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The UW Nonlocal Quantum Communication Experiment. John G. Cramer Professor of Physics. Physics 324A August 14, 2007. At a News Stand Near You …. New Scientist September 30, 2006. Seattle Post Intelligencer November 15, 2006. Quantum Nonlocality. “Spooky Action-at-a-Distance”
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The UW Nonlocal Quantum Communication Experiment John G. Cramer Professor of Physics Physics 324A August 14, 2007
At a News Stand Near You … New ScientistSeptember 30, 2006 Seattle Post IntelligencerNovember 15, 2006
Quantum Nonlocality “Spooky Action-at-a-Distance” Albert Einstein
Measurement 1 M1 Entangled photon 1 Entangled Photon Source Nonlocal Connection Entangled photon 2 M2 Measurement 1 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.”
The Freedman-Clauser Experiment, Phys. Rev. Letters 28, 938-942 (1972). 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-entangled photons affect the outcome of measurements performed on the other entangled photon.
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.
Two-slitInterference Pattern No Two-slitInterference Pattern H V Turning InterferenceOn and Off
“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.
LiIO3 Crystal 351.1 nmUV Laser Beam l1=702.2 nm BeamStop l2=702.2 nm Entangled Photons: Energy and Momentum Conservation: Down-Conversion with LiIO3
Note the useof coincidence. Starting Point: Dopfer’s Position-Momentum EPR Experiment LiIO3 Down-ConversionCrystal “Heisenberg” Lens f = 86 cm “Heisenberg”Detector D1 UV LaserBeam 28.2o Laser BeamStop 28.2o f 2f Auxiliary Lens Double Slit System a= 75 mm, d = 255 mm Momentum Position Double-SlitDetector D2 CoincidenceCircuit or Birgit Dopfer PhD Thesis U. Innsbruck, 1998. f 2f
Dopfer’s Results Receive: Observe Interference at D1? Send: Move Detector D2 Position
Can We Eliminate the Coincidence Requirement? University of Washington test of nonlocal quantum communication. Differences from Dopfer Experiment: • Use collinear Type 2 downconversion in BBO. • Separate entangled beams with polarizing splitter. • Image slit pairs with an upstream lens at distance f. • Use fiber optics in to switch on/off the which-way measurement.
BBO V 351 nm IR PassFilter PolarizingSplitter V 702 nm Argon Ion Laser H 702 nm IR PassFilter Slits Camera 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.
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
1 km 1 km 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
BBO V 351 nm 10 km IR PassFilter IR PassFilter D S V 702 nm PolarizingSplitter Switch D C S S2 S1 Argon Ion Laser D H 702 nm 10 km IR PassFilter Camera 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.
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?
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.”
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.
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.
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.
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.
Possible Fix 1: Insert a converging lens in path to slits. Path length differences at slit positions can be greater than l/2, shifting and washing out the interference pattern “signal”. The Thick-Source Effect (1) Volume of LiIO3down-conversioncrystal illuminatedby UV laser beam.
Lxc= 1 m Lxc= 3 m Lxc= 10 m 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.
