Scott Eklund / Seattle Post-Intelligencer file
University of Washington physicist John Cramer, seen here in a 2007 photo, has been working on a laser experiment to test whether causality can work backward in time.
Five years have flown by since University of Washington physicist John Cramer set out to determine whether the chain of cause and effect can go backward in time, with $40,000 from curious contributors — but the experiment is still stuck in suspended animation.
"No results to report," Cramer told me today in an email. He said the experiment, which looks into a theoretical phenomenon known as nonlocal quantum communication, is currently "mothballed" while he's traveling. Work will resume sometime after he returns to Seattle at the end of the month.
The experiment is spooky enough to qualify as a Halloween tale: It plays off the concept in quantum physics that you can link two photons in such a way that what you do to one photon is reflected in the state of the other photon as well, no matter how far apart the photons end up. Albert Einstein didn't like the concept, and called it "spooky action at a distance." But recent experiments have shown that the phenomenon is real — even when one of the photons has been sent across a distance of, say, 89 miles.
Cramer envisioned setting up a complicated laser apparatus to entangle two photons, send them along separate paths, and then manipulate one of them so that it takes on the definite characteristics of a wave or a particle. Whatever is done to that one photon should be reflected in the characteristics of the other photon as well. Thus, if you use a movable detector to observe one photon as a wave, the other photon should be detected as a wave as well. That's the "nonlocal communication" part of the experiment.
The spookier part of the experiment would involve sending the photon that's manipulated through an extra six miles (10 kilometers) of optical cable, so that it goes through Cramer's manipulating apparatus about 50 microseconds after the other photon hits the detector. Would the state of the manipulated photon be reflected in the other photon, 50 microseconds before the manipulation took place? It seems like something of a Catch-22, and Cramer suspects that something will always stand in the way of "quantum retrocausality." He just isn't sure what form the obstacle could take.
Maybe the universe is telling him something: Cramer has found that his equipment was too "noisy" to pick up the subtle signals from a single photon. He's run into other technical and financial difficulties as well. But there's still hope of finding an answer. Here's how Cramer explained the situation in his email:
"The status of the experiment is that my $10,000 Sacher 405 nm grating-stabilized laser failed in March. It would have cost $6,000 to have Sacher fix it, and I didn't have the funding for that. Therefore, my student and I spent April-June developing a new low-cost replacement. That is mostly done, and in early July I had a new high-power 405 nm 500 mW diode laser going, just before I left Seattle. ... It puts out more power than the old laser and can, in principle, be tuned to precisely 405.0 nm (required by the ppKTP down-conversion crystal) by stabilizing the diode's operating temperature to about 40 [degrees] C with feedback, but that isn't done yet.
"The principal experimental roadblock to a real test of nonlocal communication remains the fact that the avalanche photodiodes we had been using as detectors of the 810 nm entangled photons are simply too noisy for single photon detection. We tried cooling them all the way down to liquid nitrogen temperatures, but we could not beat down the noise enough. As the temperature drops, the avalanche threshold voltage goes down with the noise voltage, so the noise-detection rate does not change much.
"Fortunately, there is a new technology developed at NIST [the National Institute of Standards and Technology], superconducting transition edge detection, that involves driving superconducting junctions to normal conduction with the energy deposited by a single photon. The system has zero noise, and energy sensitivity good enough that they can distinguish detection events involving one photon, two photons, three photons, etc., from the deposited energy. My efforts when I get back to Seattle will be directed toward trying to get a pair of those detectors. It will not be cheap, but I think it's the only way the experiment might succeed."
Is it just coincidence that other researchers concluded in a recently published paper that quantum noise would head off any paradoxes associated with backward causality? It would be interesting to see whether those "zero-noise" devices suddenly start producing noisy results in Cramer's experiment. If Cramer is still having trouble in 2017, I just might start thinking that Stephen Hawking's "Chronology Protection Agency" has been hard at work.
More about quantum physics:
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.
First published October 5 2012, 6:28 PM