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arXiv:0705.1898 · Physical Review Letters

Experimental Test of the No-Signaling Theorem

Tiziano De Angelis, Eleonora Nagali, Fabio Sciarrino, F. De Martini

Published Nov 7, 2007• 14 citations
arXivHTMLPDF

Abstract

In 1981 N. Herbert proposed a gedanken experiment in order to achieve by the "First Laser-Amplified Superluminal Hookup" (FLASH) a faster-than-light (FTL) communication by quantum nonlocality. The present work reports the first experimental realization of that proposal by the optical parametric amplification of a single photon belonging to an entangled EPR pair into an output field involving N=5 x 10{3} photons. A theoretical and experimental analysis explains in general and conclusive terms the precise reasons for the failure of the FLASH program as well as of any similar FTL proposals.

DOI
10.1103/physrevlett.99.193601

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In 1981 N. Herbert proposed a gedanken experiment in order to achieve by the "First Laser-Amplified Superluminal Hookup" (FLASH) a faster-than-light (FTL) communication by quantum nonlocality. The present work reports the first experimental realization of that proposal by the optical parametric amplification of a single photon belonging to an entangled EPR pair into an output field involving N=5 x 10{3} photons. A theoretical and experimental analysis explains in general and conclusive terms the precise reasons for the failure of the FLASH program as well as of any similar FTL proposals.
The paper is a Chapter of a book. In it an exhaustive review of the proposals to send faster than light signals resorting to quantum nonlocality and the reduction process is presented, together with a critical analysis and rebuttal of all proposals. The most interesting part of the Chapter is the one in which the problem of quantum cloning is discussed. Actually, a proposal of superluminal signaling based on an hypothetical cloning machine has been presented by N. Herbert. The proposal does not work just because of the assumption that one can clone nonorthogonal states. The fact that such a machine is incompatible with quantum theory (i.e., the so-called No-Cloning Theorem) has been proved, for the first time, by the author of the present chapter in his referee's report to Herbert's paper. In the final part of the paper some recent (different but similarly not correct) proposals are analyzed and the reasons for which they are basically wrong are presented.
The Bell/EPR experiments are widely interpreted as supporting the no-signaling theorem inquantum entanglement by utilizing detector settings adjusted faster than light-travel timeto test locality [Aspect et al., 1982, Giustina et al., 2015]. This family of experiments, however, reveals a critical conceptual oversight: it presupposes faster-than-light (FTL) influencesas finite-speed propagation processes, rather than considering instantaneous state updatesinherent to non-local correlations as proposed in early quantum interpretations [Einsteinet al., 1935]. This assumption renders the timing of detector switches irrelevant, as entangled state correlations are established instantaneously upon state preparation. This paperelucidates this oversight, demonstrating that these experiments do not conclusively ruleout instantaneous transmission of information under all interpretations, thereby challengingtheir standard conclusions. We propose that new discrete-state experiments be designed tospecifically test for instantaneous state updates, advancing our understanding of quantumnon-locality
A recently proposed gedanken experiment involving the (gravitational or electromagnetic) interaction between two objects---one placed in a state of quantum superposition of two locations---seems to allow for faster-than-light communication. However, it has been argued that, if the mediating fields are endowed with quantum properties, then the possibility for superluminal signaling is fully avoided. Moreover, in the gravitational case, this conclusion has been used to argue for the view that the gravitational field must be quantized. In this work, we clarify and complement some aspects of the discussion. In particular, by focusing on the way in which entanglement spreads across the components of the system, we offer some insights into the fundamental quantum features behind the impossibility of superluminal signaling and we provide a more general proof of such an impossibility in this and related protocols.
Faster-than-light communication is possible via quantum entanglement in an EPR experiment if one can overcome nature's "quantum modesty" and gain knowledge concerning the quantum state of a single photon. Using a special EPR source that produces precisely timed doublets of EPR photons (biphotons), I show how one can gain additional information about the photon's polarization state--information not accessible using the conventional one-photon-at-a-time EPR setup. Using the Hong-Ou-Mandel effect that describes how two identical photons combine at a neutral beamsplitter (NBS), I show how Bob can obtain enough polarization information about his B photons to unambiguously ascertain the polarization basis of Alice's distant measurement device which she has deployed to measure her A photons. I am proposing, in short, that EPR + HOM = FTL. Version 2 concludes with a refutation of its FTL claim due to Lev Vaidman.
We propose a thought experiment for classical superluminal signal transmission based on the quantum nonlocal influence of photons on their momentum entangled EPR twins. The signal sender measures either position or momentum of particles in a pure ensemble of the entangled pairs, leaving their twins as localized particles or plane waves. The signal receiver distinguishes these outcomes interferometrically using a double slit interferometer modified by a system of optical filters. Since the collapse of the wavefunction is postulated to be instantaneous, this signal can be transmitted superluminally. We show that the method circumvents the no-signalling theorem because the receiver is able to modify the disentangled wavefunction before his measurement. We propose a plan for the possible practical realization of a superluminal quantum telegraph based on the thought experiment.
Present understanding of non-possibility of Quantum communication rests on analysis of possibility of signalling within a Quantum system. Is it possible to achieve signalling by a setup where the system is changed from one to another where the act of change serves as the signal and the change in the measurement probabilities would be detected and deciphered at the receiver end? A proposal is made where a variant of Delayed Choice Quantum Eraser experiment setup of Kim et al is used to achieve the same. The setup is also designed such that the speed of communication can be faster than light. NonLocality and Quantum communication Nonlocality is a feature of Quantum Mechanics that has been firmly established through experiments [Aspect et al, 1981] and well accepted now. Entangled particles separated by a distance still are part of one entity and when that entity changes state by a measurement in one part of it the change happens instantaneously across the domain the entity pervades. Is it possible to use this instantaneous state change to transmit information across at high speeds, faster than light (FTL)? This issue has been dealt with from the time the Einstein raised the EPR paradox [Einstein et al,1935] and has been claimed to be impossible and shown to be so by Peres, [Peres and Tenko, 2004] through the no-communication theorem which shows that any amount of signalling by Alice through measurements done at one end of the quantum system would not be sufficient for Bob to detect and decipher at the other end of the system. However the discussions in literature (as above) always deal with information transfer within a stable quantum system by making measurements at one end and analysing the possibility of detecting it at the other end. Peres and Tenko analyse a Quantum System S of mixed-state where Alice and Bob make measurements at their ends of the system. The authors derive the probability that Bob’s outcome is ν as