Physics Today

Demolishing quantum nondemolition

Christopher R. Monroe

Published Dec 30, 2010• 12 citations

Abstract

Most physicists are taught that quantum physics has two parts: coherent wave mechanics, in which quantum states evolve according to the reversible Schrödinger wave equation, and the measurement process, in which quantum states irreversibly collapse into a particular state given by the details of the measurement. So when a quantum superposition state such as a|0〉 + b|1〉 is measured with a device that discriminates perfectly between |0〉 and |1〉, the system evolves to the definite state |0〉 or |1〉 with respective probabilities |a|2 and |b|2. Of course, when such an ideal measurement is repeated on the surviving state, the same definite state will appear again and again. Many of us, starting with Albert Einstein, do not like the apparent disconnect between those two rules and may not even subscribe to that conventional “Copenhagen interpretation” of quantum mechanics. It is nevertheless useful because it correctly predicts the results of all experiments performed to date, even though it gives the appearance that the observer necessarily affects the system.In the past two decades, the advent of quantum information science and the ability of experiments to control quantum systems such as single atoms have provided the happy opportunity to once again ponder the disconnect between the fundamental quantum concepts of coherent evolution and measurement. Quantum systems can now be measured with nearly perfect efficiency, meaning that the observed distribution of a measurement is given almost exactly by the probabilities of the underlying superposition, as prescribed by the conventional Copenhagen rules that we are taught.However, a new term, the quantum nondemolition, or QND, has crept into the literature these past decades. Also known as “quantum back-action-evading measurement,” it’s a needlessly confusing term that seems to highlight the mundane aspect of a perfect quantum measurement: If one already knows that the system is in a particular eigenstate of the measuring device, then, obviously, a measurement on the system will produce that eigenstate and leave the system intact. Zero information is gained from the repeated measurement. On the other hand, when the system is not in an eigenstate of the measuring device, the quantum state can be thought to collapse to one of its eigenstates. In that case, information is gained from the system, and the QND measurement most certainly demolishes the system. The concept of QND measurement adds nothing to the usual rules of quantum measurement, regardless of interpretation. But worse, the term confounds students and newcomers who may even be comfortable with the prescriptions of quantum physics.Not all measurements are perfect, of course, and some people may wish to use the QND nomenclature to distinguish the efficiency of a measuring device. However, in the spirit of quantum decoherence theory (see, for example, Wojciech Zurek’s article in PHYSICS TODAY, October 1991, page 36), we can still think of any measurement as a QND measurement plus a possible interaction with a reservoir that can either mask the information obtained from the system or even uncontrollably change the quantum state. The interaction with the reservoir is what can make quantum problems difficult, and it is there that we confront the conceptual difficulties with quantum mechanics and the quantum–classical divide.As a common example of an imperfect measurement, consider photodetection. When a single photon strikes a photomultiplier tube and generates a pulse of photocurrent, the photon is lost forever. Or is it? The photocurrent may interact with a macroscopic system of a bulk conductor, and we may measure the resulting voltage across the conductor. Sure, the photon has disappeared, but if our detector indicates that we had one photon, we can always create another and get the same answer again and again, exactly like a QND measurement. Even if we begin with a coherent superposition of zero and one photon, which is not an eigenstate of our measurement device, that approach works. After the first measurement collapses the state into |0〉 or |1〉, we know which one came out and we either prepare zero or one photon forever after. A real photodetector is not 100% efficient, of course, meaning that a single photon striking the detector could register nothing, or that even in the absence of photons the noisy detector fires anyway. Again we can think of this as a QND measurement in which we have lost information to the environment. In practice, we trace over the environment and then must describe the system in terms of a density matrix containing the probabilities of various outcomes.In every case, the concept of QND measurement is confusing and unnecessary. It wasn’t needed in the formative years of quantum mechanics; and now with quantum information theory, entanglement, and decoherence theory, we still do not need it. Marvelous advances in many experimental quantum systems, such as individual atoms and superconducting circuits, are allowing quantum measurements to be performed almost exactly like in textbooks, as most of us have learned. Why not demolish the term “QND”?© 2011 American Institute of Physics.

