DOCSLIB.ORG

Quantum Theory Cannot Consistently Describe the Use of Itself

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Quantum Theory Cannot Consistently Describe the Use of Itself Quantum theory cannot consistently describe the use of itself Daniela Frauchiger and Renato Renner∗ Institute for Theoretical Physics, ETH Zurich, 8093 Zurich; Switzerland Abstract Quantum theory provides an extremely accurate description of fundamental processes in physics. It thus seems likely that the theory is applicable beyond the, mostly microscopic, domain in which it has been tested experimentally. Here we propose a Gedankenexperiment to investigate the question whether quantum theory can, in principle, have universal validity. The idea is that, if the answer was yes, it must be possible to employ quantum theory to model complex systems that include agents who are themselves using quantum theory. Analysing the experiment under this presumption, we find that one agent, upon observing a particular measurement outcome, must conclude that another agent has predicted the opposite outcome with certainty. The agents' conclusions, although all derived within quantum theory, are thus inconsistent. This indicates that quantum theory cannot be extrapolated to complex systems, at least not in a straightforward manner. Introduction Direct experimental tests of quantum theory are mostly restricted to microscopic domains. Nevertheless, quantum theory is commonly regarded as beging (almost) universally valid. It is not only used to describe fundamental processes in particle and solid state physics, but also, for instance, to explain the cosmic microwave background or the radiation of black holes. The presumption that the validity of quantum theory extends to larger scales has remarkable con- sequences, as noted already in 1935 by Schr¨odinger[1]. His famous example consisted of a cat that is brought into a state corresponding to a superposition of two macroscopically entirely different states, one in which it is dead and one in which it is alive. Schr¨odinger pointed out, however, that such macroscopic superposition states do not represent anything contradictory in themselves. This view was not shared by everyone. In 1967, Wigner proposed an argument, known as the Wigner's Friend Paradox, which should show that \quantum mechanics cannot have unlimited valid- ity" [2]. His idea was to consider the views of two different observers in an experiment analogous to the one depicted by Fig.1. One observer, called agent F, measures the vertical polarisation z of a spin 1 one-half particle S, such as a silver atom. Upon observing the outcome, which is either z = 2 or 1 − z = + 2 , agent F would thus say that S is in state = or = ; (1) S j#iS S j"iS respectively. The other observer, agent W, has no direct access to the outcome z observed by his friend F. Agent W could instead model agent F's