I first met David Bohm in the early spring of 1976. At the time I had the idea that, given suitable experimental conditions, certain kinds of superconducting circuits should display manifestly quantum mechanical behaviour over macroscopic length scales. I hoped that such circuits would provide a new experimental vehicle for investigating quantum mechanics. David Bohm was, and is, one of the great authorities on the foundations of quantum mechanics so it seemed natural that I should seek his advice. I found him very approachable and he listened with great patience to my attempts to explain my still rather rudimentary physical views of these circuits. His encouragement then was a great morale booster for our group at Sussex. Eventually we were able to justify that encouragement. We now have superconducting circuits roughly a centimetre across which behave as single quantum objects. I would like to think that in due course these circuits will be of use in testing some of David Bahm's remarkable ideas concerning the nature of quantum mechanics.
I first met David Bohm in the early spring of 1976. At the time I had the idea that, given suitable experimental conditions, certain kinds of superconducting circuits should display manifestly quantum mechanical behaviour over macroscopic length scales. I hoped that such circuits would provide a new experimental vehicle for investigating quantum mechanics. David Bohm was, and is, one of the great authorities on the foundations of quantum mechanics so it seemed natural that I should seek his advice. I found him very approachable and he listened with great patience to my attempts to explain my still rather rudimentary physical views of these circuits. His encouragement then was a great morale booster for our group at Sussex. Eventually we were able to justify that encouragement. We now have superconducting circuits roughly a centimetre across which behave as single quantum objects. I would like to think that in due course these circuits will be of use in testing some of David Bahm's remarkable ideas concerning the nature of quantum mechanics.
Macroscopic quantum phenomena refer to quantum features in objects of 'large' sizes, systems with many components or degrees of freedom, organized in some ways where they can be identified as macroscopic objects. This emerging field is ushered in by several categories of definitive experiments in superconductivity, electromechanical systems, Bose-Einstein condensates and others. Yet this new field which is rich in open issues at the foundation of quantum and statistical physics remains little explored theoretically (with the important exception of the work of A J Leggett [1], while touched upon or implied by several groups of authors represented in this conference. Our attitude differs in that we believe in the full validity of quantum mechanics stretching from the testable micro to meso scales, with no need for the introduction of new laws of physics.) This talk summarizes our thoughts in attempting a systematic investigation into some key foundational issues of quantum macroscopic phenomena, with the goal of ultimately revealing or building a viable theoretical framework. Three major themes discussed in three intended essays are the large N expansion [2], the correlation hierarchy [3] and quantum entanglement [4]. We give a sketch of the first two themes and then discuss several key issues in the consideration of macro and quantum, namely, a) recognition that there exist many levels of structure in a composite body and only by judicious choice of an appropriate set of collective variables can one give the best description of the dynamics of a specific level of structure. Capturing the quantum features of a macroscopic object is greatly facilitated by the existence and functioning of these collective variables; b) quantum entanglement, an exclusively quantum feature [5], is known to persist to high temperatures [6] and large scales [7] under certain conditions, and may actually decrease with increased connectivity in a quantum network [8]. We use entanglement as a measure of quantumness here and pick out these somewhat counter-intuitive examples to show that there are blind spots worthy of our attention and issues which we need to analyze closer. Our purpose is to try to remove the stigma that quantum only pertains to micro, in order to make way for deeper probes into the conditions whereby quantum features of macroscopic systems manifest.
