Notes on a Paulian Idea Foundational, Historical, Anecdotal and Forward-Looking Thoughts on the Quantum Selected Correspondence, 1995–2001 Christopher A. Fuchs Computing Science Research Center Bell Labs, Lucent Technologies Room 2C-420, 600–700 Mountain Ave. Murray Hill, New Jersey 07974, U.S.A. 10 May 2001 °c 2001, Christopher A. Fuchs Introduction This document is the first installment of three in the Cerro Grande Fire Series. The Cerro Grande Fire left many in the Los Alamos community acutely aware of the impor- tance of backing up the hard drive. I could think of no better instrument for the process than LANL itself. This is a collection of letters written to various friends and colleagues (several of whom regularly circuit this archive), including Howard Barnum, Paul Be- nioff, Charles Bennett, Herbert Bernstein, Doug Bilodeau, Gilles Brassard, Jeffrey Bub, Carlton Caves, Gregory Comer, Robert Griffiths, Adrian Kent, Rolf Landauer, Hideo Mabuchi, David Mermin, David Meyer, Michael Nielsen, Asher Peres, John Preskill, Joseph Renes, Mary Beth Ruskai, R¨udigerSchack, Robert Schumann, Abner Shimony, William Wootters, Anton Zeilinger, and many others. In a way, I hope this book evokes images of the kind of dusty, pipe-smoke infused gems one sometimes finds in the far corner of a used bookstore—something not unlike the copy of William James’s collected letters I once owned. It takes a funny person to read such a book: one who is willing to dig in the far corner. But I would not want that imagery taken too far. For despite its nod to the happenstance that brought it about, this is not a book about the past, but about our open future. Its singular theme is the quantum. Without exception, every letter in the book is devoted to coming to grips with and extolling the virtues of our quantum world. The content ranges from the foundational, to the historical, to the anecdotal, but every piece sings or whispers of the quantum. For myself, I see some of the letters as my best efforts to date at defining a vague thought that keeps creeping into my mind—the Paulian idea. To the extent I have communicated its faint shadow to my correspondents and seen a head turn, it seemed worthwhile to try to give it more life. The idea pleads to be made precise. But for this, it must possess the souls of more than me. ii Foreword On September 18, 1996 I received email that began Dear Dr. Mermin, I encountered your ``Ithaca Interpretation'' paper this morning on the quant-ph archive ... and, I must say, I've been walking around with a nice feeling since. There are some things in it that I like very much! Although I didn’t really know my correspondent, it was clear from what followed that he had thought hard — probably harder than I had — about many of the matters I was trying to address. His scholarship and intelligence were so evident that I started walking around with a nice feeling myself at having received unsolicited praise from so thoughtful a source. Little did I know that those heartwarming words of appreciation were just bait to lure me into a long critical exchange which, to my pleasure and enlightenment, has been going on ever since. A part of my subsequent education (there have also been several invariably instructive and delightful real-life meetings) can be found in Chapter 18 below. If Chris Fuchs (rhymes with “books”) did not exist then God would have been remiss in not inventing him. Foundations of quantum mechanics is a unique blend of poetry and analysis. Without the poetic vision the analysis tends to chase its own tail into more and more convoluted realms of intricate triviality. Without the analysis the poetry easily degenerates into self-indulgent doggerel. To achieve an adequate understanding of how quantum mechanics captures our efforts to express the “relations between the manifold aspects of our experience” [Bohr] requires both the poetry and the analysis. Analysis is needed to pin down the structure of the relations; poetry is required to characterize how they reflect the experience. Nobody today writing about quantum mechanics combines poetry and analysis to better effect than Chris Fuchs. By deliberate authorial choice, the 500 pages that follow are long on poetry and short on analysis, but a search on “Fuchs” will uncover in these Archives many examples where the balance is more even. The past decade has seen the growth of intense interest in applications of quantum mechanics to information processing, brought about by its deep intellectual richness, in fortuitous but sociologically significant resonance with our cultural obsession with keeping secrets. Chris Fuchs is the conscience of the field. He never loses sight of the real aim of these pursuits, and if you yourself thought it had to do with secure data transmission, RSA code cracking, fast