DOW-UAP-D133, AAWSAP DIRD, The Space Communication Implications of Quantum Entanglement and Nonlocality, March 2010

This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD reviews quantum entanglement and nonlocality with a focus on whether those phenomena might be usable as a means of faster-than-light communication between observers, especially for real-time space operations over interplanetary distances. The report surveys the relevant quantum experiments and no-signal theorems, then examines proposed communication schemes based mainly on momentum-entangled photons, including scenarios involving superluminal and retro-causal signaling. However, it repeatedly acknowledges that the central question remains unresolved experimentally, and it gives substantial attention to the coherence-versus-entanglement tradeoff and other features of standard quantum mechanics that may prevent usable signaling even if non-local correlations are experimentally validated. Overall, the document is an exploratory analysis of whether quantum nonlocality could conceivably support a practical communications application rather than a demonstration of prospective utility.

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Defense
Intelligence
Reference
Document
Acquisition Threat Support
30 March 2010
ICOD: 1 December 2009
DIA-08-1003-016
The Space-Communication
Implications of Quantum
Entanglement and Nonlocality
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The Space-Communication Implications of Quantum
Entanglement and Nonlocality
Prepared by:
Acquisition Support Division (DW0-3)
Defense Warning Office
Directorate for Analysis
Defense Intelligence Agency
Author:
AAP Person 76
Administrative Note
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This product is one in a series of advanced technology reports produced in FY 2009
under the Defense Intelligence Agency, Defense Warning Office's Advanced Aerospace
Weapon System Applications (AAWSA) Program. Comments or questions pertaining to
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this document should be addressed to !AAP Person 1 AAWSA Program
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Contents
Foreword and Introduction ....................................................................................iv
I. Quantum Entanglement, Nonlocality, and EPR Experiments ............................... 1
II. The Quantum No-Signal Theorems .................................................................... 4
III. Nonlocality Versus Special Relativity? ............................................................. 5
IV. Momentum Domain Entanglement and EPR Experiments.................................. 5
V. Coherence-Entanglement Complementarity ..................................................... 10
VI. Nonlocal Communication Versus Signaling ..................................................... 11
VII. A Transactional Analysis of the Nonlocal Communication Test ...................... 13
VIII. Superluminal and Retrocausal Nonlocal Communication ............................. 17
IX. Paradoxes and Nonlocal Communication ........................................................ 18
X. Superluminal Communication without Paradoxes ............................................ 19
XI. Example: Real-Time Earth Control of Mars Rover............................................ 20
XII. Another Superluminal Possibility: Nonlinear Quantum Mechanics................. 22
XIII. Conclusion ................................................................................................... 23
Appendix: Glossary .............................................................................................. 24
Figures
Figure 1. Schematic of the 1972 Freedman-Clauser Experiment ............................ 2
Figure 2. Schematic of the 1995 Ghost Interference Experiment of the Shih
Group ...................................................................................................... 6
Figure 3. Ghost Interference Position Distributions at X2 ....................................... 7
Figure 4. "Unfolding" the Ghost Interference Experiment...................................... 8
Figure 5. Schematic of the 1998 Dopfer Experiment .............................................. 9
Figure 6. Thick-Source Effect ............................................................................... 10
Figure 7. Slit-Imaging Coincidence-Free Version of the Ghost Interference
Experiment to Demonstrate Nonlocal Communication ........................... 12
Figure 8. Transactional Interpretation Diagrams for Case 1................................. 15
Figure 9. Transactional Interpretation Diagrams for Case 2................................. 16
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Figure 10. Slit-Imaging Coincidence-Free Version of the Ghost Interference
Experiment Demonstrating Superluminal and Retrocausal Signaling .. 17
Figure 11. A Superluminal Nonlocal Communication System in Which the
Communication Spans a Spacelike Interval ........................................ 20
Figure 12. Schematic of Earth-to-Mars Real-Time Control of a Rover................... 21
iii
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The Space-Communication Implications of Quantum
Entanglement and Nonlocality
Foreword and Introduction
This paper reviews quantum entanglement and nonlocality and considers the
possibility that this phenomenon could be used for sending observer-to­
observer signals. Such a demonstration would break several quantum "no­
signal theorems" in the physics literature. Nonlocal quantum signaling would
have far-reaching implications as an enabling technology for superluminal and
retrocausal signaling. Scenarios that might lead to nonlocal quantum
communication are described, and applications to retrocausal signaling and
real-time space communication are considered. Also considered briefly is the
nonlocal communication implications of nonlinear quantum mechanics.
