[LINK] Quantum Shadows

[stephen at melbpc.org.au]

Quantum shadows: The mystery of matter deepens

07 January 2013 by Anil Ananthaswamy
<http://www.newscientist.com/article/mg21728971.600-quantum-shadows-the-
mystery-of-matter-deepens.html?full=true> (Free registration required)


Forget particles and waves. When it comes to the true guise of material 
reality, what's out there is beyond our grasp

"If you haven't found something strange during the day," John Archibald 
Wheeler is said to have remarked, "It hasn't been much of a day." But then, 
strangeness was Wheeler's stock in trade. 

As one of the 20th century's leading theoretical physicists, the things he 
dealt with every day - the space-and-time-bending warpings of Einstein's 
relativity, the fuzzy uncertainties and improbabilities of quantum physics 
- were the sort to boggle the minds of most mere mortals.

http://www.newscientist.com/special/instant-expert-general-relativity
http://www.newscientist.com/article/mg20627596.000-seven-wonders-of-the-
quantum-world.html

Even so, one day in 1978 must have been quite something for Wheeler. That 
was when he first lit on a very strange idea to test how photons might be 
expected to behave. Half a century earlier, quantum physics had produced 
the startling insight that light - everything in the quantum world, in fact 
- has a dual character. Sometimes it acts as if made of discrete chunks of 
stuff that follows well-defined paths - particles. At other times, it 
adopts the more amorphous, space-filling guise of a wave. That led to a 
question that exercised Wheeler: what makes it show which side, and when?

It took a while for the test Wheeler devised to become experimental 
reality. When it finally did, the answer that came was strange enough. Now, 
though, the experiment has been redone with a further quantum twist. And 
it's probably time to abandon any pretence of understanding the outcome. 
Forget waves, forget particles, forget anything that's one or the other. 
Reality is far more inscrutable than that.

For centuries, light has illuminated our ideas of the material world. The 
debate about its nature, wave or particle, goes back to the philosophers of 
ancient Greece, and has featured luminaries such as Newton, Descartes and 
Einstein on one side or the other. By the dawn of the 20th century, the 
result was best described as a scoring draw, with both sides having 
gathered significant support (see diagram).

http://www.newscientist.com/data/images/archive/2897/28971601.jpg

The central mystery

Quantum physics broke the deadlock essentially by saying that everyone was 
right. The apparent proof comes with a quantum version of an experiment 
first performed by the English physicist Thomas Young in 1803, ironically 
to support the wave theory of light. Young shone light on a screen with two 
tiny, parallel slits in it. On another screen a distance behind the first, 
he saw alternating vertical fringes of light and dark that seemed 
incontrovertible proof of light's wave character. Water waves passing 
through two narrow openings in a sea wall diffract and interfere in a 
similar way, sometimes constructively amplifying and sometimes 
destructively reducing each other beyond.

The strangeness starts when you lower the light intensity to the point at 
which only a single photon enters the experimental setup at any one time. 
In 1905, Einstein had strongly suggested that a single photon is a 
particle, and indeed, place a detector at one or other of the slits and you 
hear the beep, beep of single particles hitting it. But remove the particle 
detector and place a light-collecting screen - a kind of long-exposure 
camera - a distance behind the slits, and the same pattern of light and 
shade that Young had observed slowly builds up. It is as if each photon is 
an interfering wave that passes simultaneously through both slits. The same 
happens with other quantum particles: electrons, neutrons, atoms and even 
60-carbon-atom buckyballs.

For Niels Bohr, the great Danish pioneer of quantum physics, this "central 
mystery" was nothing less than a principle of the new theory, one he called 
the complementarity principle. Quantum objects such as photons simply have 
complementary properties - being a wave, being a particle - that can be 
observed singly, but never together. And what determines which guise an 
object adopts? Bohr laid out a first outline of an answer at a grand 
gathering of physicists at the Istituto Carducci on the shores of Lake Como 
in Italy in September 1927: we do. Look for a particle and you'll see a 
particle. Look for a wave and that's what you'll see.