DOI: 10.1063/1.3541926

Related papers

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 TodayJul 1, 20112 citations

THE ROAD NOT TAKEN: A Different Kind of Physics

Rowan Brad Quni-Gudzinas

Quantum mechanics is our most successful physical theory. It predicts the behavior of atoms, molecules, and light to ten decimal places. No experiment has ever contradicted it. It also contains a crack at its foundation. The theory’s central equation—the Schrodinger equation—describes a smooth, continuous, perfectly predictable evolution of the quantum state. But whenever anyone performs a measurement, they get a single, definite result. Which result they get appears to be random. The transition from smooth evolution to random outcome is called the “collapse of the wavefunction,” and after nearly a century of analysis, there is no agreed-upon explanation for what causes it, when it occurs, or whether it occurs at all. This is the measurement problem. The measurement problem is not a minor puzzle at the margins of the theory. It is a failure to explain the most basic operation in science: looking at the result of an experiment. Around it cluster other puzzles. Light behaves like a wave in some experiments and like a particle in others—two contradictory descriptions. Two particles that have interacted remain correlated across arbitrary distances, as though communicating faster than light. The probabilities that govern measurement outcomes appear as a postulate—the Born rule—inserted by hand with no deeper justification. And the theory offers no unified picture of reality: quantum mechanics, general relativity, and the standard model of particle physics speak incompatible mathematical languages. A century of effort has produced many interpretations of quantum mechanics. Copenhagen. Many-worlds. De Broglie-Bohm. Objective collapse. Quantum Bayesianism. None has achieved consensus. The measurement problem remains. This document proposes that the source of these puzzles is not quantum mechanics itself. It is a single, invisible assumption made at the very beginning of the theory in 1900—an assumption so deeply embedded that nobody noticed they were making it. Change that assumption, and the puzzles do not get solved. They dissolve. They stop being puzzles. They become predictable consequences of geometry.

May 3, 20260 citations

Copenhagen quantum mechanics

Timothy J. Hollowood

In our quantum mechanics courses, measurement is usually taught in passing, as an ad-hoc procedure involving the ugly collapse of the wave function. No wonder we search for more satisfying alternatives to the Copenhagen interpretation. But this overlooks the fact that the approach fits very well with modern measurement theory with its notions of the conditioned state and quantum trajectory. In addition, what we know of as the Copenhagen interpretation is a later 1950s development and some of the earlier pioneers like Bohr did not talk of wave function collapse. In fact, if one takes these earlier ideas and mixes them with later insights of decoherence, a much more satisfying version of Copenhagen quantum mechanics emerges, one for which the collapse of the wave function is seen to be a harmless book keeping device. Along the way, we explain why chaotic systems lead to wave functions that spread out quickly on macroscopic scales implying that Schrödinger cat states are the norm rather than curiosities generated in physicists’ laboratories. We then describe how the conditioned state of a quantum system depends crucially on how the system is monitored illustrating this with the example of a decaying atom monitored with a time of arrival photon detector, leading to Bohr’s quantum jumps. On the other hand, other kinds of detection lead to much smoother behaviour, providing yet another example of complementarity. Finally we explain how classical behaviour emerges, including classical mechanics but also thermodynamics.

