∣2, respectively. That description is a relic from the times when quantum theory was expected to apply only to isolated atoms, electrons, and so forth. (It does not seem to be a genuine part of the Copenhagen interpretation, which describes measurement outcomes in classical terms and often assumes that the wavefunction loses its meaning thereafter.) In most situations, though, one has to include other interacting systems in the unitary description. In fact, all of the much-discussed weirdness of quantum theory has been derived by using entangled quantum states for all involved systems. Therefore, most of the inflationary confusing vocabulary of quantum weirdness—including spooky action, quantum teleportation, quantum eraser, quantum Darwinism, sudden death, quantum information, and even quantum jumps—could be abandoned if one used instead a consistent description in terms of entangled wavefunctions, at least as far as this entanglement remains relevant.11. See, for example, H. D. Zeh, Foundations of Physics 40 9-10 1476 (2010). https://doi.org/10.1007/s10701-009-9383-9 Usually this is so until irreversible decoherence, which was itself derived from unitary interaction with the environment, has occurred in the chain of interactions leading to observation. However, I think it is definitely wrong to say that decoherence theory allows us to “think of any measurement as a [quantum nondemolition] measurement plus a possible interaction with a reservoir.” Although I personally prefer the assumption that unitary dynamics is universal, I agree that some stochastic dynamics has to be used somewhere along the chain of interactions that ends at the observer. It does not matter whether you describe this observed indeterminism in terms of a collapse or a branching of a universal wavefunction into dynamically autonomous components (unlike the collapse, the branching does not require any new dynamical postulate; it is a consequence of the Schrödinger equation with local interactions). Such a real or apparent collapse is an important part of the effective dynamics of “our quantum world”—our “branch,” if you like—that should be carefully analyzed rather than being used ad hoc in a pragmatic and confusing manner. In neither case would it describe a mere increase of information about an initially incompletely known quantum state of the involved systems; the indeterminism is described by a change of a state, which is known to be different from an ensemble of potential states.REFERENCESSection:ChooseTop of pageREFERENCES <<1. See, for example, H. D. Zeh, Foundations of Physics 40 9-10 1476 (2010). https://doi.org/10.1007/s10701-009-9383-9 , Google ScholarCrossref, ISI© 2011 American Institute of Physics.
In his June 2022 Quick Study (page 62), David Mermin argues that “the quantum state of a system expresses only the belief of the particular physicist who assigns it to the system.” Applying that to quantum measurements, he finds that “the acquisition of further information by that physicist … can lead to an abrupt change in those probabilities and thus to an updating of the quantum state that the physicist uses to represent them. There is no quantum measurement problem.”But wavefunctions have been collapsing ever since the Big Bang, with no assistance from physicists. An apparatus’s display of a measurement outcome occurs even if the experimenters happen to be out of the room. When a cosmic-ray proton strikes a sand grain on Mars and moves the grain, a quantum measurement occurs and the proton’s wavefunction collapses regardless of the absence of humans.Roger Carpenter and Andrew Anderson of the University of Cambridge performed a “Schrödinger’s cat” experiment that demolishes Mermin’s interpretation. Instead of connecting a Geiger counter to a cat-killing device, they mercifully connected it to a hammer that would fall without harm. Their strategy was to split information about the experimental result between two observers in such a way that neither observer can know the outcome. The observers learn the outcome later by sharing their information. The question is then, Did the hammer fall at the time of the experiment or later, when the observers became conscious of the outcome? The result: The hammer fell when the nucleus decayed, not later when the observers became conscious of the outcome. I think nearly all physicists would have predicted that. My hat is off to Carpenter and Anderson, who reported it with a straight face.11. R. H. S. Carpenter, A. J. Anderson, Ann. Fond. Louis de Broglie 31, 45 (2006).Humans and their consciousness have nothing to do with quantum physics. Photons, electrons, and the like, as well as their states, are real configurations of fields that have existed throughout the universe since the Big Bang.22. A. Hobson, Am. J. Phys. 81, 211 (2013). https://doi.org/10.1119/1.4789885Nevertheless, I agree with Mermin’s title: There is no quantum measurement problem, because quantum physics, with no special interpretation and without a collapse postulate, logically implies that superpositions collapse nonlocally to a single definite outcome.33. A. Hobson, Quantum Eng. 2022, 1 (2022). https://doi.org/10.1155/2022/5889159ReferencesSection:ChooseTop of pageReferences <<CITING ARTICLES1. R. H. S. Carpenter, A. J. Anderson, Ann. Fond. Louis de Broglie 31, 45 (2006). Google Scholar2. A. Hobson, Am. J. Phys. 81, 211 (2013). https://doi.org/10.1119/1.4789885, Google ScholarCrossref, ISI3. A. Hobson, Quantum Eng. 2022, 1 (2022). https://doi.org/10.1155/2022/5889159, Google ScholarCrossref© 2022 American Institute of Physics.
This paper resolves the problem of definite outcomes, also known as "Schrodinger's cat" and the "problem of wavefunction collapse." It's the problem of understanding how nature goes from a coherent superposition of possible outcomes to a single definite outcome. This "measurement problem" has not to my knowledge been solved. The analysis stems from an insight into entanglement demonstrated in two nonlocality experiments involving pairs of momentum-entangled photons. These show that an entangled state of two subsystems is not a superposition of products of paired states, but instead a superposition of correlations between paired states. The "premeasurement state" is just such a superposition with the proviso that one subsystem is the detector of the other subsystem. This state is not a paradoxical superposition of macroscopic detector states; it is instead a superposition of correlations between detector states and microscopic quantum states. Thus, Schrodinger's cat isn't "smeared" between dead and alive but instead represents a nonlocal superposition of correlations between states of the cat and states of the nucleus: This is what we want. We also critique one previously claimed resolution of the measurement problem and seven previously claimed proofs that the measurement problem is unsolvable.