Communication in space at the scale of the solar system is severely limited by
the space-time scale set by the speed of light. Light signals, whether in the
form of radio waves, microwaves, visible light, X-rays, or gamma rays, require
about 3.3 microseconds to travel a distance of 1 kilometer. A light signal sent
from Earth requires about 1.3 seconds to reach the Moon, between 4.4 and 20
minutes to reach Mars, and between 4 and 4.3 hours to reach Neptune,
depending on their orbital positions. This time delay makes real-time control
of remote space-based devices impossible and leads to the need for pre­
programmed robotic devices with enough "intelligence" to perform limited
operations with a minimum of remote control.
The burden of these limitations raises the question of whether there is some
way to speed up the space communications link. The conventional answer is
"No!," because the well-established special theory of relativity is viewed as
limiting signal transmission speed to the speed of light, with superluminal
communications strictly forbidden. However, as will be discussed in Section
III, relativity prohibits only certain forms of superluminal communication,
while other forms are not in conflict with relativity. One phenomenon that
appears, at least superficially, to exhibit superluminal aspects while
preserving compatibility with special relativity is quantum nonlocality, the
ability of quantum phenomena to enforce correlations between quantum
states over large separations in space-time.
When two photons emerge from a single quantum event, the state of one
photon may be subtly connected to that of the other. The classical view is that,
once separated, such photon states must be fixed according to mechanics and
conservation relations that act at the point of their origin, so that modifying
one later will not affect the other. In quantum physics, however, as borne out
by experiment (Reference 1, 2), the outcome of a measurement of the state of
one of the photons, even well after their point of joint creation, can affect the
state of the other photon. This connection is referred to as quantum
entanglement, a phrase first coined by Erwin Schrodinger (Reference 3).
Questions raised by the phenomenon of quantum entanglement are: (1) what
is the causal connection between entangled states, and (2) can the
phenomenon possibly be used for sending observer-to-observer signals? This
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paper attempts to address these questions by taking a close look at quantum
entanglement, quantum nonlocality, the experiments that have explored them,
and proposed experiments to test the causal and faster-than-light
communication issues evoked by such physics.
Quantum entanglement describes the condition of separated parts of the same
quantum system in which each of the parts can be described only by
referencing the state of other parts. This is one of the most counterintuitive
aspects of quantum mechanics, because classically one would expect system
parts out of "local" 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 nonclassical consequences
for the outcome of measurements performed on the other (possibly very
distant) part of the same system. This quantum connectedness acting in
entangled quantum systems is called quantum nonlocality.
Nonlocality was first highlighted by Albert ~instein and his coworkers Boris
_fodolsky and Nathan ,Rosen in their famous EPR paper (Reference 4). They
argued that the nonlocal connectedness of quantum systems was unphysical
in that it implied a faster-than-light connection in apparent conflict with
special relativity. Despite their objection, quantum nonlocality has now been
demonstrated (see Section I) in many quantum systems (Reference 1, 2). In
the physics community, it is now generally acknowledged to be implicit in the
quantum formalism as applied to entangled systems, although there remain a
few Copenhagen "holdouts" who would require an explicit demonstration of
nonlocal signaling before admitting that nonlocality can be considered a real
quantum phenomenon.
The question investigated in this paper is whether quantum nonlocality is the
private domain of nature or whether it can be used in experimental situations
to send signals from one observer to another. As we will see, there is at
present no compelling answer to this question. However, it is clear that if such
nonlocal observer-to-observer communication were possible, it would have
far-reaching implications. In particular, it would represent an enabling
technology for superluminal (and retrocausal) signaling and communications,
and perhaps make possible the real-time exploration of the universe.