arXiv:1511.01069Contemporary PhysicsNov 30, 20155 citations

More on the quantum measurement problem

Gregory N. Derry

Although I have neither an objection to nor an endorsement of David Mermin’s major premise in his Quick Study, “There is no quantum measurement problem” (Physics Today, June 2022, page 62), I would like to make several contextual points.Mermin’s argument has an extensive history, along with a comparable history of rebuttal. Five decades ago Max Jammer wrote that “measurements in quantum mechanics are no more or less problematic than in classical physics, for the Hilbert space vector is only a purely formal device for relating the statistics associated with these arrangements to the physics of observations in classical physics.” That seems like a condensed version of the argument Mermin presents. On the following page, however, Jammer notes that a “problematic aspect whose serious implications were only gradually understood was the fact that as long as a quantum mechanical one-body or many-body system does not interact with macroscopic objects, as long as its motion is described by the deterministic Schrödinger time-dependent equation, no events could be considered to take place in the system.”11. M. Jammer, The Philosophy of Quantum Mechanics: The Interpretations of Quantum Mechanics in Historical Perspective, Wiley (1974), pp. 473, 474.Niels Bohr giving a lecture in Copenhagen in April 1929. (Photograph by Samuel Goudsmit, courtesy of the AIP Emilio Segrè Visual Archives, Goudsmit Collection.)PPT|High resolutionRegarding the quotations from Niels Bohr, Mermin’s point is entirely valid and represents Bohr’s position accurately, but Bohr’s thought is always subtle and difficult to capture completely. Another quotation from Bohr, which is consistent with everything else he wrote, is, “The decisive point is that in neither case does the appropriate widening of our conceptual framework imply any appeal to the observing subject, which would hinder unambiguous communication of experience.”22. N. Bohr, Essays 1958–1962 on Atomic Physics and Human Knowledge, Ox Bow Press (1987), p. 7. That quotation seems inconsistent with Mermin’s paragraph that includes the passage “Why should I insist that my interpretation of science, which I use to make sense of the world that I experience, should never make any mention of me?” Bohr always insisted that his framework was a rational “generalization” of classical objectivity.The last and most conceptually important point concerns the ontological status of probability. I don’t necessarily disagree with Mermin and Bruno de Finetti, but it is by no means obvious that they are correct either. David Hawkins, for example, begins 50 pages of rigorous analysis with a brief statement of a major premise.The obvious and natural interpretation of probability in this context is that it introduces, not something that has to do with the nature of the physical system, but something that has to do with the incompleteness of our knowledge….But this opinion turns out to be false. Ignorance of dynamical regularity may be a motive for resorting to probabilities, but it does not explain the probabilities to which we resort.33. D. Hawkins, The Language of Nature: An Essay in the Philosophy of Science, W. H. Freeman (1964), p. 135.Another example can be found in Jammer’s well-known historical account of quantum theory: “For Einstein the notion of probability … was the traditional conception of classical physics, a mathematical objectivization of the human deficiency of complete or exact knowledge but ultimately a creation of the human mind…. For Born probability, as far as it was related to the wave function, was not merely a mathematical fiction but something endowed with physical reality.”44. M. Jammer, The Conceptual Development of Quantum Mechanics, McGraw-Hill (1966), p. 286.Of course, such examples don’t mean that Mermin’s position is incorrect, but I do think that they demand a better argument than a comparison to “Fairies and Witches.” I am confident that Mermin has such an argument, and I look forward to reading it as I continue working toward a coherent position on the unresolved interpretational issues of quantum theory.ReferencesSection:ChooseTop of pageReferences <<CITING ARTICLES1. M. Jammer, The Philosophy of Quantum Mechanics: The Interpretations of Quantum Mechanics in Historical Perspective, Wiley (1974), pp. 473, 474. Google Scholar2. N. Bohr, Essays 1958–1962 on Atomic Physics and Human Knowledge, Ox Bow Press (1987), p. 7. Google Scholar3. D. Hawkins, The Language of Nature: An Essay in the Philosophy of Science, W. H. Freeman (1964), p. 135. Google Scholar4. M. Jammer, The Conceptual Development of Quantum Mechanics, McGraw-Hill (1966), p. 286. Google Scholar© 2022 American Institute of Physics.