lab as a big quantum system, L S D F, which contains the spin S as a subsystem, another subsystem, D, for the friend's measurement≡ ⊗ devices⊗ and everything else connected to them, as well as a subsystem F that includes the friend herself. Suppose that, from agent W's perspective, the lab L is initially in a pure state and that it remains isolated during agent F's spin measurement experiment. (One may object that these assumptions are unrealistic [3], but, crucially, the laws of quantum theory do not preclude that they be satisfied to arbitrarily good ∗[email protected] 1 <latexit sha1_base64="XvskfxKCf57ZWVUyjcLkv0bxg3Q=">AAACEHicbVC7TsMwFL0pr1JeKUgsLBEVElOVsMBYwcLA0Er0ITVRcFynteo4ke2Aqig/wc4Kv8CGWPkDFmY+A6ftAC1HsnR0zn35BAmjUtn2p1FaWV1b3yhvVra2d3b3zOp+R8apwKSNYxaLXoAkYZSTtqKKkV4iCIoCRrrB+Krwu/dESBrzWzVJiBehIachxUhpyTerblNSP3MjpEYiym7y3Ddrdt2ewlomzpzUGoetrzsAaPrmtzuIcRoRrjBDUvYdO1FehoSimJG84qaSJAiP0ZD0NeUoItLLpqfn1olWBlYYC/24sqbq744MRVJOokBXFifKRa8Q//U4xSQUCC/sV+GFl1GepIpwPFsfpsxSsVWkYw2oIFixiSYIC6p/YOER0mOUzrCio3EWg1gmnbO6Y9edllNrXMIMZTiCYzgFB86hAdfQhDZgeIAneIYX49F4Nd6M91lpyZj3HMAfGB8/uUifdQ==</latexit>sha1_base64="hjRNlJ7vmUcVO2RAuC4xSEThMTM=">AAACEHicbVA7T8MwGHTKq5RXChILi0WFxFQlLDBWZWFgaCX6kJoqclyntWo7ke2Aqih/gp0VJnY2hMTEP2Bh5mfgtB2g5SRLp7vv5QtiRpV2nE+rsLK6tr5R3Cxtbe/s7tnl/baKEolJC0cskt0AKcKoIC1NNSPdWBLEA0Y6wfgy9zu3RCoaiRs9iUmfo6GgIcVIG8m3y15DUT/1ONIjydPrLPPtilN1poDLxJ2TSu2w+UWf6+8N3/72BhFOOBEaM6RUz3Vi3U+R1BQzkpW8RJEY4TEakp6hAnGi+un09AyeGGUAw0iaJzScqr87UsSVmvDAVOYnqkUvF//1BMUklAgv7NfhRT+lIk40EXi2PkwY1BHM04EDKgnWbGIIwpKaH0A8QmaMNhmWTDTuYhDLpH1WdZ2q23QrtTqYoQiOwDE4BS44BzVwBRqgBTC4Aw/gETxZ99aL9Wq9zUoL1rznAPyB9fEDMMChMQ==</latexit>sha1_base64="U6ArxhaKakGP6NDZTaBFadF8pXE=">AAACEHicbVC7TsMwFL0pr1JeKYwsFhUSU5WwwFjBwsBQJPqQ2ihyXKe16jiR7YCqKD/Bzgq/wIZY+QP+gM/AaTNAy5EsHZ1zXz5BwpnSjvNlVdbWNza3qtu1nd29/QO7fthVcSoJ7ZCYx7IfYEU5E7Sjmea0n0iKo4DTXjC9LvzeA5WKxeJezxLqRXgsWMgI1kby7fqwrZifDSOsJzLKbvPctxtO05kDrRK3JA0o0fbt7+EoJmlEhSYcKzVwnUR7GZaaEU7z2jBVNMFkisd0YKjAEVVeNj89R6dGGaEwluYJjebq744MR0rNosBUFieqZa8Q//UEIzSUmCzt1+GllzGRpJoKslgfphzpGBXpoBGTlGg+MwQTycwPEJlgM0abDGsmGnc5iFXSPW+6TtO9cxutqzKkKhzDCZyBCxfQghtoQwcIPMIzvMCr9WS9We/Wx6K0YpU9R/AH1ucPAQKdkA==</latexit> S<latexit sha1_base64="GW+BpxDswORBjNoNFjjKGTYw75s=">AAACB3icbVC7TsMwFL0prxJeBUYWiwqJqUpYYEFUsDAWQR+ijSrHdVqrthPZDlIVVWJlZ4VfYEOsTHwDf8Bn4LQdoHAkS0fn3JdPmHCmjed9OoWFxaXlleKqu7a+sblV2t5p6DhVhNZJzGPVCrGmnElaN8xw2koUxSLktBkOL3K/eUeVZrG8MaOEBgL3JYsYwcZKtx2BzUCJ7HrcLZW9ijcB+kv8GSmffbin9wBQ65a+Or2YpIJKQzjWuu17iQkyrAwjnI7dTqppgskQ92nbUokF1UE2uXiMDqzSQ1Gs7JMGTdSfHRkWWo9EaCvzC/W8l4v/epIRGilM5vab6CTImExSQyWZro9SjkyM8lBQjylKDB9Zgoli9geIDLAdY2x0ro3Gnw/iL2kcVXyv4l/55eo5TFGEPdiHQ/DhGKpwCTWoAwEJj/AEz86D8+K8Om/T0oIz69mFX3DevwFnhZwO</latexit>sha1_base64="qjsk0YoVb0VEDsckOZ7w095kORk=">AAACB3icbVC7TsMwFHXKq4RXgZHFokJiqhIWWBAVLIxF0Idoq8pxb1qrthPZDlIV9QPYWWHhA9gQKxOfgPgDPgOn7QAtR7J0dM59+QQxZ9p43peTW1hcWl7Jr7pr6xubW4XtnZqOEkWhSiMeqUZANHAmoWqY4dCIFRARcKgHg4vMr9+B0iySN2YYQ1uQnmQho8RY6bYliOkrkV6POoWiV/LGwPPEn5Li2Yd7Gj9/upVO4bvVjWgiQBrKidZN34tNOyXKMMph5LYSDTGhA9KDpqWSCNDtdHzxCB9YpYvDSNknDR6rvztSIrQeisBWZhfqWS8T//UkoxAqQmf2m/CknTIZJwYknawPE45NhLNQcJcpoIYPLSFUMfsDTPvEjjE2OtdG488GMU9qRyXfK/lXfrF8jibIoz20jw6Rj45RGV2iCqoiiiR6QI/oybl3XpxX521SmnOmPbvoD5z3H3kMnYI=</latexit>sha1_base64="5cdJPwtUKMBeIawMtLaHqQhkZ8s=">AAACB3icbVC7TsMwFL0pr1JeBUYWiwqJqUpYYKxgYSyCtog2qhz3prXqOJHtIFVRP4CdFX6BDbHyGfwBn4HTZoCWI1k6Oue+fIJEcG1c98sprayurW+UNytb2zu7e9X9g7aOU8WwxWIRq/uAahRcYstwI/A+UUijQGAnGF/lfucRleaxvDOTBP2IDiUPOaPGSg+9iJqRirLbab9ac+vuDGSZeAWpQYFmv/rdG8QsjVAaJqjWXc9NjJ9RZTgTOK30Uo0JZWM6xK6lkkao/Wx28ZScWGVAwljZJw2Zqb87MhppPYkCW5lfqBe9XPzXk5xhqChb2G/CCz/jMkkNSjZfH6aCmJjkoZABV8iMmFhCmeL2B4SNqB1jbHQVG423GMQyaZ/VPbfu3Xi1xmURUhmO4BhOwYNzaMA1NKEFDCQ8wwu8Ok/Om/PufMxLS07Rcwh/4Hz+ANCgmkE=</latexit> L <latexit sha1_base64="pyW9Bkf9Bd9lWADQi2StAQVtzGw=">AAACEHicbVC7TsMwFL0pr1JeKYwwRFRITFXCAmMFC2Mr6ENqoshxndaq40S2A6qi/AQ7K/xCN8TKH7Ax8hk4bQdoOZKlo3PuyydIGJXKtj+N0tr6xuZWebuys7u3f2BWDzsyTgUmbRyzWPQCJAmjnLQVVYz0EkFQFDDSDcY3hd99IELSmN+rSUK8CA05DSlGSku+WXUTSf3MjZAaiSi7y3PfrNl1ewZrlTgLUmucTFtfAND0zW93EOM0IlxhhqTsO3aivAwJRTEjecVNJUkQHqMh6WvKUUSkl81Oz60zrQysMBb6cWXN1N8dGYqknESBrixOlMteIf7rcYpJKBBe2q/CKy+jPEkV4Xi+PkyZpWKrSMcaUEGwYhNNEBZU/8DCI6THKJ1hRUfjLAexSjoXdceuOy2n1riGOcpwDKdwDg5cQgNuoQltwPAIz/ACr8aTMTXejPd5aclY9BzBHxgfP4NOn/w=</latexit>sha1_base64="47CEc2b6x1X1o5Q2tMlOTSh49ck=">AAACEHicbVC7TsMwFHXKq5RXCiMIWVRITFXCAmMFC2Mr6ENqo8hxndaq7US2A6qijPwAOyv8QjfEyh/wB0j8BE7bAVqOZOnonPvyCWJGlXacT6uwsrq2vlHcLG1t7+zu2eX9looSiUkTRyySnQApwqggTU01I51YEsQDRtrB6Dr32/dEKhqJOz2OicfRQNCQYqSN5NvlXqyon/Y40kPJ09ss8+2KU3WmgMvEnZNK7WjS+H48ntR9+6vXj3DCidCYIaW6rhNrL0VSU8xIVuolisQIj9CAdA0ViBPlpdPTM3hqlD4MI2me0HCq/u5IEVdqzANTmZ+oFr1c/NcTFJNQIrywX4eXXkpFnGgi8Gx9mDCoI5inA/tUEqzZ2BCEJTU/gHiIzBhtMiyZaNzFIJZJ67zqOlW34VZqV2CGIjgEJ+AMuOAC1MANqIMmwOABPIMX8Go9WRPrzXqflRasec8B+APr4weAV6Fi</latexit>sha1_base64="kZSxv6+jz4yaFlDqQw3zqPObrwc=">AAACEHicbVC7TsMwFL0pr1JeKYwsFhUSU5WwwFjBwlgEfUhtFDmu01p1nMh2QFWUn2BnhV9gQ6z8AX/AZ+C0GaDlSJaOzrkvnyDhTGnH+bIqa+sbm1vV7drO7t7+gV0/7Ko4lYR2SMxj2Q+wopwJ2tFMc9pPJMVRwGkvmF4Xfu+BSsVica9nCfUiPBYsZARrI/l2fZgo5mfDCOuJjLK7PPfthtN05kCrxC1JA0q0fft7OIpJGlGhCcdKDVwn0V6GpWaE07w2TBVNMJniMR0YKnBElZfNT8/RqVFGKIyleUKjufq7I8ORUrMoMJXFiWrZK8R/PcEIDSUmS/t1eOllTCSppoIs1ocpRzpGRTpoxCQlms8MwUQy8wNEJtiM0SbDmonGXQ5ilXTPm67TdG/dRuuqDKkKx3ACZ+DCBbTgBtrQAQKP8Awv8Go9WW/Wu/WxKK1YZc8R/IH1+QNBZp23</latexit> S z<latexit sha1_base64="diKaSBnv4kWIs/MA5TN6QETczgI=">AAAB/nicbVDLSgNBEOyNrxhfUY9eBoPgKex60YsY9OIxAfOAZAmzk95kyOzsMjMrxCWQu1f9BW/i1T/wG/wDP8PJ46DRgoaiqnt6uoJEcG1c99PJrayurW/kNwtb2zu7e8X9g4aOU8WwzmIRq1ZANQousW64EdhKFNIoENgMhjdTv3mPSvNY3plRgn5E+5KHnFFjpdpDt1hyy+4M5C/xFqR09VG4nABAtVv86vRilkYoDRNU67bnJsbPqDKcCRwXOqnGhLIh7WPbUkkj1H42++iYnFilR8JY2ZKGzNSfExmNtB5Fge2MqBnoZW8q/utJzjBUlC3tN+GFn3GZpAYlm68PU0FMTKZZkB5XyIwYWUKZ4vYCwgbUPmNsYgUbjbccxF/SOCt7btmreaXKNcyRhyM4hlPw4BwqcAtVqAMDhEd4gmdn4rw4r87bvDXnLGYO4Rec928WK5f+</latexit>sha1_base64="2sApYc+yanwWBCp6RgSllyeoXnE=">AAAB/nicbVDLSsNAFL2prxpfVZdugkVwVRI3uhGLbly2YB/QhjKZ3rRDJ5MwMxFqKLh3q18guBO3/oGfIP6Bn+H0sdDWAxcO59w7d+4JEs6Udt0vK7e0vLK6ll+3Nza3tncKu3t1FaeSYo3GPJbNgCjkTGBNM82xmUgkUcCxEQyuxn7jFqVisbjRwwT9iPQECxkl2kjVu06h6JbcCZxF4s1I8eLDPk+eP+1Kp/Dd7sY0jVBoyolSLc9NtJ8RqRnlOLLbqcKE0AHpYctQQSJUfjb56Mg5MkrXCWNpSmhnov6eyEik1DAKTGdEdF/Ne2PxX08wiqEkdG6/Ds/8jIkk1SjodH2YckfHzjgLp8skUs2HhhAqmbnAoX1intEmMdtE480HsUjqJyXPLXlVr1i+hCnycACHcAwenEIZrqECNaCA8ACP8GTdWy/Wq/U2bc1Zs5l9+APr/Qcnsply</latexit>sha1_base64="e/nwL20XZrQwgFiRhif+T1j26BI=">AAAB/nicbVDLTgJBEOzFF+IL9ehlIjHxRHa9yJHoxSMk8khgQ2aHXpgwO7uZmTXBDYl3r/oL3oxXf8U/8DMcYA8KVtJJpap7erqCRHBtXPfLKWxsbm3vFHdLe/sHh0fl45O2jlPFsMViEatuQDUKLrFluBHYTRTSKBDYCSa3c7/zgErzWN6baYJ+REeSh5xRY6Xm46BccavuAmSdeDmpQI7GoPzdH8YsjVAaJqjWPc9NjJ9RZTgTOCv1U40JZRM6wp6lkkao/Wzx0Rm5sMqQhLGyJQ1ZqL8nMhppPY0C2xlRM9ar3lz815OcYagoW9lvwpqfcZmkBiVbrg9TQUxM5lmQIVfIjJhaQpni9gLCxtQ+Y2xiJRuNtxrEOmlfVT236jW9Sv0mD6kIZ3AOl+DBNdThDhrQAgYIz/ACr86T8+a8Ox/L1oKTz5zCHzifP39GljE=</latexit>
Load more

Recommended publications
  • Quantum Theory Cannot Consistently Describe the Use of Itself
    ARTICLE DOI: 10.1038/s41467-018-05739-8 OPEN Quantum theory cannot consistently describe the use of itself Daniela Frauchiger1 & Renato Renner1 Quantum theory provides an extremely accurate description of fundamental processes in physics. It thus seems likely that the theory is applicable beyond the, mostly microscopic, domain in which it has been tested experimentally. Here, we propose a Gedankenexperiment 1234567890():,; to investigate the question whether quantum theory can, in principle, have universal validity. The idea is that, if the answer was yes, it must be possible to employ quantum theory to model complex systems that include agents who are themselves using quantum theory. Analysing the experiment under this presumption, we find that one agent, upon observing a particular measurement outcome, must conclude that another agent has predicted the opposite outcome with certainty. The agents’ conclusions, although all derived within quantum theory, are thus inconsistent. This indicates that quantum theory cannot be extrapolated to complex systems, at least not in a straightforward manner. 1 Institute for Theoretical Physics, ETH Zurich, 8093 Zurich, Switzerland. Correspondence and requests for materials should be addressed to R.R. (email: [email protected]) NATURE COMMUNICATIONS | (2018) 9:3711 | DOI: 10.1038/s41467-018-05739-8 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/s41467-018-05739-8 “ 1”〉 “ 1”〉 irect experimental tests of quantum theory are mostly Here, | z ¼À2 D and | z ¼þ2 D denote states of D depending restricted to microscopic domains. Nevertheless, quantum on the measurement outcome z shown by the devices within the D “ψ ”〉 “ψ ”〉 theory is commonly regarded as being (almost) uni- lab.