The idea that physical reality depends on an observer's whim bothered the 
likes of Einstein no end. "No reasonable definition of reality could be 
expected to permit this," he huffed in a famous paper he co-authored in 
1935 with Boris Podolsky and Nathan Rosen (Physical Review, vol 47, p 777). 

http://prola.aps.org/abstract/PR/v47/i10/p777_1

Einstein favoured an alternative idea of an underlying but as-yet 
inaccessible layer of reality containing hidden influences that "told" the 
photon about the nature of the experiment to be performed on it, changing 
its behaviour accordingly.

http://www.newscientist.com/article/mg20928011.100-reality-check-closing-
the-quantum-loopholes.html

There is more to this than wild conspiracy theory. Imagine an explosion 
that sends two pieces of shrapnel in opposite directions. The explosion 
obeys the law of conservation of momentum, and so the mass and velocity of 
the pieces are correlated. But if you know nothing of momentum 
conservation, you could easily think that measuring the properties of one 
fragment determines the properties of the other, rather than both being set 
at the point of explosion. Was a similar hidden reality responsible for 
goings on in the quantum world?

This is where Wheeler's thought experiment came in. Its aim was to settle 
the issue of what told the photon how to behave, using an updated version 
of the double-slit experiment. Photons would be given a choice of two paths 
to travel in a device known as an interferometer. At the far end of the 
interferometer, the two paths would either be recombined or not. If the 
photons were measured without this recombination - an "open" interferometer 
- that was the equivalent of putting a detector at one or other of the 
slits. You would expect to see single particles travelling down one path or 
the other, all things being equal, splitting 50:50 between the two (see 
"Neither one nor the other").

Alternatively, the photons could be measured after recombination - a 
"closed" setting. In this case, what you expect to see depends on the 
lengths of the two paths through the interferometer. If both are exactly 
the same length, the peaks of the waves arrive at the same time at one of 
the detectors and interfere constructively there: 100 per cent of the hits 
appear on that detector and none on the other. By altering one path length, 
however, you can bring the wave fronts out of sync and vary the 
interference at the first detector from completely constructive to totally 
destructive, so that it receives no hits. This is equivalent to scanning 
across from a bright fringe to a dark one on the interference screen of the 
double slit experiment.

Wheeler's twist to the experiment was to delay choosing how to measure the 
photon - whether in an open or a closed setting - until after it had 
entered the interferometer. That way, the photon couldn't possibly "know" 
whether to take one or both paths, and so if it was supposed to act as a 
particle or a wave.

Or could it?

It was almost three decades before the experiment could actually be done. 
To make sure there was no hidden influence of the kind favoured by 
Einstein, you needed a very large interferometer, so that no word of the 
choice of measurement could reach the photon, even if the information 
travelled at light speed (anything faster was expressly forbidden by 
Einstein's own theory of relativity). In 2007, Alain Aspect and his team at 
the Institute of Optics in Palaiseau, France, built an interferometer with 
arms 48 metres long. The result? Whenever they chose at the last instant to 
measure the photons with a closed interferometer, they saw wave 
interference. Whenever they chose an open interferometer, they saw 
particles (Science, vol 315, p 966).

http://www.sciencemag.org/content/315/5814/966.abstract

There was no getting round it. Wave and particle behaviours really do seem 
to be two sides of one coin representing material reality. As to which way 
it flips - well, you decide. "Isn't that beautiful?" said Aspect in a 
public lecture at the Physics at FOM conference in Veldhoven, the Netherlands, 
last year. "I think there is no other conclusion to draw from this 
experiment."

http://www.youtube.com/watch?
feature=player_detailpage&v=6dvQFXIny1w#t=3611s

Unless, of course, you make things even stranger. In December 2011, Radu 
Ionicioiu of the Institute for Quantum Computing in Waterloo, Canada, and 
Daniel Terno of Macquarie University in Sydney, Australia, proposed 
extending Wheeler's thought experiment (Physical Review Letters, vol 107, p 
230406).  http://arxiv.org/abs/1103.0117


Their new twist was that the decision of how to measure the photon, as a 
particle or as a wave, should itself be a quantum-mechanical one - not a 
definite yes or no, but an indeterminate, fuzzy yes-and-no.

Infinite shades of grey

There is a way to do that: you use light to control the detector designed 
to probe the light. First you prepare a "control" photon in a quantum 
superposition of two states. One of these states switches the 
interferometer to an open, particle-measuring state, and the other to a 
closed, wave-measuring state. Crucially, you only measure the state of the 
control photon after you have measured the experimental "system" photon 
passing through the interferometer. As far as you are concerned, the system 
photon is passing through an interferometer that is both open and closed; 
you don't know whether you are setting out to measure wave or particle 
behaviour (see diagram). So what do you measure?