Physics TodayNov 1, 20222 citations

Measured responses to quantum Bayesianism

Michael Nauenberg

Some time ago, John Bell and I wrote a tongue-in-cheek article on the interpretation of quantum mechanics;11. J. S. Bell, M. Nauenberg, in Preludes in Theoretical Physics, A. de Shalit, H. Feshbach, L. van Hove, eds., North Holland, Amsterdam (1966), p. 270;reprinted in J. S. Bell, Speakable and Unspeakable in Quantum Mechanics, Cambridge U. Press, New York (1987), p. 24. in it we remarked that only a minority of physicists had any interest in the topic and that the typical physicist thought he would understand it “if ever he can spare twenty minutes to think about it.” According to David Mermin, however, that situation has changed dramatically, and “new interpretations [of quantum mechanics] appear every year.”Actually, most textbooks on the subject continue to be based on the well-established Born interpretation of quantum mechanics, which postulates that the absolute square of the Schrödinger wavefunction, projected onto a particular quantum state, gives the probability for the occurrence of that state after an appropriate measurement has been made. Mermin indicated that the problem with the Born interpretation involves a confusion, “prevalent among physicists,” regarding the nature of probability. He presented his preference for a “personalist Bayesian view,” which leads separate “agents,” like Eugene Wigner and his friend, to make different probability assignments based on their beliefs. But as Richard Feynman remarked, “Nature does not know what you are looking at, and she behaves the way she is going to behave whether you bother to take down the data or not.”22. R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Lectures on Physics, vol. 3, Addison-Wesley, Redwood City, CA (1989), p. 3–7.Indeed, as applied to experiments, probability simply corresponds to the relative frequency for the occurrence of a given final state after identical states are prepared initially and measurements are made by appropriate detectors—devices that, through an irreversible macroscopic process, provide a permanent record of the outcome. Hence, the results of an experiment are independent of any observers, whose only role is to set up appropriate measurement devices, calculate the probabilities from quantum theory, and then write papers based on the recorded results.As Max Born also emphasized, in classical mechanics the situation is analogous, when one takes into account that in practice initial conditions in an experiment are known only imprecisely.33. M. Born, Dan. Mat. Fys. Medd. 30 (2), 1 (1955). Hence the observed evolution of the system is also determined by a probability function that satisfies the Liouville equation. But in that case, after a measurement is performed, no one talks about the “collapse” of this classical function, whereas in quantum mechanics it is common to use that controversial expression for the Schrödinger wavefunction.Mermin discussed a “shifty split” between the classical and the quantum domain, a split that supposedly “haunts quantum mechanics.” He claimed that “regardless of what is split from what, all versions of the shifty split are vague and ambiguous.” However, that is not always the case, as a simple example shows. Consider an initial quantum wavepacket for a particle under an attractive central force that varies inversely with the square of the radial distance—the classic problem of planetary motion from the viewpoint of quantum mechanics, for example, or an excited electron in a Rydberg atom. For a finite time, the wavepacket will follow a classical Keplerian elliptic orbit, but like the Liouville distribution for the analogous classical problem, from the outset it will disperse due to the spread in initial position and momentum.For the classical distribution to be in close correspondence with the quantum wavepacket, its initial spread is assumed to satisfy the uncertainty relation. Eventually, however, the head of the wavepacket, like in the classical distribution, will catch up with its tail. Afterwards, further spreading of the wavepacket leads to interference effects between the two components, and the classical–quantum correspondence ceases to be valid. The time when such interference effects first appear is of order nτ/3, where n is the mean principal quantum number of the wavepacket and τ is the Kepler period.44. M. Nauenberg, Phys. Rev. A 40, 1133 (1989). https://doi.org/10.1103/PhysRevA.40.1133 Experimentally, that transition has been observed to agree with theory in Rydberg atoms55. M. Nauenberg, C. Stroud, J. Yeazell, Sci. Am., June 1994, p. 44. https://doi.org/10.1038/scientificamerican0694-44 for n = 100, τ = 152 picoseconds, and nothing shifty about this classical–quantum correspondence is found. For planetary orbits, however, n is of order 1070, and therefore the answer to Albert Einstein’s often repeated remark “Is the moon there when nobody looks?” (see David Mermin, Physics Today, April 1985, page 38) is yes.REFERENCESSection:ChooseTop of pageREFERENCES <<1. J. S. Bell, M. Nauenberg, in Preludes in Theoretical Physics, A. de Shalit, H. Feshbach, L. van Hove, eds., North Holland, Amsterdam (1966), p. 270; Google Scholarreprinted in J. S. Bell, Speakable and Unspeakable in Quantum Mechanics, Cambridge U. Press, New York (1987), p. 24. Google Scholar2. R. P. Feynman, R. B. Leighton, M. Sands, The Feynman Lectures on Physics, vol. 3, Addison-Wesley, Redwood City, CA (1989), p. 3–7. Google Scholar3. M. Born, Dan. Mat. Fys. Medd. 30 (2), 1 (1955). Google Scholar4. M. Nauenberg, Phys. Rev. A 40, 1133 (1989). https://doi.org/10.1103/PhysRevA.40.1133, Google ScholarCrossref, CAS5. M. Nauenberg, C. Stroud, J. Yeazell, Sci. Am., June 1994, p. 44. https://doi.org/10.1038/scientificamerican0694-44, Google ScholarCrossref© 2012 American Institute of Physics.