    [Show full text]
  • Appendix: the Interaction Picture
    Appendix: The Interaction Picture The technique of expressing operators and quantum states in the so-called interaction picture is of great importance in treating quantum-mechanical problems in a perturbative fashion. It plays an important role, for example, in the derivation of the Born–Markov master equation for decoherence detailed in Sect. 4.2.2. We shall review the basics of the interaction-picture approach in the following. We begin by considering a Hamiltonian of the form Hˆ = Hˆ0 + V.ˆ (A.1) Here Hˆ0 denotes the part of the Hamiltonian that describes the free (un- perturbed) evolution of a system S, whereas Vˆ is some added external per- turbation. In applications of the interaction-picture formalism, Vˆ is typically assumed to be weak in comparison with Hˆ0, and the idea is then to deter- mine the approximate dynamics of the system given this assumption. In the following, however, we shall proceed with exact calculations only and make no assumption about the relative strengths of the two components Hˆ0 and Vˆ . From standard quantum theory, we know that the expectation value of an operator observable Aˆ(t) is given by the trace rule [see (2.17)], & ' & ' ˆ ˆ Aˆ(t) =Tr Aˆ(t)ˆρ(t) =Tr Aˆ(t)e−iHtρˆ(0)eiHt . (A.2) For reasons that will immediately become obvious, let us rewrite this expres- sion as & ' ˆ ˆ ˆ ˆ ˆ ˆ Aˆ(t) =Tr eiH0tAˆ(t)e−iH0t eiH0te−iHtρˆ(0)eiHte−iH0t . (A.3) ˆ In inserting the time-evolution operators e±iH0t at the beginning and the end of the expression in square brackets, we have made use of the fact that the trace is cyclic under permutations of the arguments, i.e., that Tr AˆBˆCˆ ··· =Tr BˆCˆ ···Aˆ , etc.
    [Show full text]
  • Quantum Violation of Classical Physics in Macroscopic Systems
    Quantum VIOLATION OF CLASSICAL PHYSICS IN MACROSCOPIC SYSTEMS Lucas Clemente Ludwig Maximilian University OF Munich Max Planck INSTITUTE OF Quantum Optics Quantum VIOLATION OF CLASSICAL PHYSICS IN MACROSCOPIC SYSTEMS Lucas Clemente Dissertation AN DER Fakultät für Physik DER Ludwig-Maximilians-Universität München VORGELEGT VON Lucas Clemente AUS München München, IM NoVEMBER 2015 TAG DER mündlichen Prüfung: 26. Januar 2016 Erstgutachter: Prof. J. IGNACIO Cirac, PhD Zweitgutachter: Prof. Dr. Jan VON Delft WEITERE Prüfungskommissionsmitglieder: Prof. Dr. HarALD Weinfurter, Prof. Dr. Armin Scrinzi Reality is that which, when you stop believing in it, doesn’t go away. “ — Philip K. Dick How To Build A Universe That Doesn’t Fall Apart Two Days Later, a speech published in the collection I Hope I Shall Arrive Soon Contents Abstract xi Zusammenfassung xiii List of publications xv Acknowledgments xvii 0 Introduction 1 0.1 History and motivation . 3 0.2 Local realism and Bell’s theorem . 5 0.3 Contents of this thesis . 10 1 Conditions for macrorealism 11 1.1 Macroscopic realism . 13 1.2 Macrorealism per se following from strong non-invasive measurability 15 1.3 The Leggett-Garg inequality . 17 1.4 No-signaling in time . 19 1.5 Necessary and sufficient conditions for macrorealism . 21 1.6 No-signaling in time for quantum measurements . 25 1.6.1 Without time evolution . 26 1.6.2 With time evolution . 27 1.7 Conclusion and outlook . 28 Appendix 31 1.A Proof that NSIT0(1)2 is sufficient for NIC0(1)2 . 31 2 Macroscopic classical dynamics from microscopic quantum behavior 33 2.1 Quantifying violations of classicality .
