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arXiv:1005.3449 · Physica Scripta

Entanglement, intractability and no-signaling

R. Srikanth

Published May 12, 2010• 4 citations
arXivHTMLPDF

Abstract

We consider the problem of deriving the no-signaling condition from the assumption that, as seen from a complexity theoretic perspective, the universe is not an exponential place. A fact that disallows such a derivation is the existence of polynomial superluminal gates, hypothetical primitive operations that enable superluminal signaling but not the efficient solution of intractable problems. It therefore follows, if this assumption is a basic principle of physics, either that it must be supplemented with additional assumptions to prohibit such gates or, improbably, that no-signaling is not a universal condition. Yet a gate of this kind is possibly implicit, although not recognized as such, in a decade-old quantum optical experiment involving position–momentum entangled photons. Here, we describe a feasible modified version of the experiment that appears to explicitly demonstrate the action of this gate. Some obvious counter-claims are shown to be invalid. We believe that the unexpected possibility of polynomial superluminal operations arises because some practically measured quantum optical quantities are not describable as standard quantum mechanical observables.

DOI
10.1088/0031-8949/81/06/065002

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We consider the problem of deriving the no-signaling condition from the assumption that, as seen from a complexity theoretic perspective, the universe is not an exponential place. A fact that disallows such a derivation is the existence of polynomial superluminal gates, hypothetical primitive operations that enable superluminal signaling but not the efficient solution of intractable problems. It therefore follows, if this assumption is a basic principle of physics, either that it must be supplemented with additional assumptions to prohibit such gates or, improbably, that no-signaling is not a universal condition. Yet a gate of this kind is possibly implicit, although not recognized as such, in a decade-old quantum optical experiment involving position–momentum entangled photons. Here, we describe a feasible modified version of the experiment that appears to explicitly demonstrate the action of this gate. Some obvious counter-claims are shown to be invalid. We believe that the unexpected possibility of polynomial superluminal operations arises because some practically measured quantum optical quantities are not describable as standard quantum mechanical observables.
We consider the problem of deriving the no-signaling condition from the assumption that, as seen from a complexity theoretic perspective, the universe is not an exponential place. A fact that disallows such a derivation is the existence of {\em polynomial superluminal} gates, hypothetical primitive operations that enable superluminal signaling but not the efficient solution of intractable problems. It therefore follows, if this assumption is a basic principle of physics, either that it must be supplemented with additional assumptions to prohibit such gates, or, improbably, that no-signaling is not a universal condition. Yet, a gate of this kind is possibly implicit, though not recognized as such, in a decade-old quantum optical experiment involving position-momentum entangled photons. Here we describe a feasible modified version experiment that appears to explicitly demonstrate the action of this gate. Some obvious counter-claims are shown to be invalid. We believe that the unexpected possibility of polynomial superluminal operations arises because some practically measured quantum optical quantities are not describable as standard quantum mechanical observables.
The quantum foundations literature contains dozens of "no-signaling" arguments that rely on the Born rule or, more generally, the positive operator-valued measure (POVM) framework to show that the resources of non-relativistic quantum mechanics (NRQM) cannot be used to send superluminal signals. What these arguments typically establish, however, is not a prohibition on superluminal signaling per se, but the seemingly stronger claim that signaling at any speed between mutually isolated systems is forbidden. This paper gives a precise formulation of this claim, the no-signaling hypothesis, and investigates the following converse question: under what additional assumptions does it imply that all experimental outcome probabilities are described by POVMs? Surprisingly few are needed: given the standard NRQM representation of composite experiments and two textbook claims about marginal probabilities, the no-signaling hypothesis is equivalent to the universality of the POVM framework. More broadly, by formalizing the relevant concepts and deriving the logical relationships between them, the paper shows that the no-signaling hypothesis can be used to derive most, though not all, of the probability-generating fragment of NRQM. The resulting map of implications clarifies which common postulates of NRQM are logically interrelated and which can be modified independently.
We show that the basic dynamical rules of quantum physics can be derived from its static properties and the condition that superluminal communication is forbidden. More precisely, the fact that the dynamics has to be described by linear completely positive maps on density matrices is derived from the following assumptions: (1) physical states are described by rays in a Hilbert space, (2) probabilities for measurement outcomes at any given time are calculated according to the usual trace rule, and (3) superluminal communication is excluded. This result also constrains possible nonlinear modifications of quantum physics.
Two of the key properties of quantum physics are the no-signaling principle and the Grover search lower bound. That is, despite admitting stronger-than-classical correlations, quantum mechanics does not imply superluminal signaling, and despite a form of exponential parallelism, quantum mechanics does not imply polynomial-time brute force solution of NP-complete problems. Here, we investigate the degree to which these two properties are connected. We examine four classes of deviations from quantum mechanics, for which we draw inspiration from the literature on the black hole information paradox. We show that in these models, the physical resources required to send a superluminal signal scale polynomially with the resources needed to speed up Grover's algorithm. Hence the no-signaling principle is equivalent to the inability to solve NP-hard problems efficiently by brute force within the classes of theories analyzed.
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.
We investigate whether the inner product structure of quantum mechanics can be modified without violating fundamental physical principles. We consider a generalized inner product defined by a positive operator and assume local unitary dynamics, existence of entangled states and the no-signalling principle. We show that any nontrivial choice of inner product different from standard one inevitably leads to superluminal signalling, in contradiction with relativistic causality. Therefore, the standard Hilbert-space inner product is uniquely enforced by no-signalling.