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I. Quantum Entanglement, Nonlocality, and EPR
Experiments
In the quantum mechanical description of elementary entities like photons, there is a
duality between the description as a particle and as a wave. Photons can be thought of
as traveling through space as waves but delivering energy (and other conserved
quantities) at detection as particles. By choosing the kinds of measurements made on
such objects, one can force wave-like or particle-like behavior to be exhibited in the
measurements results. Between the entangled parts of a quantum system (for
example, the emission of a pair of entangled photons), this wave-like or particle-like
behavior in a measurement on one part of the system may force similar behavior in the
other part. This is considered further in Section IV below.
The quantum entanglement condition is usually a consequence of some conservation
law acting within the system, so that the subsystems are connected by the conserved
quantities. For example, if two photons are emitted back to back in a joint state that
has zero angular momentum and positive parity, then whatever linear or circular
polarization state one photon is measured to have, the other photon must have an
identical polarization if measured in the same basis (linear or circular). This condition
must exist to ensure that the net angular momentum of the two photon states is zero.
In this situation, if the photons are measured for circular polarization, they must both
be in states of right circular polarization or in states of left circular polarization. Because
linear polarization is a coherent superposition of circular polarization states, if measured
in the vertical/horizontal linear polarization basis, they must be in the same vertical or
horizontal polarization state, and in the 45° left or right linear polarization basis, they
must be in the same 45° left/right polarization state.
Classically, such a polarization correlation condition could in principle exist in some
particular polarization basis but not in all of the many possible polarization bases
simultaneously. This is the underlying physics of the Bell Inequalities (Reference 8),
which deal with the falloff rate of the correlations as the polarization basis of one of the
measurements is rotated in angle. The Bell Inequalities demonstrate mathematically
that the predictions of semi-classical local hidden-variable theories are inconsistent with
those of standard quantum mechanics. Tests of such polarization correlations have
been the basis for a number of Bell-Inequality tests (or so-called EPR experiments), in
which the validity of the predictions of quantum mechanics and the inadequacies of
semi-classical local hidden-variable theories have been demonstrated to high statistical
precision (Reference 1, 2) .
It was later demonstrated (Reference 5, 6) that the issues surrounding a violation of
the Bell Inequalities could be separated into violations of either parameter
independence (the outcome probability of a measurement on one of a pair of entangled
particles is independent of the choice of parameters of a measurement performed on
the other member of the entangled pair) and violations of outcome independence (the
outcome probability of a measurement on one of a pair of entangled particles is
independent of the outcome of a measurement performed on the other member of the
entangled pair). The observation of a violation of the Bell Inequalities indicates a
violation of either parameter independence or outcome independence (or both).
Outcome independence is fairly evident in the quantum formalism, while parameter
independence is more elusive and depends on specific assumptions. Below, the
1
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implications of this dichotomy are considered in the context of the "no-signal"
theorems.
It is noted that there is some misinformation in the literature concerning the chronology
of successful EPR polarization correlation experiments, and here we wish to set the
record at least somewhat straighter. The experimental measurement that first
demonstrated a polarization correlation related to EPR nonlocality was performed by C.
S. Wu and I. Shanknov in 1949 (Reference 7), well before Bell's work and the
subsequent interest in testing Bell's Inequality. Wu and Shanknov showed that the
linear polarizations of back-to-back entangled gamma rays from electron-positron
annihilation (an L=O negative parity state) were anticorrelated, for example, if one
photon was polarized vertically, then the other was polarized horizontally. They did not,
however, investigate the falloff of the correlation with polarimeter angle, which is the
basis of Bell Inequality tests, nor did they depict their results as a consequence of
quantum nonlocality.
Almost two decades passed before the publication of John Bell's pivotal work (Reference
8) in 1964 and 1966. In 1972, Freedman and Clauser (Reference 1) performed the first
definitive Bell inequality