    [Show full text]
  • Particle Or Wave: There Is No Evidence of Single Photon Delayed Choice
    Particle or wave: there is no evidence of single photon delayed choice. Michael Devereux* Los Alamos National Laboratory (Retired) Abstract Wheeler supposed that the way in which a single photon is measured in the present could determine how it had behaved in the past. He named such retrocausation delayed choice. Over the last forty years many experimentalists have claimed to have observed single-photon delayed choice. Recently, however, researchers have proven that the quantum wavefunction of a single photon assumes the identical mathematical form of the solution to Maxwell’s equations for that photon. This efficacious understanding allows for a trenchant analysis of delayed-choice experiments and denies their retrocausation conclusions. It is now usual for physicists to employ Bohr’s wave-particle complementarity theory to distinguish wave from particle aspects in delayed- choice observations. Nevertheless, single-photon, delayed-choice experiments, provide no evidence that the photon actually acts like a particle, or, instead, like a wave, as a function of a future measurement. And, a recent, careful, Stern-Gerlach analysis has shown that the supposition of concurrent wave and particle characteristics in the Bohm-DeBroglie theory is not tenable. PACS 03.65.Ta, 03.65.Ud 03.67.-a, 42.50.Ar, 42.50.Xa 1. Introduction. Almost forty years ago Wheeler suggested that there exists a type of retrocausation, which he called delayed choice, in certain physical phenomena [1]. Specifically, he said that the past behavior of some quantum systems could be determined by how they are observed in the present. “The past”, he wrote, “has no existence except as it is recorded in the present” [2].
    [Show full text]
  • The Consistent Histories Approach to the Stern-Gerlach Experiment
    The Consistent Histories Approach to the Stern-Gerlach Experiment Ian Wilson An undergraduate thesis advised by Dr. David Craig submitted to the Department of Physics, Oregon State University in partial fulfillment of the requirements for the degree BSc in Physics Submitted on May 8, 2020 Acknowledgments I would like to thank Dr. David Craig, for guiding me through an engaging line of research, as well as Dr. David McIntyre, Dr. Elizabeth Gire, Dr. Corinne Manogue and Dr. Janet Tate for developing the quantum curriculum from which this thesis is rooted. I would also like to thank all of those who gave me the time, space, and support I needed while writing this. This includes (but is certainly not limited to) my partner Brooke, my parents Joy and Kevin, my housemate Cheyanne, the staff of Interzone, and my friends Saskia, Rachel and Justin. Abstract Standard quantum mechanics makes foundational assumptions to describe the measurement process. Upon interaction with a “classical measurement apparatus”, a quantum system is subjected to postulated “state collapse” dynamics. We show that framing measurement around state collapse and ill-defined classical observers leads to interpretational issues, and artificially limits the scope of quantum theory. This motivates describing measurement as a unitary process instead. In the context of the Stern-Gerlach experiment, the measurement of an electron’s spin angular momentum is explained as the entanglement of its spin and position degrees of freedom. Furthermore, the electron-environment interaction is also detailed as part of the measurement process. The environment plays the role of a record keeper, establishing the “facts of the universe” to make the measurement’s occurrence objective.
    [Show full text]
  • Theoretical Physics Group Decoherent Histories Approach: a Quantum Description of Closed Systems
    Theoretical Physics Group Department of Physics Decoherent Histories Approach: A Quantum Description of Closed Systems Author: Supervisor: Pak To Cheung Prof. Jonathan J. Halliwell CID: 01830314 A thesis submitted for the degree of MSc Quantum Fields and Fundamental Forces Contents 1 Introduction2 2 Mathematical Formalism9 2.1 General Idea...................................9 2.2 Operator Formulation............................. 10 2.3 Path Integral Formulation........................... 18 3 Interpretation 20 3.1 Decoherent Family............................... 20 3.1a. Logical Conclusions........................... 20 3.1b. Probabilities of Histories........................ 21 3.1c. Causality Paradox........................... 22 3.1d. Approximate Decoherence....................... 24 3.2 Incompatible Sets................................ 25 3.2a. Contradictory Conclusions....................... 25 3.2b. Logic................................... 28 3.2c. Single-Family Rule........................... 30 3.3 Quasiclassical Domains............................. 32 3.4 Many History Interpretation.......................... 34 3.5 Unknown Set Interpretation.......................... 36 4 Applications 36 4.1 EPR Paradox.................................. 36 4.2 Hydrodynamic Variables............................ 41 4.3 Arrival Time Problem............................. 43 4.4 Quantum Fields and Quantum Cosmology.................. 45 5 Summary 48 6 References 51 Appendices 56 A Boolean Algebra 56 B Derivation of Path Integral Method From Operator
    [Show full text]
  • Consistent Histories
    Chapter 12 Local Weak Property Realism: Consistent Histories Earlier, we surmised that weak property realism can escape the strictures of Bell’s theorem and KS. In this chapter, we look at an interpretation of quantum mechanics that does just that, namely, the Consistent Histories Interpretation (CH). 12.1 Consistent Histories The basic idea behind CH is that quantum mechanics is a stochastic theory operating on quantum histories. A history a is a temporal sequence of properties held by a system. For example, if we just consider spin, for an electron a history could be as follows: at t0, Sz =1; at t1, Sx =1; at t2, Sy = -1, where spin components are measured in units of h /2. The crucial point is that in this interpretation the electron is taken to exist independently of us and to have such spin components at such times, while in the standard view 1, 1, and -1 are just experimental return values. In other words, while the standard interpretation tries to provide the probability that such returns would be obtained upon measurement, CH provides the probability Pr(a) that the system will have history a, that is, such and such properties at different times. Hence, CH adopts non-contextual property realism (albeit of the weak sort) and dethrones measurement from the pivotal position it occupies in the orthodox interpretation. To see how CH works, we need first to look at some of the formal features of probability. 12.2 Probability Probability is defined on a sample space S, the set of all the mutually exclusive outcomes of a state of affairs; each element e of S is a sample point to which one assigns a number Pr(e) satisfying the axioms of probability (of which more later).