Physics TodayNov 30, 20120 citations

Consciousness and Quantum Mechanics

Jonathan Simon

This book is a treat, a collection of essays about the relationship between consciousness and quantum mechanics that is guaranteed to move the needle: serious, original, and relatively accessible. The essays develop cutting-edge views by thought-leading authors on questions that matter for the philosophy of physics and the philosophy of mind, and while to follow along you need to know what supervenience is and what an eigenstate is, the majority of essays should be accessible to advanced undergraduates (i.e., those who have taken a rigorous introductory course in quantum mechanics as well as a course or two in the philosophy of mind).Why care about the relationship between consciousness and quantum mechanics? If you are coming from physics, your point of entry is probably the measurement problem. When you let quantum systems evolve according to the Schrödinger equation, they enter into superpositions of mutually incompatible states. How can this be reconciled with the testimony of our senses that such systems are always in one state or the other when we measure them (with a probability distribution given by the Born rule)? This is the measurement problem, and it concerns consciousness because it draws from the testimony of our senses.If you are coming from the philosophy of mind, your primary concern is probably the mind-body problem: the task of accounting for how consciousness arises out of or fits into physical reality. If consciousness is reducible, how might the details of quantum mechanics make it harder, or easier, to establish that reduction? If consciousness is irreducible, how might the details of quantum mechanics make it harder, or easier, to explain why consciousness exists, or to find some work for it to do in the world? You may also be concerned with the structure and function of consciousness: for example, you may hold that minds can do things that classical digital computers cannot, and take it that we must look to the quantum realm to discover how.If you are not already entrenched, you might just wonder what kind of world we live in. The mechanistic philosophy of centuries past seemed to disenchant the world, rendering it a place of barren corpuscles banging into one another, leaving no room for mind or meaning, except perhaps as patterns in the collisions, or useless epiphenomena. But quantum mechanics and its relativistic generalizations force a rethinking of the mechanistic picture from the ground up. Once the dust has settled—if it ever does—we may find reasons to relegate consciousness once again to the shadows. But we may also find reasons to give it pride of place. If you are after scientific reasons to reenchant the world, this is a good place to look.The entries in this volume address these questions and others besides. The volume is organized into three parts. The first part contains four essays about the idea that consciousness causes (or constrains) quantum collapse. The second part contains eight essays discussing other ways that consciousness might contribute to theories of quantum mechanics. The third part contains five essays about how quantum mechanics might contribute to an ultimate theory of consciousness. I’ll follow the book’s order.In chapter 1, David Chalmers and Kelvin McQueen develop a new consciousness-triggered collapse interpretation of quantum mechanics. On their approach, consciousness has a strict physical correlate (perhaps given by quantum integrated information theory), it is approximately superposition-resistant, and collapse dynamics are continuous (à la Philip Pearle). They respond to the standard objections that a consciousness-collapse link makes the collapse mechanism imprecise and requires dualism, as well as to worries about Quantum Zeno, among others. Crucially, their approach allows that superpositions of consciousness are possible: their motivation is to give the view a run for its money and to find work for consciousness to do in the universe, not to rid the world of all traces of superposed consciousness. One question: In taking consciousness itself to be superposition-resistant (rather than the environmental systems it asks questions about, as Henry Stapp suggests), we appear to favor internalism over externalism. What would an externalist variant look like? Another question: How exactly would experimental results—for example, that simple enough systems with maximal Φ remain in coherent superposition—constrain our theoretical options? If to accommodate this we have to impose an experimentally determined threshold for the minimal absolute Φ for consciousness, will that mean that on the physicalist version of the view, the property we privilege for superposition-resistance is just an arbitrary collection of atoms?In chapter 2, Elias Okon and Miguel Angel Sebastian defend a consciousness-collapse theory that they call the Subjective-Objective collapse model. Like Chalmers and McQueen, Okon and Sebastian invoke a Pearle-style continuous collapse dynamics and think of consciousness as only weakly superposition-resistant. A key contribution is the treatment of a worry about multiple realizability (i.e., the way that some property of biological neurons and some other property of artificial neurons might both realize the state of being in pain). The problem is that it is not straightforward to define Hermitian operators whose eigenstates are disjunctions of the eigenstates of other Hermitian operators: you must ensure orthogonality (“being red or being a circle” is not orthogonal to “being red or being square”). In answer, the authors point out that we don’t need each multiply realizable phenomenal state to be an eigenstate of our collapse operator; it suffices that each way of realizing each phenomenal state is an eigenstate. One question is whether