Decades of experiments have verified the quirky laws of quantum theory again and again. So when scientists in Germany announced in 2012 an apparent violation of a fundamental law of quantum mechanics, a physicist at the University of Rochester was determined to find an explanation.

"You don't destroy the laws of quantum mechanics that easily," said Robert Boyd, professor of optics and of physics at Rochester and the Canada Excellence Research Chair in Quantum Nonlinear Optics at the University of Ottawa.

In their 2012 version of the famous Young two-split experiment, Ralf Menzel and his colleagues at the University of Potsdam simultaneously determined a photon's path and observed high contrast interference fringes created by the interaction of waves from the two slits.

"This result was extremely surprising, as one of the basic tenets of quantum mechanics holds that there should be no quantum interference when it is known through which slit the particle (a photon in this case) had passed," said Boyd.

Inspired by these intriguing results, Boyd and his colleagues replicated the Menzel experiment. Their findings were recently published online in an early edition of the *Proceedings of the National Academy of Sciences*.

"The data of the Menzel experiment were very clean, so we weren't surprised to obtain the same initial result," said Boyd. "My coworkers and I asked what could explain this apparent violation of a key principle of quantum mechanics. What we found is that the resolution of the problem requires great subtly in the way that one needs to analyze the data for this type of measurement."

Following the method of Menzel and the Potsdam researchers, Boyd's group generated an entangled pair of photons, one called a signal and the other called an idler. By measuring the position of the idler photon, they thereby determined through which slit the signal photon had passed. They then observed that the signal photons produced an interference pattern, in agreement with the results of the Potsdam group and in apparent conflict with the duality principle.

A careful examination of the results shows that the visibility of the interference pattern is stronger in some places and weaker in others. In particular, the strongest recorded visibility was much higher than the average visibility of the entire pattern.

Wave-particle duality suggests that elementary particles, like electrons and photons, cannot be completely described as either waves or particles, because they exhibit both types of properties. In the double-slit experiment, observing a photon pass through one of the two slits is an example of a particle-like property; a particle can only pass through one or the other. When two waves converge to form an interference pattern, the photon must have passed through both slits simultaneously—a wave-like property. Trying to measure both types of properties simultaneously, however, is problematic. The interference pattern disappears as soon as it is known through which slit the photon has passed.

Boyd and his colleagues discovered that the German physicists had inadvertently sampled the sections of high visibility with greater probability than the other sections. While only a handful of photons produced high visibility interference, they used the entire set of photons to determine the predictability of knowing through which slit they had passed.

This phenomenon, called biased sampling, occurs when certain measurements of a system are selected with a higher probability than others, and that subset of measurements is mistakenly taken to be representative of the entire system. In this case, the high visibility photon subsystem was more likely to be sampled. When Boyd's team "fairly" sampled each variable—giving each subsystem an equal opportunity to be detected and sampled—the problem went away and the results were consistent with the standard interpretation of quantum mechanics.

Boyd emphasizes that the Menzel group had interpreted its data just as anyone else would have. The results were both "strange" and "incredible," but it took Boyd and his colleagues nearly a year and a half to figure out what was going on. He said in some ways everyone is relieved that our understanding of quantum laws has been reaffirmed.

**Explore further:**
Quantum physics first: Researchers observe single photons in two-slit interferometer experiment

**More information:**
*Proceedings of the National Academy of Sciences*, www.pnas.org/content/early/2014/08/06/1400106111.abstract

## Doug_Huffman

## Vyhea

Have you not been reading the accident reports? Your "working mechanism" is irreparably broken. Your paving mechanism slid off the mountain, along with the entire roadway, years ago. The ill-conceived route required an impossible-to-design foundation and was doomed from the start. The NTSB recommends you bury the "mechanism" where it lay, choose a more plausible route and stop designing in a vacuum.

## bloopotheclown

## antialias_physorg

Dang it. That's one subtle piece of business. Good job by Boyd and his group on figuring out the flaw. That sounds like some serious piece of detective work.

## Jixo

Aug 27, 2014## swordsman

## johanfprins

Now if you are insanely stupid, you will conclude the photon-wave has only moved through one slit.

## Jixo

Aug 27, 2014## Jixo

Aug 27, 2014## Jixo

Aug 27, 2014## Hakan1997

## Jixo

Aug 27, 2014## Jixo

Aug 27, 2014## johanfprins

PLEASE go and learn calculus and solve Maxwell's equations: THERE IS NO AETHER.!!

## Jixo

Aug 27, 2014## no fate

At the quantum level there are no particles, just waves and fields. You would think that the necessity to catalogue every "particle" by its vibrational energy or oscillation frequency would be a tip off...or the fact that it isn't called "the cosmic microparticle background".

## johanfprins

Maxwell only WRONGLY interpreted that waves can ONLY move in an aether. If he had solved his own equations for stationary EM -waves in a reflecting cavity, it would have hit him like a sledgehammer that these waves cannot move within a medium: For if they did it would violate energy-conservation.

PLEAAAASE Zephyr: Stop spouting when you cannot even solve a differential equation but can only imagine ducks farting in a non-existing aether.

## Aligo

Aug 27, 2014## johanfprins

He did not know about the constancy of the speed of light within all inertial reference- frames: So how the HELL could he know better? You are an arsehole!!

## Aligo

Aug 28, 2014## Aligo

Aug 28, 2014## johanfprins

Wrong again: His theory DID predict it: He just did not realise that it did. Maxwell's equations when solved within any inertial reference frame, gives the SAME speed for light c=1/(SRT(eps0*mu0)) where eps0 and mu0 are constants which are THE SAME in any and all inertial reference frames. So PLEAAASE Zephyr: Stop spouting claptrap.

## johanfprins

## Jixo

Aug 29, 2014## johanfprins

Oh but they do this. You can directly derive the Lorentz equations from them since they predict that light speed must be c in all IRF's. One can then derive the de Broglie wavelength from the Lorentz transformation, which gives for any moving matter object with mass m a frequency f=(m*c^2)/h. Note that h has NOTHING to do with a matter quantum.

continued

## johanfprins

E=M*c^2-m*c^2=h*F-h*f=h(F-f)=h*nu, where nu is the frequency that the light wave must have.

The light-wave does not need to consist of light-quanta since the matter-wave, like any antenna (for examplefor radio-waves) only absorbs a part of the energy from the light-wave, notwithstanding that the light-wave is continuous energy, as it must be when it has been emitted by a coherent, harmonic source. This wave-energy is stupidly called a photon-"particle". IT IS NOT A "PARTICLE" , but still a wave that has continous energy as demanded by Maxwell's equations. Only its energy is restricted to h*nu, since the electron absorber changes in energy by E=h*(F-f)=h*nu.

Thus Maxwell's equations and quantum mechanics dovetail seamlessly!

## Da Schneib

I certainly wouldn't call it decided either way, but this result puts a lot of the burden back on Afshar to show the result cannot be due to a biased measurement like the one in this article.

## Jixo

Sep 01, 2014## Da Schneib

## Aligo

Sep 13, 2014## Aligo

Sep 13, 2014