Good order is the foundation of all good things. E. Burke, Reflections on the Revolution in France , 1790 Josephson effect Superconductivity: A crash course The transition from the theoretical description of hypothetical building blocks of a quantum coherent device to something which can be actually fabricated and controlled is made much easier by the existence of superconductivity . This phenomenon, roughly speaking, allows a macroscopic quantum coherent flow of electrons in a sufficiently cold piece of an appropriate material by establishing a specific long-range order among them. Due to this one can, for example, use macroscopically different states of a superconductor as quantum states of a qubit, with the obvious advantage over “microscopically quantum” systems (like actual atoms) from the point of view of control, measurement and, last but not least, fabrication of structures with the desired parameters and on the desired scale. The phenomenon of superconductivity – the history of its discovery, experimental manifestations, theoretical explanation, open questions, relevance to other branches of physics, and technological applications – requires a thorough treatment, which can be found in any number of books. For our purposes Tinkham (2004) will provide more than sufficient background. We will not wander into the field of exotic/high-temperature superconductors for two simple reasons: the quantum coherent behaviour of the kind we need to realize qubits or other quantum coherent devices has not yet been properly and/or routinely achieved in these systems; and the fabrication of such devices is not yet reliable enough or even feasible.
There can be few physicists who have delved as deeply into the philosophical implications of their subject as has David Bohm. It is therefore with some trepidation that I am paying my respects to him on the occasion of his retirement with an attempt of my own to relate questions of philosophy to those of the foundations of physics. The viewpoint that I am putting forward here attempts to relate questions that have been two of Bohm's major interests over the years; namely, a possible breakdown of quantum mechanics on a macroscopic scale and the physical basis of conscious thought. It must be evident to the reader that in both these topics I am venturing into dangerously speculative territory. Moreover others more expert that I (such as Wigner 1, as well as Bohm 2 himself) have put forward strongly-reasoned views which relate these topics. Yet I do feel that my own viewpoint is both well worth describing and sufficiently different from these others that the reader may find something of value here. But my account must necessarily be somewhat truncated, for reasons of space and of time. It is my intention to expand these thoughts at much greater length elsewhere.
It is widely believed that the microscopic, that is, atomic-scale or smaller, world is governed by quantum mechanics while the macroscopic world, where our direct and daily experiences reside, is governed by classical mechanics. But why? Why do we not observe quantum phenomena such as quantum tunneling and quantum superposition in the macroscopic world? Where is the boundary between the microscopic and macroscopic worlds when it comes to the governing physical principles? Macroscopic quantum phenomena were first examined theoretically by A. Leggett in the 1970s. This boldly audacious idea was experimentally demonstrated in a series of groundbreaking works on Josephson junctions by John Clarke, Michel Devoret, and John Martinis in 1985, that have been celebrated by the 2025 Nobel Prize in Physics. The trio’s work spurred the development of various qubit types based on superconducting circuits, ultimately leading to the current advancement of superconducting quantum computers.
If you have a restless intellect, it is very likely that you have played at some point with the idea of investigating the meaning and conceptual foundations of quantum mechanics. It is also probable (albeit not certain) that your intentions have been stopped on their tracks by an encounter with some version of the "Shut up and calculate" command. You may have heard that everything is already understood. That understanding is not your job. Or, if it is, it is either impossible or very difficult. Maybe somebody explained to you that physics is concerned with "hows" and not with "whys"; that whys are the business of "philosophy" ---you know, that dirty word. That what you call "understanding" is just being Newtonian; which of course you cannot ask quantum mechanics to be. Perhaps they also added some norms: The important thing a theory must do is predict; a theory must only talk about measurable quantities. It may also be the case that you almost asked "OK, and why is that?", but you finally bit your tongue. If you persisted in your intentions and the debate got a little heated up, it is even possible that it was suggested that you suffered of some type of moral or epistemic weakness that tend to disappear as you grow up. Maybe you received some job advice such as "Don't work in that if you ever want to own a house". I have certainly met all these objections in my short career, and I think that they are all just wrong. In this somewhat personal document, I try to defend that making sense of quantum mechanics is an exciting, challenging, important and open scientific endeavor. I do this by compulsively quoting Feynman (and others), and I provide some arguments that you might want to use the next time you confront the mentioned "opinions". By analogy with the anti-rationalistic Copenhagen command, all the arguments are subsumed in a standard answer to it: "Shut up and let me think"