    [Show full text]
  • New Journal of Physics the Open–Access Journal for Physics
    New Journal of Physics The open–access journal for physics The rise and fall of redundancy in decoherence and quantum Darwinism C Jess Riedel1,2,5, Wojciech H Zurek1,3 and Michael Zwolak1,4 1 Theoretical Division/CNLS, LANL, Los Alamos, NM 87545, USA 2 Department of Physics, University of California, Santa Barbara, CA 93106, USA 3 Santa Fe Institute, Santa Fe, NM 87501, USA 4 Department of Physics, Oregon State University, Corvallis, OR 97331, USA E-mail: [email protected] New Journal of Physics 14 (2012) 083010 (20pp) Received 9 March 2012 Published 10 August 2012 Online at http://www.njp.org/ doi:10.1088/1367-2630/14/8/083010 Abstract. A state selected at random from the Hilbert space of a many-body system is overwhelmingly likely to exhibit highly non-classical correlations. For these typical states, half of the environment must be measured by an observer to determine the state of a given subsystem. The objectivity of classical reality—the fact that multiple observers can agree on the state of a subsystem after measuring just a small fraction of its environment—implies that the correlations found in nature between macroscopic systems and their environments are exceptional. Building on previous studies of quantum Darwinism showing that highly redundant branching states are produced ubiquitously during pure decoherence, we examine the conditions needed for the creation of branching states and study their demise through many-body interactions. We show that even constrained dynamics can suppress redundancy to the values typical of random states on relaxation timescales, and prove that these results hold exactly in the thermodynamic limit.
    [Show full text]
  • High Energy Physics Quantum Information Science Awards Abstracts
    High Energy Physics Quantum Information Science Awards Abstracts Towards Directional Detection of WIMP Dark Matter using Spectroscopy of Quantum Defects in Diamond Ronald Walsworth, David Phillips, and Alexander Sushkov Challenges and Opportunities in Noise‐Aware Implementations of Quantum Field Theories on Near‐Term Quantum Computing Hardware Raphael Pooser, Patrick Dreher, and Lex Kemper Quantum Sensors for Wide Band Axion Dark Matter Detection Peter S Barry, Andrew Sonnenschein, Clarence Chang, Jiansong Gao, Steve Kuhlmann, Noah Kurinsky, and Joel Ullom The Dark Matter Radio‐: A Quantum‐Enhanced Dark Matter Search Kent Irwin and Peter Graham Quantum Sensors for Light-field Dark Matter Searches Kent Irwin, Peter Graham, Alexander Sushkov, Dmitry Budke, and Derek Kimball The Geometry and Flow of Quantum Information: From Quantum Gravity to Quantum Technology Raphael Bousso1, Ehud Altman1, Ning Bao1, Patrick Hayden, Christopher Monroe, Yasunori Nomura1, Xiao‐Liang Qi, Monika Schleier‐Smith, Brian Swingle3, Norman Yao1, and Michael Zaletel Algebraic Approach Towards Quantum Information in Quantum Field Theory and Holography Daniel Harlow, Aram Harrow and Hong Liu Interplay of Quantum Information, Thermodynamics, and Gravity in the Early Universe Nishant Agarwal, Adolfo del Campo, Archana Kamal, and Sarah Shandera Quantum Computing for Neutrino‐nucleus Dynamics Joseph Carlson, Rajan Gupta, Andy C.N. Li, Gabriel Perdue, and Alessandro Roggero Quantum‐Enhanced Metrology with Trapped Ions for Fundamental Physics Salman Habib, Kaifeng Cui1,
    [Show full text]
  • Quantum Trajectories for Measurement of Entangled States
    Quantum Trajectories for Measurement of Entangled States Apoorva Patel Centre for High Energy Physics, Indian Institute of Science, Bangalore 1 Feb 2018, ISNFQC18, SNBNCBS, Kolkata 1 Feb 2018, ISNFQC18, SNBNCBS, Kolkata A. Patel (CHEP, IISc) Quantum Trajectories for Entangled States / 22 Density Matrix The density matrix encodes complete information of a quantum system. It describes a ray in the Hilbert space. It is Hermitian and positive, with Tr(ρ)=1. It generalises the concept of probability distribution to quantum theory. 1 Feb 2018, ISNFQC18, SNBNCBS, Kolkata A. Patel (CHEP, IISc) Quantum Trajectories for Entangled States / 22 Density Matrix The density matrix encodes complete information of a quantum system. It describes a ray in the Hilbert space. It is Hermitian and positive, with Tr(ρ)=1. It generalises the concept of probability distribution to quantum theory. The real diagonal elements are the classical probabilities of observing various orthogonal eigenstates. The complex off-diagonal elements (coherences) describe quantum correlations among the orthogonal eigenstates. 