Quantum entanglement, a phrase first coined by Erwin Schrödinger in 1935, describes a condition of the separated parts of the same quantum system in which each of the parts can only be described by referencing the state of other part. This is one of the most counterintuitive aspects of quantum mechanics, because classically one would expect system parts out of speed-of-light contact to be completely independent.  Thus, entanglement represents a kind of quantum “connectedness” in which measurements on one isolated part of an entangled quantum system have non-classical consequences for the outcome of measurements performed on the other (possibly very distant) part of the same system.  This quantum connectedness that enforces the measurement correlation and state-matching in entangled quantum systems has come to be called quantum nonlocality.

Nonlocality was first highlighted by Albert Einstein and his coworkers Boris Podolsky and Nathan Rosen in their famous 1935 EPR paper. They argued that the nonlocal connectedness of quantum systems requires a faster-than-light connection that appears to be in conflict with special relativity.  This criticism of quantum mechanic was ignored by most of the physics community until 1964, when John S. Bell, a theoretical physicists working at the CERN laboratory in Geneva, used the formalism of quantum mechanics to show that certain experimental tests could distinguish the predictions of quantum mechanics from those of alternative theories that were “local”, in the sense that nonlocality was eliminated.  Bell based his calculations not on measurements of position and momentum, the focus of Einstein's arguments, but on measurements of the states of polarization of photons of light.

In a propagating light wave, if the electric field oscillates in the vertical or horizontal direction the light is said to be linearly polarized vertically or horizontally.  If the electric vector corkscrews through space in a counterclockwise or clockwise direction, as viewed from the front, the light is said to be right or left circularly polarized.  A mixture of linear and circular polarization is called elliptical polarization.  As an example, my sunglasses pass light that has vertical linear polarization and block light that has horizontal linear polarization because the latter is produced by reflection and glare.

Bell showed, essentially, that when pairs of polarization-entangled photons are measured for linear polarization in particular directions, quantum mechanics predicts that the coincidence rates between detections of the entangled pair vs. the angle between the polarization measurements can be used to generate a quantity that has a value of 2.8 according to quantum mechanics, while “realistic local” theories predict that the same quantity of 2.0 must be less than.  This difference occurs because the coincidence rate predicted by quantum mechanics (and the classical Malus Law of polarization) falls off as the square of angle between the measured linear-polarization directions, while all local theories predict a linear falloff.  The mathematical expression of this dichotomy is called Bell’s Inequalities or Bell’s Theorem.

Since the 1970s, experimentalists have performed many “EPR experiments” based on Bell’s Theorem.  (See my AV Column “Einstein's Spooks and Bell's Theorem”, Analog, January-1990).  These experiments have consistently found with high statistical precision that Bell’s quantity has a value of 2.8, demonstrating the validity of the nonlocal predictions of quantum mechanics and falsifying alternative theories that are local and “realistic” (see below).

But do such EPR experiments actually demonstrate the existence of quantum nonlocality?  As it turns out, there is more than one way of interpreting the implications of the EPR experimental results, and there is dispute as to whether it is locality or “realism” (the objective observer-independent reality of external events) that has been refuted by the EPR measurements.  To put it another way, to accommodate the results of the EPR experiments, either one has to accept that there is some mysterious “nonlocal influence” that acts across space-time to force the results of separated measurements to be consistent with each other and with conservation laws, or one has to relinquish the concept of objective reality, the idea that the universe exists with well-defined properties, independent of what we choose to observe and measure.  Local realistic theories have been falsified by the EPR experiments based on Bell’s Theorem, but is it locality or realism (or both) that has been eliminated?

The reaction of the general physics community to these Bell’s Inequality test results has been either (a) to ignore them altogether (as the majority of working physicists seem consistently to do) or (b) to assume objective reality is OK and to admit grudgingly that nonlocality is perhaps an inseparable aspect of quantum mechanics.

However, Noble Laureate Tony Leggett of the University of Illinois has recently pushed this issue somewhat farther.  He has demonstrated that by focusing on the falloff of correlations with elliptical polarization rather than the linear polarization of the Bell Inequality EPR experiments, one can compare the predictions of quantum mechanics with a class of nonlocal realistic theories.  The resulting Leggett Inequalities can be used in the same way as the Bell Inequalities, but to test nonlocal realism instead of local realism.

A group of experimentalists at the Institute for Quantum Optics and Quantum Information (IQOQI) in Vienna has now performed an EPR experiment that is a definitive test of the Leggett Inequalities, and their results have recently been published in the British science journal Nature.  They show that in EPR measurements with elliptically polarized entangled photons, the Leggett Inequalities in two observables are violated by 3.6 and by 9 standard deviations.  This in interpreted as a statistically significant falsification of the whole class of nonlocal realistic theories studied by Leggett.

The group summarizes the implications of their results with the statement, “We believe that our results lend strong support to the view that any future extension of quantum theory that is in agreement with experiments must abandon certain features of realistic descriptions.”  In other words, quantum mechanics and reality appear to be incompatible.

Is the case against objective reality truly so strong?  To answer this question, we must examine in more detail the nonlocal realistic theories that Leggett studied.  This class of theories assumes that when entangled photons emerge from their emission source, they are in a definite state of polarization.  It is well know that when that assumption (and no others) is made, one does not observe the quantum mechanical prediction of Malus’s Law for the correlations of the photon pair.