searches, fighting decoherence, concocting ever more ingenious tricks, and such, you should look up from your beautiful algorithms or candidate qubits for a few hours every now and then to browse through these pages. The real issue is nothing less than how you and I can each construct a representation of the manifold aspects of our individual experiences (loosely known as a world), and the constraints that my representation imposes on yours, and vice-versa. By focusing explicitly on the strange information-processing capabilities inherent in the quantum mechanical description of physical reality, the new discipline of quantum information offers an opportunity to put on a sound foundation what was only hinted at in the convoluted prose of Bohr, the facile sensationalism of Heisenberg, the aphorisms of Pauli, and the poetic mysticism of Schr¨odinger.If it hasn’t occurred to you that this is iii the real justification for your quantum information-theoretic pursuits, then you owe it to yourself to pause and peruse these pages. Those who do not come at the subject from the perspective of quantum information theory may well be irritated or even outraged by some of the views expressed herein about their fondest notions. But you would have to be an exceptionally conservative Bohmian, a dry-as-dust consistent historian, a stubbornly literal-minded dynamical collapsian, a supremely surrealistic many-worldsian, or an irresponsibly post-modern correlations-without-correlatan not to be at least entertained and amused and even, every now and then, instructed by this highly readable commingling of scholarship, intellectual passion, philosophical vision, and biographical glimpses into the daily life of a young scientist. Indeed, one of the difficulties I have encountered at a busy time in writing this Foreword with dispatch, is the distracting presence, in another window on my screen, of the text itself, constantly luring me away from my own painful efforts at expression, into its charming, and by no means fully explored, byways. The death of letters as a high literary form brought about by the telephone turns out to have been only a lengthy coma — a 20th century aberration. Clearly the rise of email had, from the beginning, the potential to resuscitate the patient. Now we have an existence proof. The thought-provoking pages that follow, which can either be read like a Nabokov novel, or dipped into from time to time, like a collection of poems or short stories, gloriously provide an early 21st century demonstration that the art form is once again alive and well — and also, of course, that there remain profound questions to ask and to strive to answer about the real meaning of quantum mechanics. N. David Mermin Ithaca, New York May 8, 2001 iv DISCLAIMERS: I. This document represents a unique, and hopefully entertaining, method to communi- cate some happy thoughts on the quantum. For precisely this reason, however, it carries a great danger to my friends. It is after all a collection of correspondence. There are two things that should not be mistaken: 1) The potential of my memory to be faulty when re- porting the views of others, and 2) that the quotes taken from my correspondents were composed in anything other than a casual manner for private use only. With regard to the latter, I assert the right of my corre- spondents to deny—without apologies!—that their quotes represent accurate accounts of their thoughts. I have tried to guard against misrepresentation by keeping the number of quotes and correspondent-replies to a mini- mum: The ones that are used, are used mainly as springboards for my tendentiousness. II. Various deletions of text have been made to the original letters. The purpose of the vast majority of these is to spare the reader of the “merely personal” in my life. A smaller frac- tion are for the sake of protecting the inno- cent, protecting the correspondents, and pro- tecting the illusion that I am good-natured. The same holds as well for a small number of explicit changes of phrase (in my own writ- ings, never the correspondents). In most cases, I have tried to make the process look as seamless as possible, with no evidence that the text may have been otherwise. In my own writings, bare ellipses should be interpreted as punctuation; bracketed ellipses indicate true editorial changes. III. There is no claim that all the ideas pre- sented here are coherent. The hope is instead that the incoherent ones will earn their keep by their entertainment value. ACKNOWLEDGEMENTS: I thank Todd Brun, Jeff Bub, Carl Caves, Steven van Enk, and David Mermin for the subtle influence that gave this project life. I thank my correspondents for the use of their quotes. Most importantly, I thank my wife Kiki for allowing me to type in the middle of the night.