1 Feb 2018, ISNFQC18, SNBNCBS, Kolkata A. Patel (CHEP, IISc) Quantum Trajectories for Entangled States / 22 Density Matrix The density matrix encodes complete information of a quantum system. It describes a ray in the Hilbert space. It is Hermitian and positive, with Tr(ρ)=1. It generalises the concept of probability distribution to quantum theory. The real diagonal elements are the classical probabilities of observing various orthogonal eigenstates. The complex off-diagonal elements (coherences) describe quantum correlations among the orthogonal eigenstates. For pure states, ρ2 = ρ and det(ρ)=0. Any power-series expandable function f (ρ) becomes a linear combination of ρ and I .
    [Show full text]
  • DECOHERENCE AS a FUNDAMENTAL PHENOMENON in QUANTUM DYNAMICS Contents 1 Introduction 152 2 Decoherence by Entanglement with an En
    DECOHERENCE AS A FUNDAMENTAL PHENOMENON IN QUANTUM DYNAMICS MICHAEL B. MENSKY P.N. Lebedev Physical Institute, 117924 Moscow, Russia The phenomenon of decoherence of a quantum system caused by the entangle- ment of the system with its environment is discussed from di®erent points of view, particularly in the framework of quantum theory of measurements. The selective presentation of decoherence (taking into account the state of the environment) by restricted path integrals or by e®ective SchrÄodinger equation is shown to follow from the ¯rst principles or from models. Fundamental character of this phenomenon is demonstrated, particularly the role played in it by information is underlined. It is argued that quantum mechanics becomes logically closed and contains no para- doxes if it is formulated as a theory of open systems with decoherence taken into account. If one insist on considering a completely closed system (the whole Uni- verse), the observer's consciousness has to be included in the theory explicitly. Such a theory is not motivated by physics, but may be interesting as a metaphys- ical theory clarifying the concept of consciousness. Contents 1 Introduction 152 2 Decoherence by entanglement with an environment 153 3 Restricted path integrals 155 4 Monitoring of an observable 158 5 Quantum monitoring and quantum Central Limiting Theorem161 6 Fundamental character of decoherence 163 7 The role of consciousness in quantum mechanics 166 8 Conclusion 170 151 152 Michael B. Mensky 1 Introduction It is honor for me to contribute to the volume in memory of my friend Michael Marinov, particularly because this gives a good opportunity to recall one of his early innovatory works which was not properly understood at the moment of its publication.
    [Show full text]
  • How Quantum Is Quantum Counterfactual Communication? Arxiv
    View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Explore Bristol Research Hance, J. R. , Ladyman, J., & Rarity, J. (Accepted/In press). How Quantum is Quantum Counterfactual Communication? arXiv. Peer reviewed version Link to publication record in Explore Bristol Research PDF-document University of Bristol - Explore Bristol Research General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms How Quantum is Quantum Counterfactual Communication? Jonte R. Hance,1, ∗ James Ladyman,2 and John Rarity1 1Quantum Engineering Technology Laboratories, Department of Electrical and Electronic Engineering, University of Bristol, Woodland Road, Bristol, BS8 1US, UK 2Department of Philosophy, University of Bristol, Cotham House, Bristol, BS6 6JL Quantum Counterfactual Communication is the recently-proposed idea of using quantum mechan- ics to send messages between two parties, without any particles travelling between them. While this has excited massive interest, both for potential `un-hackable' communication, and insight into the foundations of quantum mechanics, it has been asked whether this phenomena is truly quantum, or could be performed classically. We examine counterfactuality, both classical and quantum, and the protocols proposed so far, and conclude it must be quantum, at least insofar as it requires particle quantisation. I. INTRODUCTION municated to counterfactually (e.g. knowing a car's en- gine works by the `check engine' light being off; knowing Quantum Counterfactual Communication is the no-one is calling you by your phone not ringing; or being combination of counterfactual circumstances (where sure your house is not on fire by hearing no alarm).
    [Show full text]