The%20UW%20Nonlocal%20Quantum%20Communication%20Experiment

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The UW Nonlocal Quantum Communication Experiment

• John G. Cramer
• Professor of Physics

Physics 324A August 14, 2007
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At a News Stand Near You
New ScientistSeptember 30, 2006
Seattle Post IntelligencerNovember 15, 2006
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Quantum Nonlocality
Spooky Action-at-a-Distance Albert Einstein
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Entanglement and Nonlocality
Entanglement The separated but entangled
parts of the same quantum system can only be
described by referencing the state of other
part. The possible outcomes of measurement
M2 depend of the results of measurement M1, and
vice versa. This is usually a consequence of
conservation laws. Nonlocality This
connectedness between the separated system
parts is called quantum nonlocality. It should
act even of the system parts are separated by
light years. Einstein called this spooky
actions at a distance.
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EPR Experiments
A series of EPR experiments, beginning with
the 1972 Freedman-Clauser experiment, have
demonstrated convincingly that measurements
performed on one of a pair of polarization-entangl
ed photons affect the outcome of measurements
performed on the other entangled photon.
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Can Quantum Nonlocality be Used to Send Signals?

• It is now well established that quantum
  nonlocality really does connect the separated
  parts of the same quantum mechanical system (c.f.
  Freedman-Clauser, Aspect, etc.)
• There are several No-Signal Theorems in the
  literature (c.f. P. Eberhard, A. Shimony, )
  showing that quantum nonlocal signaling is
  impossible, e.g., a change on one measurement has
  no observable effect on the other, in the absence
  of coincidence links.
• However, Peacock and Hepburn have argued that
  these proofs are tautological and that certain
  key assumptions (e.g., measurements are local)
  are inconsistent with the quantum formalism
  (e.g., Bose-Einstein symmetrization).
• Therefore, the question of nonlocal signaling
  remains open (at least a crack) and should be
  tested.

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Interference and Entanglement
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Interference of Waves
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One-Slit Diffraction
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Two-Slit Interference
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Turning InterferenceOn and Off
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Ghost Interference
In their 1994 Ghost Interference
experiment, the Shih Group at the University of
Maryland in Baltimore County demonstrated that
causing one member of an entangled-photon pair to
pass through a double slit produces a double slit
interference pattern in the position distribution
of the other member of the pair also. If
one slit is blocked, however, the two slit
interference pattern is replaced by a single-slit
diffraction pattern in both detectors. Note
that a coincidence was required between the two
photon detections.
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Can We UseQuantum Nonlocality for Communication?
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Down-Conversion with LiIO3
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Starting Point Dopfers Position-Momentum EPR
Experiment
LiIO3 Down-ConversionCrystal
Heisenberg Lens f 86 cm
HeisenbergDetector D1
UV LaserBeam
28.2o
Laser BeamStop
28.2o
f
2f
Auxiliary Lens
Double Slit System a 75 mm, d 255 mm
Position
Momentum
Double-SlitDetector D2
CoincidenceCircuit
or
Birgit Dopfer PhD Thesis U. Innsbruck, 1998.
f
2f
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Dopfers Results
Receive Observe Interference at D1?
Send Move Detector D2 Position
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Whats Going On?
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TestingNonlocal Communication
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Can We Eliminate the Coincidence Requirement?
University of Washington test of nonlocal quantum
communication.
Differences from Dopfer Experiment

1. Use collinear Type 2 downconversion in BBO.
2. Separate entangled beams with polarizing
   splitter.
3. Image slit pairs with an upstream lens at
   distance f.
4. Use fiber optics in to switch on/off the
   which-way measurement.

