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Copyright: © 2011
|Pages: 37

DOI: 10.4018/978-1-60960-525-4.ch007

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TopAfter conquering his general theory of relativity regarding space, time, and gravity, there must be few physical phenomena left to freak out Albert Einstein. Yet quantum entanglement caused Einstein to use the word *“spooky.”* Take a pair of entangled photons, for instance, it seems that performing an experiment on one of them instantaneously affects another no matter how far apart are the two, whether they are in the same room, or at two opposite ends of the universe. The simultaneous multiple appearances are called *superposition*. Einstein once called this intuitively supernatural behavior *“spooky action at a distance.”*

The EPR paradox (Einstein, Podolsky & Rosen, 1935) challenged long-held ideas about the relation between the observed values of physical quantities and the values that can be accounted for by a physical theory. It was their theoretical possibility or impossibility that led Einstein to reject the idea that quantum mechanics might be a fundamental physical law. Instead, Einstein deemed it an incomplete theory. The missing part has been referred to as *“hidden variables”* in the literature.

Three decades later, Bell’s theorem (Bell, 1964) extended the argument of the EPR paradox and proved the validity of quantum entanglement from a statistical perspective with probability distributions. Despite its practical significance in quantum computing, Bell's theorem so far hasn’t led to a logical unification of general relativity and quantum mechanics. The so-called *“hidden variables”* never really surfaced in simple logically definable terms.

Thus, quantum mechanics at the Planck scale needs to be further reconciled with general relativity. That is the goal of quantum gravity. Until this day, however, the searching for quantum gravity has failed to find a decisive battleground; quantum computing is still years away from reality. Nevertheless, since the 1980s, many successful experimental results have shown the physical existence of quantum entanglement.

Besides the quantum observability problem, another major obstacle in quantum information processing is the difficulty or even impossibility of cloning an unknown quantum state. While classical information can be copied or cloned for storage and retrieval, the quantum *“no cloning”* theorem (Dieks, 1982; Wootters & Zurek, 1982) asserts the impossibility of cloning an unknown quantum state. Without being copied or cloned quantum information cannot be stored for retrieval.

Consequently, despite numerous reported experimental successes in testing quantum entanglement (e.g. Furusawa *et al.,* 1998; Buchanan, 1998; Ghosh *et al.,* 2003; Salart et al., 2008; Jost *et al.,* 2009), the quantum observability controversy and no cloning dilemma remain unresolved. Now many physicists have subscribed to the instrumentalist interpretation of quantum mechanics with the slogan *“Shut up and calculate!”* Some others including several of the best living theoretical physicists feel compelled to question the basic assumptions of relativity and quantum theory. As described by theoretical physicist Lee Smolin they *“learn it, and they can carry out its arguments and calculations as well as anyone. But they don’t believe it.”* (Smolin, 2006, p. 319).

Nevertheless, the *“spooky”* quantum phenomenon has become a fundamental concept in quantum computing even though, until recently, physicists have only been able to demonstrate quantum entanglement through either highly esoteric examples or under extreme conditions. Without a decisive victory in the quest for quantum gravity, it is fair to say that something fundamental must still be missing from the big picture.

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