this problematizes the dancing qualia argument. A second worry is that we still must find a suitable basis for Hilbert space. But theories of consciousness may constrain both position and momentum: for example, both distances between neurons and how energy is transferred. We can talk about wave packets, but these will not get us eigenstates of a Hermitian operator.In chapter 3, J. Acacio de Barros and Carlos Montemayor suggest that if a mentality-triggered collapse theory is what you want, you should opt for intentionality rather than phenomenality. This is because, first, we have evidence from psychophysics that there is unconscious perception. Second, the first-personal data that motivate collapse theory concern intentionality: we remember only recording definite values; our decisions about what to measure affect the outcomes we see. First question: Can the reasons we have for thinking that there is unconscious intentionality extend into a case for unconscious primitive intentionality? Another question is how the proposal here would differ from Stapp’s treatment of what questions a mind asks of the universe. A third is how we contend with Quantum Zeno, the effect that forces both Chalmers-McQueen and Okon-Angel Sebastian to say that consciousness is only weakly superposition-resistant. Those authors note the possibility that the relevant superposed states of consciousness might overflow access consciousness (in Ned Block’s sense). We may have an easier time ensuring that access conscious states only rarely enter into superposition, if we set them back from our front line of superposition-resistant states.In chapter 4, Adrian Kent explores objective collapse theories—mass-dependent continuous spontaneous localization models. Consciousness may not be in the spotlight on these theories, but they should at least ensure that collapse happens faster than the time it takes for a stimulus (e.g., a system of several photons) to make it from a retinal impact to consciousness. Angelo Bassi, Dirk-André Deckert, and Luca Ferialdi (2010) argue that for a range of theories this constraint is respected, with collapse for a system of several photos happening at around 100 ms. Kent problematizes their argument, showing that their estimation may be off by an order of magnitude or more. Kent’s point is compelling: more work is needed to refine the estimate and to conceptually clarify the import of the constraints from perception.Part 2 consists of eight essays on various ways that consciousness might figure in or constrain quantum theories. In chapter 5, Phillip Goff argues that 3N-dimensional wave function monism cannot accommodate ground truths about consciousness, pressing on worries that the view imposes too few structural constraints to allow for the unambiguous recovery of ordinary 3D reality. While I agree that wave function monistic accounts of ordinary reality (e.g., Albert’s functionalism or Ney’s priority monism) yield an explanatory gap (even given the 3D-ish structure of GRW jumps), I was puzzled here. All type-B physicalists confront an explanatory gap where consciousness is concerned. Isn’t this a point in support of wave function monism, since it amounts to a companions-in-guilt argument against opposition from other type-B materialists?In chapter 6, Peter Lewis raises some sensible worries about the use of phenomenal consciousness as an unanalyzed primitive or special case in quantum theory. He objects to the suggestion made by some Bohmians that we can be directly aware of the particles in our brains. Another target is John von Neumann, who appeals to intuitions about psychophysical connections to justify a collapse hypothesis. These are nice points: I would be curious to know whether Lewis thinks they apply to the justifications for mentalistic collapse theories from part 1.In chapter 7, Jenann Ismael first argues that the hard problem of consciousness is of no concern to physics per se: “If consciousness enters the problem space of physics, it does so by making a difference to the behavior of physical objects.” She then offers some clarifying remarks about how we might reconcile Van Gelder-style dynamicist conceptions of mentality with (virtual) machine functionalism. I find much to agree with here, though I do wonder how strictly to take Ismael’s claims about the irrelevance of the hard problem. If there is a tie between two theories of physics over predictive accuracy, elegance, and naturalness, but one yields a more satisfying analysis of consciousness than the other, are we not even allowed to treat that as a tie-breaker?In chapter 8, Shan Gao defends a mentalistic characterization of the measurement problem in reply to Andrea Oldofredi (2019). The question is whether the key premise of the measurement problem should be that measurements take on definite values or only that we (definitely) experience them as taking on definite values. Of course, our reasons for thinking that they do arise from our seeing or remembering that they do, but we can distinguish between the claim and our evidence for it. Gao argues that the underlying mentalistic claims are more inextricable from the dilemma than that. Many dilemmas involve premises for which the evidence is perceptual, though they make no mention of perception. Here, however, solutions to the problem require special attention to the nature of that evidence.In chapter 9, Paul Skokowski illustrates one reason that we must pay special attention to the nature of that evidence. Jeffrey A. Barrett’s (1999) bare theory is a minimalistic reading of Everett on which measurement outcomes are only definite if they obtain on all branches. One question is whether this means that observers have blatantly false disjunctive beliefs: let someone measure an electron’s spin and then ask, Do you believe that you got a definite measurement outcome, either “UP” or “DOWN”? The observer will say yes, because on each branch they saw one and not the other. However, by the lights of the theory, this disjunctive belief is false. Skokowski’s insight is that while the observer definitely says yes, it does not follow (by the lights of the bare theory) that the observer has a (definite) false belief, since the neural realizer for the disjunctive belief will be different on each branch and therefore not correspond to a common eigenstate (compare the worry from chapter 2 about multiple realizability). But isn’t this just to move from the frying pan to the fire, as it seems now that our observer has no (definite) beliefs at all?In chapter 10, Michael Silberstein and W. M. Stuckey advocate for a psi-epistemicist perspective that vindicates many phenomenological intuitions, including the one that experimental outcomes are definite (and intersubjectively consistent). The approach requires that we rethink naïve realism about the wave function and that we rethink mind-body duality, in favor of a neutral monist ontology governed by constraints. This is an exciting approach, though I’m not fully clear on how to differentiate neutral monism from its idealist, dual aspect, or panpsychist cousins.In chapter 11, Michael Bitbol suggests a phenomenology-first approach to interpreting quantum mechanics. Bitbol favors a QBist participatory realism, tempered by phenomenology, that could perhaps also be described as neutral monist. He notes that the phenomenology of embodiment (à la Merleau-Ponty) acquaints us with how something can be both active and passive, both minded and incarnate, and he traces the error of believing in a separate, physical world (an error participatory realism corrects) to the tendency of self-separation: the tendency to try to project ourselves beyond ourselves, discussed in Heidegger, Sartre, and Michel Henry. These phenomenological insights are enlightening, though I am still not quite clear on how the underlying (neutral?) ontology works. Who, precisely, are the observers? Do electrons have feelings, or are they just fictions we use to cope with ours?In chapter 12, Lucien Hardy describes an experiment in which humans might be used to determine the settings in a Bell experiment. Hardy suggests that the outcome may be a violation of quantum theory consistent with the Bell inequality. He argues that this result would support (superdeterministic) mind-body dualism and also could serve as a new more objective Turing test. This is a fascinating proposal: it remains only to do the experiment.This brings us to part 3: accounts that move from quantum mechanics to consciousness, rather than conversely. In chapters 13 and 14, Roger Penrose and Stuart Hameroff give a comprehensive account of the current state of their ORCH-OR theory of quantum collapse and consciousness. Among other things we find here a nice presentation of Goodstein’s theorem as an example of a theorem that is noncomputable but easy enough for humans to understand, and a nice discussion of how retroactivity might equip us with something to say to Benjamin Libet about the role of consciousness in action selection. I’ll note, though, that I asked ChatGPT to explain Goodstein to me and it did a fairly good job, suggesting something like understanding. On Libet, I’m not sure what exactly we need retrocausality for: Why not just content ourselves that awareness of the action plan leaves time for a veto, as Libet and others have suggested?In chapter 15, Basil J. Hiley and Paavo Pylkkänen explore another route to a comprehensive theory of consciousness, drawing from the materials of a theory of quantum reality. Here, that theory is the Bohmian implicate order picture, with special attention to the notion of active information that arises there. This is another approach with neutral monist overtones, but here the wave function of a system behaves something like a customized hylomorphic form for the system while at the same time imbuing it with intentionality. There are interesting claims here about the continuity between life and mind, the unity of consciousness, and protopanpsychism, as well as some speculation of how the theory might align with the various scientific theories of consciousness inspired by psychophysics, like higher-order or global workspace theories.In chapter 16, William Seager offers a helpful discussion of the kinds of intelligibility at issue in these debates, distinguishing between mundane, mathematical, and metaphysical intelligibility, weaving together insights from Leibniz and others to argue that a Bohmian approach offers the best bet for a metaphysically intelligible world view. I sympathize with the positive aspects of his argument, though I think that other papers in this volume showcase the metaphysical intelligibility of rival approaches.Finally, in chapter 17, Lee Smolin describes a completion of quantum mechanics, also echoing Leibniz, which could support a theory of quantum gravity and which offers a new conception of consciousness: the causal theory of views, alongside the principle of precedence. This yields a novel sort of protopanpsychism in which the world is a collection of (background-independent) causal views, where views are conscious when they have a few extra features, including being unprecedented and being maximal. This is an exciting approach. One question is how we reconcile the claim that experiential states are maximal with the (mental) problem of the many, the problem that there are ties between composites (that differ over a stray ion or two) over which has best claim to be me.In closing, this is a fantastic volume. I commend Shan Gao for his editorial work: he would make a wonderful DJ. There are subtle symmetries in the way that each entry segues to the next, and the collection has a remarkable coherence.

The Philosophical ReviewJul 1, 20240 citations