However, Leggett cures that problem by assuming an unspecified nonlocal connection mechanism between the detection systems that fixes the discrepancy.  In effect, the two measurements talk to each other nonlocally in such a way that the detected linearly polarized photons obey Malus’ Law and produce the same linear polarization correlations predicted by quantum mechanics calculations.  Leggett then shows that this nonlocal “fix” cannot be extended into the realm of elliptical polarization, and that quantum mechanics and this type of nonlocal realistic theories give differing predictions for the elliptic polarization correlations.  In other words, the “reality” that is being tested is whether the photon source is initially emitting the entangled photons in a definite state of polarization.  It is this version of reality that has been falsified by the IQOQI measurements.

We can clarify what is going on in these experimental tests by applying the Transactional Interpretation (TI) of quantum mechanics (see my column “The Quantum Handshake” in Analog 11/86) to these Leggett Inequality tests..  As some of you readers may know, I originated the TI in 1986, and it is considered to be one of the leading alternatives to the orthodox Copenhagen Interpretation of quantum mechanics.

From the point of view of the TI, Leggett’s assumption that the entangled photons are emitted in definite states of polarization is wrong.  The “offer wave” for each photon that emerges from the source includes all possible polarization states.  These offer waves travel to downstream detectors, and time-reversed “confirmation waves” travel back up the time-stream to the source, arriving at the instant of emission.  A three-way transaction then forms between the source and the two detections that matches the confirmation waves to a mutually consistent overall state that satisfies appropriate conservation laws (in this case, conservation of angular momentum).  The final result is a completed transaction with the two photons in definite states, but this definite state was not present in the initial emission of the offer waves, and that is the part of the process described in detail by the wave-mechanics formalism of quantum mechanics.  We note that the TI does not in itself make any predictions about the linear or elliptical polarization correlations of the entangled photon pair.  It only describes the quantum formalism that is making the predictions that the IQOQI group has observed to be consistent with their experiment, but it clarifies what is going on in those predictions.

Does this mean that the TI (and the quantum formalism it describes) are not “realistic”, i.e. inconsistent with an objective reality that is independent of the observer’s choice of measurements?  I don’t think so.  The transactions that form in quantum processes arise from a “handshake” between the past and future across space-time, but they are not specifically the result of measurements or observer choices.  The latter are only a small subset of the transactions that form as the universe evolves in space-time.  The message of the Leggett Inequality tests, from the point of view of the TI, is that the assumption of emission in a definite polarization state is too restrictive.  I would argue that initial emission without a definite polarization state is not inconsistent with objective reality and is consistent with the quantum formalism.

The TI description of the quantum formalism is realistic and nonlocal, in at least some definitions of those terms, and it is completely consistent with the IQOQI results.  To put it another way, Leggett has set up a straw man that has been demolished by the IQOQI tests, but that is only an indication that his version of “realism” is too naïve.  And this theory and experiment can be viewed as another demonstration of the value and power of the TI in understanding the peculiar predictions and intrinsic weirdness of quantum mechanics.

Since this is an SF magazine, we should as usual consider the science fictional implications of this work.  Ignoring my remarks above about the TI, these experimental results could be viewed as reinforcement for the “observer-created quantum reality” (i.e., non-realism) that is an important theme in contemporary SF.  This theme goes back at least as far as Greg Bear’s pivotal novella Blood Music, a story that concludes with the formation of a planet-wide group mind, an entity that acts as the proverbial 1,000 pound gorilla and can collapse wave functions any way it damn well pleases.  Perhaps the “ansible” used by LeGiun, Card, and others might be considered a SF use of nonlocality. With that exception, there has not been much SF written that uses the weirdness of quantum nonlocality in a central way (although I may write some of that soon).  The IQOQI tests could be viewed as calling quantum nonlocality into question, but I would argue against that view using the transactional analysis of the IQOQI experiment described above.  But, as usual, SF authors are free to work all sides of the street, when it comes to quantum phenomena, and we readers reap the benefits of that diversity.

John G. Cramer's 2016 nonfiction book (Amazon gives it 5 stars) describing his transactional interpretation of quantum mechanics, The Quantum Handshake - Entanglement, Nonlocality, and Transactions, (Springer, January-2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406 or https://www.amazon.com/dp/3319246402.

SF Novels by John Cramer: Printed editions of John's hard SF novels Twistor and Einstein's Bridge are available from Amazon at https://www.amazon.com/Twistor-John-Cramer/dp/048680450X and https://www.amazon.com/EINSTEINS-BRIDGE-H-John-Cramer/dp/0380975106. His new novel, Fermi's Question may be coming soon.

Alternate View Columns Online: Electronic reprints of 212 or more "The Alternate View" columns by John G. Cramer published in Analog between 1984 and the present are currently available online at: http://www.npl.washington.edu/av .

References:

Reality Test:

“An experimental test of non-local realism”, S. Gröblacher, T. Paterek, R. Kaltenbaek, C. Brukner, M. Zukowski, M. Aspelmeyer, and A. Zeilinger, Nature 446, 871-875 (2007); available online at http://www.arxiv.org/pdf/0704.2529 .

Anthony J. Leggett, Foundations of Physics 33, 1469 (2003); available online at http://www.springerlink.com/content/r23275410u4p5q72/fulltext.pdf .

The Transactional Interpretation of QM:

John G. Cramer, Reviews of Modern Physics 58, 647 (1986) ; available online at: http://npl.washington.edu/npl/int_rep/tiqm .

John G. Cramer, International Journal of Theoretical Physics 27, 227 (1988) ; available online at: http://npl.washington.edu/npl/int_rep/ti_over .

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Exit to the website.  This page was created by John G. Cramer on 08/27/2008.