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Interference Fringes Visible?
The first issue to be addressed
experimentally is whether a 2-slit interference
pattern can be observed with the entangled
photons from the thick BBO crystal
down-conversion source. Using collinear
downconversion eliminates longitudinal
thick-source variations. Dealing with
transverse variations may require extra distance
or some compensation for the thick-source effect
(e.g., a diverging lens) so that the wave fronts
from the crystal arriving at the slits are
parallel to the slit system, with a minimum of
phase variation introduced by differences in the
point of production within the crystal.
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Demonstrating Nonlocal Quantum Communication
The University of Washington test of nonlocal
quantum communication.
Send
To demonstrate nonlocal quantum
communication, one simply changes the switch and
observes a change in the interference pattern at
the camera. That would constitute a breakthrough
discovery.
Receive
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Faster than LightBackwards in Time
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A Demonstration ofSuperluminal Signaling
In this test, we would string equal lengths
of fiber optics cables to separate the two ends
of the experiment by a line-of-sight distance of
1.4 km. We would then send bits at a photon
rate of 10 MHz over this link. Assuming a
10-photon decoding latency, this would
demonstrate a signal transmission speed of about
5 times the speed of light.
Send
Receive
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A Demonstration ofRetrocausal Signaling
Send
Receive
In this test, we leave 10 km of optical
fiber coiled up in the corner of the laboratory,
and pass the entangled Transmitter photons
through this path. The Receiver photons
have no such optical delay, and the signal is
received as soon as these photons are detected at
D1, which is about 50 ms before the signal is
transmitted, when the twin entangled photons
arrive at D2.
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Time-TravelParadoxes
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The Bilking Paradox
Suppose that we constructed a million
connected retrocausal links of the type just
shown (or used 107 km of fiber optics). Then the
transmitted message would be received 50 seconds
before it was sent. Now suppose that a tricky
observer receives a message from himself 50
seconds in the future, but then he decides not to
send it. This produces an inconsistent timelike
loop, which has come to be known as a bilking
paradox. Could this happen? If not, what would
prevent it?
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Anti-Bilking
Discussions of such bilking paradoxes have
been published in the physics literature in the
1940s by Wheeler and Feynman (advanced waves) and
in the 1990s by Kip Thorne and colleagues
(timelike wormholes). The consensus of both
discussions is that Nature will forbid
inconsistent timelike loops and will instead
require a consistent set of conditions. Thorn
and coworkers showed that nearby to any
inconsistent paradoxical situation involving a
timelike wormhole, there is self-consistent
situation that does not involve a paradox.
As Sherlock Holmes told us several times, When
the impossible is eliminated, whatever remains,
however improbable, must be the truth.
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Bilking Probability Control
These speculations suggest that equipment
failure producing a consistent sequence of events
is more likely than producing an inconsistency
between the send and receive events. The
implications of this are that bilking itself is
impossible, but that very improbable events could
be forced into existence in avoiding it.
Thus, using the threat of producing an
inconsistent timelike loop, one might bilk
Nature into producing an improbable event. For
example, you might set up a highly reliable
system that would produce an inconsistent
timelike loop unless the number for the lottery
ticket you had purchased was the winning number.
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The ImmaculateConception Paradox
The other issue raised by retrocausal
signaling might be called the immaculate
conception paradox. Suppose that you are using
the setup described above, and you receive from
yourself in the future the manuscript of a
wonderful novel with your name listed as the
author. You sell it, it is published, it becomes
a best-seller, and you become rich and famous.
When the time subsequently comes for
transmission, you duly send the manuscript back
to yourself, thereby closing the timelike loop
and producing a completely consistent set of
events. But the question is, just who actually
wrote the novel? Clearly, you did not you
merely passed it along to yourself. Yet highly
structured information (the novel) has been
created out of nothing. And in this case, Nature
should not object, because there are no
inconsistent timelike loops.
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Present Status

• The experiment has been in testing phases since
  mid-January. Our initial attempt to detect the
  down-converted photons was with a cooled CCD
  camera. We have demonstrated that this detetor
  lacked the needed sensitivity.
• The experiment has been moved from B063 (the UW
  Laser Physics Facility) to B055, to make room for
  the arrival of Prof. Gupta, the newest member of
  the UW Atomic Physics Group.
• The experiment is presently being rebuilt,
  using avalanche photodiodes as the primary
  detectors. It will continue this Fall.

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Conclusions

• There are no obvious show stoppers that would
  prevent the proposed measurements. Nevertheless,
  because of their implications, they have a low
  probability of success.
• My colleague Warren Nagourney and I have been
  working on this experiment since January, and
  because of the publicity, we now have 40k in
  contributions from foundations and individuals to
  support the work.
• This experiment is a rare opportunity to push
  the boundaries of physics with a simple tabletop
  measurement. We intend to push hard.

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TheEnd
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Transactions with D2 at 2f
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Transactions with D2 at f
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The Thick-Source Effect (1)
Volume of LiIO3down-conversioncrystal
illuminatedby UV laser beam.
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The Thick-Source Effect (2)
Fix 2 Lengthen the crystal-to-slit distance,
flattening the wave fronts.
Fix 3 Srikanth suggests spatial filtering by
crossing over the light from the crystal with two
lenses, and using an aperture to eliminate
non-parallel wave front components.
Conclusion There seem to be severalways of
solving the Thick-Source problem.