This time, it took only a few months for the experimentalists to catch up 
with the theorists. But when three independent groups, led by Chuan-Feng Li 
at the University of Science and Technology of China in Hefei, Jeremy 
O'Brien at the University of Bristol, UK, and Sébastien Tanzilli at the 
University of Nice, France, performed different versions of the experiment 
last year, the results were unnerving - even to those who consider 
themselves inured to the weirdnesses of quantum physics (Nature Photonics, 
vol 6, p 600; Science, vol 338, p 634 and p 637).

http://www.nature.com/nphoton/journal/v6/n9/full/nphoton.2012.179.html

The answer is, what you see depends on the control photon. If you look at 
the measurements of the system photons without ever checking the 
corresponding measurements of the control photons - so never knowing what 
measurement you made - you see a distribution of hits on the two detectors 
that is the signature neither of particles or waves, but some ambiguous 
mixture of the two. If particle is black and wave is white, this is some 
shade of grey.

Do the same, but this time looking at the control photon measurements as 
well, and it is like putting on a pair of magic specs. Grey separates 
clearly into black and white. You can pick out the system photons that 
passed through an open interferometer, and they are clearly particles. 
Those that passed through a closed interferometer look like waves. The 
photons reveal their colours in accordance with the kind of measurement the 
control photon said you made.

It gets yet stranger. Quantum mechanics allows you to put the control 
photon not just in an equal mix of two states, but in varying proportions. 
That is equivalent to an interferometer setting that is, say, open 70 per 
cent of the time and closed 30 per cent of the time. If we measure a bunch 
of system photons in this configuration, and look at the data before 
putting on our magic specs, we see an ambiguous signature once again - but 
this time, its shade of grey has shifted closer to particle black than wave 
white. Put on the specs, though, and we see system photons 70 per cent of 
which have seemingly - but clearly - behaved as particles, while the 
remaining 30 per cent acted as waves.

In one sense, the results leave Bohr's side of the argument about quantum 
reality stronger. There is a tight correlation between the state of the 
control photon, representing the nature of the measurement, and the system 
photon, representing the state of reality. Make for more of a particle 
measurement, and you'll measure something more like a particle, and vice 
versa. As in earlier experiments, a hidden-reality theory à la Einstein 
cannot explain the results.

But in another sense, we are left grappling for words. "Our experiment 
defies the conventional boundaries set by the complementarity principle," 
says Li. Ionicioiu agrees. "Complementarity shows only the two ends, black 
and white, of a spectrum between particle and wave," he says. "This 
experiment allows us to see the shades of grey in between."

So, has Bohr been proved wrong too? Johannes Kofler of the Max Planck 
Institute of Quantum Optics in Garching, Germany, doesn't think so. "I'm 
really very, very sure that he would be perfectly fine with all these 
experiments," he says. The complementarity principle is at the heart of the 
"Copenhagen interpretation" of quantum mechanics, named after Bohr's home 
city, which essentially argues that we see a conflict in such results only 
because our minds, attuned as they are to a macroscopic, classically 
functioning cosmos, are not equipped to deal with the quantum world. 

"The Copenhagen interpretation, from the very beginning, didn't demand any 
'realistic' world view of the quantum system," says Kofler.

http://plato.stanford.edu/entries/qm-copenhagen/

The outcomes of the latest experiments simply bear that out. "Particle" and 
"wave" are concepts we latch on to because they seem to correspond to 
guises of matter in our familiar, classical world. But attempting to 
describe true quantum reality with these or any other black-or-white 
concepts is an enterprise doomed to failure.

It's a notion that takes us straight back into Plato's cave, says 
Ionicioiu. http://plato.stanford.edu/entries/plato-metaphysics/#13


In the ancient Greek philosopher's allegory, prisoners shackled in a cave 
see only shadows of objects cast onto a cave wall, never the object itself. 
A cylinder, for example, might be seen as a rectangle or a circle, or 
anything in between. Something similar is happening with the basic building 
blocks of reality. "Sometimes the photon looks like a wave, sometimes like 
a particle, or like anything in between," says Ionicioiu. In reality, 
though, it is none of these things. What it is, though, we do not have the 
words or the concepts to express.

Now that is strange. And for quantum physicists, all in a day's work.

(By Anil Ananthaswamy, New Scientist)

--

Cheers,
Stephen