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Home Jack Sarfatti's Blog Oak Ridge physicists sought entanglement signals ten years ago

I don't know how much progress they made since then.

"The primary objective of this effort is to study the signaling potential of quantum information processing systems based on quantum entanglement. ...

The nonlocality of the correlations of two particles in quantum entanglement has no classical analog. It allows coherent effects to occur instantaneously in spatially separate locations. The question naturally arises as to whether a more general formulation of QT could provide a basis for superluminal communications. This issue has recently been the subject of considerable debate in the open literature.

There are basically two schools of thought: one, which precludes this possibility (based, for example, on conflicts with the theory of special relativity), and one which allows it under special provisions. We will discuss these issues in some detail in the sequel. First, however, we briefly highlight a few of the more significant new findings in the growing experimental and theoretical evidence of superluminal effects.

A conference on superluminal velocities took place in June 1998 in Cologne [21]. Theoretical and experimental contributions to this topic focused primarily on evanescent mode propagation and on superluminal quantum phenomena. The issues of causality, superluminality, and relativity were also examined. In the area of electromagnetic propagation, two exciting developments were addressed. Nimtz reported on experimental measurements of superluminal velocities achieved with frequency band-limited signals carried by evanescent modes [22]. Specifically, he timed a microwave pulse crossing an evanescent barrier (e.g., undersized waveguides, or periodic dielectric heterostructures) at 4.7c. He demonstrated that, as consequence of the frequency band limitation of information signals, and if all mode components are evanescent, an actual signal might travel faster than the speed of light. Capelas de Oliveira and Rodrigues introduced the intriguing theory of superluminal electromagnetic X-waves (SEXW) defined as undistorted progressive waves solutions of the relativistic Maxwell equations [23]. They present simulations of finite aperture approximations to SEXW, illustrate the signaling mechanism, and discuss supporting experimental evidence.

What are the key arguments put forward against the possibility of superluminal signaling? Chiao and Steinberg analyze quantum tunneling experiments and tachyon-like excitations in laser media [24]. Even though they find the evidence conclusive that the tunneling process is superluminal, and that tachyon-like excitations in a population-inverted medium at frequencies close to resonance give rise to superluminal wave packets, they argue that such phenomena can not be used for superluminal information transfer. In their view, the group velocity can not be identified as the signal velocity of special relativity, a role they attribute solely to Sommerfeld’s front velocity. In that context, Aharonov, Reznik, and Stern have shown that the unstable modes, which play an essential role in the superluminal group velocity of analytical wave packets, are strongly suppressed in the quantum limit as they become incompatible with unitary time evolution [25].

At the Cologne symposium [21] Mittelstaedt reviewed the arguments that had been put forward in recent years in order to show that non-local effects in quantum systems with EPR-like correlations can not be used for superluminal communications. He demonstrated that most of these arguments are based on circular proofs. For instance, a “locality principle” can not be used to exclude superluminal quantum signals and to justify quantum causality, since the locality principle itself is justified by either quantum causality or an equivalent “covariance postulate” [32]. In a similar vein, van Enk shows that the proof given by Westmoreland and Schumacher in [33] that superluminal signaling violates the quantum no-cloning theorem is in fact incorrect [34]. Hegerfeld uses the formalism of relativistic quantum mechanics to show that the wave function of a free particle initially in a finite volume instantaneously spreads to infinity and, more importantly, that transition probabilities in widely separated systems may also become nonzero instantaneously [35]. His results hold under amazingly few assumptions (Hilbert space framework and positivity of the energy). Hegerfeld observes that, in order to retain Einstein causality, a mechanism such as “clouds of virtual particles or vacuum fluctuations” would be needed. To conclude this review, we note a recent suggestion of Mittelstaedt [36]. If the existence of superluminal signals is assumed ab initio (viz. [22] and [35]), and consequently a new space-time metric (different from the Minkowskian metric) is adopted, all the paradoxes and difficulties discussed above would immediately disappear.

Let us now examine EPR-based superluminal schemes. Furuya et al analyze a paradigm proposed by Garuccio, in which one of the photons of a polarization-entangled EPR pair is incident upon a Michelson interferometer in which a phase-conjugation mirror (PCM) replaces one of the mirrors [26]. The sender (located at the source site) can superluminally communicate with a receiver (located at the detector site), based on the presence or absence of interferences at the detector. The scheme uses the PCM property that a reflected photon has the same polarization as the incident photon (contrary to reflection by an ordinary mirror), allowing to distinguish between circular and linear polarization. In a related context, Blaauboer et al also proposed [27] a connection between optical phase conjugation and superluminal behavior. Furuya et al prove that Garuccio’s scheme would fail if non coherent light is used, because then the interferometer could not distinguish between unpolarized photons prepared by mixing linear polarization states or by mixing circular polarization states. They admit, however, that their counterproof would not apply to a generalized Garuccio approach, which would use coherent light states. Finally, in terms of criticism, let us mention the recent article by Peres [28], where criteria that prevent superluminal signaling are established. These criteria must be obeyed by various operators involved in classical interventions on quantum systems localized in mutually spacelike regions.

What are the arguments in favor of superluminal information transfer? Gisin shows [29] that Weinberg’s general framework [30] for introducing nonlinear corrections into quantum mechanics allows for arbitrary fast communications. It is interesting to note that, in a recent book [31], Weinberg himself states: “I could not find a way to extend the nonlinear version of quantum mechanics to theories based on Einstein’s special theory of relativity (...) both N. Gisin in Geneva and my colleague Joseph Polchinsky at the University of Texas independently pointed out that (...) the nonlinearities of the generalized theory could be used to send signals instantaneously over large distances”.

In this paper, we have presented recent progress achieved at CESAR/ORNL in the area of QT. We have also highlighted some of the formidable theoretical challenges that must be overcome if an application of this technology to communications is to become possible. The feasibility question is, in our minds, still open. To summarize, we now succinctly indicate our near-term proposed road map.

From a theory perspective, we will focus our attention on two recent proposals for superluminal communications. Greenberger has demonstrated [37] that if one can construct a macroscopic Schrodinger cat state (i.e., a state that maintains quantum coherence), then such a state can be used for sending superluminal signals. His scheme assumes that the following two requirements can be realized. First, it should be possible to entangle the signal-transmitting device with the signal itself, thereby constructing a GHZ state. Second, that non-unitary evolution can be established and controlled in a subset of the complete Hilbert space. This latter property has already been demonstrated successfully in several down conversion experiments. Greenberger uses an optical phase shifter as model for his signaling device. We believe that as of this date better alternatives are available. The second Gedankenexperiment we intend to examine was introduced by Srikanth [37]. His proposed method uses a momentum-entangled EPR source. Assuming a pure ensemble of entangled pairs, either position or momentum is measured at the sender. This leaves the counterpart in the EPR pair as either a localized particle or a plane wave. In Srikanth’s scheme, the receiver distinguishes between these outcomes by means of interferometry. Since the collapse of the wavefunction is assumed to be instantaneous, superluminal signal transmission would be established.

We intend to explore possible experimental realizations of the above paradigms. We will also continue to focus on cascaded type-II OPDC, with emphasis on walk-off, optical collimation, optimal generation efficiency, and maximal entanglement. Special attention will also be given to multi-photon entanglement."

http://www.ornl.gov/~webworks/cpr/pres/107480_.pdf

Category: MyBlog

Written by Jack Sarfatti

Published on Sunday, 31 July 2011 23:30

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