from QuantaMagazine Website
in a famous experiment designed to prove
that quantum objects don't have intrinsic properties,
three experimental groups
quickly sewed the loophole shut.
The episode closes the door on many
"hidden variable" theories.
The photon, in other words, has definite reality at the beginning and end.
But its state in the middle - the dragon's body - is nebulous.
Wheeler was espousing the view that elementary quantum phenomena are not real until observed, a philosophical position called anti-realism.
He even designed an experiment to show that if you hold on to realism - in which quantum objects such as photons always have definite, intrinsic properties, a position that encapsulates a more classical view of reality - then one is forced to concede that the future can influence the past...
Given the 'absurdity' of
backward time-travel, Wheeler's experiment became an argument
for anti-realism at the level of the quantum.
a physicist at the International Institute of Physics,
and his colleagues used the emerging field of causal modeling
to find a loophole in Wheeler's delayed-choice experiment.
They showed that Wheeler's experiment, given certain assumptions, can be explained using a classical model that attributes to a photon an intrinsic nature.
They gave the dragon a
well-defined body, but one that is hidden from the mathematical
formalism of standard quantum mechanics.
Quantum mechanics may be weird, but it's still, oddly, the simplest explanation around.
Quantum objects seem to act either like particles or waves, but never both at the same time.
This feature of quantum mechanics seems to imply that objects have no inherent reality until observed.
The phenomenon is
underscored by a special case of the famous double-slit experiment
In this case, the photon
will take either path 1 or path 2, and then go on to hit either
detector D1 or D2 with equal probability. The photon acts like an
indivisible whole, showing us its particle-like nature.
At the point where path 1 and path 2 cross, one can add a second beam splitter, which changes things. In this setup, quantum mechanics says that the photon seems to take both paths at once, as a wave would.
The two waves come back together at the second beam splitter.
The experiment can be set up so that the waves combine constructively - peak to peak, trough to trough - only when they move toward D1. The path toward D2, by contrast, represents destructive interference. In such a setup, the photon will always be found at D1 and never at D2.
Here, the photon displays its wavelike nature.
Let's assume the photon enters the interferometer without the second beam splitter in place. It should act like a particle.
One can, however, add the second beam splitter at the very last nanosecond. Both theory and experiment show that the photon, which until then was presumably acting like a particle and would have gone to either D1 or D2, now acts like a wave and goes only to D1.
To do so, it had to
seemingly be in both paths simultaneously, not one path or the
other. In the classical way of thinking, it's as if the photon went
back in time and changed its character from particle to wave.
His team wanted to explain counterintuitive aspects of quantum mechanics using a new set of ideas called causal modeling, which has grown in popularity in the past decade, advocated by computer scientist Judea Pearl and others.
Causal modeling involves establishing cause-and-effect relationships between various elements of an experiment.
Often when studying
correlated events - call them A and B - if one cannot conclusively
say that A causes B, or that B causes A, there exists a possibility
that a previously unsuspected or "hidden" third event, C, causes
both. In such cases, causal modeling can help uncover C.
They thought they would prove that the delayed-choice experiment is,
a physicist at the International Institute of Physics,
showed how a "hidden variable"
could be affecting the results of the experiment.
The task proved relatively easy. They began by assuming that the photon, immediately after it has crossed the first beam splitter, has an intrinsic state denoted by a "hidden variable."
A hidden variable, in this context, is something that's absent from standard quantum mechanics but that influences the photon's behavior in some way. The experimenter then chooses to add or remove the second beam splitter.
Causal modeling, which
prohibits backward time travel, ensures that the experimenter's
choice cannot influence the past intrinsic state of the photon.
Here was a classical,
causal, realistic explanation. They had found a new loophole.
The next step was to
figure out how to modify Wheeler's experiment in such a way that it
could distinguish between this classical hidden variable theory and
Instead, two "phase
shifts" - one near the beginning of the experiment, one toward the
end - serve the role of experimental dials that the researcher can
adjust at will.
For example, the value of the first phase shift could be such that the photon acts like a particle inside the interferometer, but the second phase shift could force it to act like a wave.
The researchers require
that the second phase shift is set after the first.
That makes six possible experimental settings in total. They calculated what they expected to see for each of these six settings. Here, the predictions of a classical hidden variable model and standard quantum mechanics differ. They then constructed a formula.
The formula takes as its input probabilities calculated from the number of times that photons land on particular detectors (based on the setting of the two phase shifts). If the formula equals zero, the classical causal model can explain the statistics.
But if the equation spits
out a number greater than zero, then, subject to some constraints on
the hidden variable, there's no classical explanation for the
Simultaneously, two teams
in China - one led by Jian-Wei Pan, an experimental physicist
at the University of Science and Technology of China (USTC) in
Hefei, China, and another by Guang-Can Guo, also at USTC -
carried out the experiment.
Guo's group stuck to the basics, using an actual Mach-Zehnder interferometer
But all three showed that the formula is greater than zero with irrefutable statistical significance. They ruled out the classical causal models of the kind that can explain Wheeler's delayed-choice experiment.
The loophole has been closed.
The formula comes with certain assumptions.
The biggest one is that the classical hidden variable used in the causal model can take one of two values, encoded in one bit of information. Chaves thinks this is reasonable, since the quantum system - the photon - can also only encode one bit of information.
(It either goes in one arm of the interferometer or the other.)
a physicist and historian at MIT,
wants to eliminate the possibility
of any unseen experimental correlations
by employing a random-number generator
based on distant astrophysical objects.
The De Broglie-Bohm theory, a deterministic and realistic alternative to standard quantum mechanics, is perfectly capable of explaining the delayed-choice experiment. In this theory, particles always have positions (which are the hidden variables), and hence have objective reality, but they are guided by a wave.
So reality is both wave and particle...
The wave goes through
both paths, the particle through one or the other. The presence or
absence of the second beam splitter affects the wave, which then
guides the particle to the detectors - with exactly the same results
as standard quantum mechanics.
Kaiser, even as he lauds the efforts so far, wants to take things further.
In current experiments, the choice of whether or not to add the second phase shift or the second beam splitter in the classic delayed-choice experiment was being made by a quantum random-number generator.
But what's being tested in these experiments is quantum mechanics itself, so there's a whiff of circularity.
To this end, Kaiser and his colleagues have built such a source of randomness using photons coming from distant quasars, some from more than halfway across the universe.
The photons were collected with a one-meter telescope at the Table Mountain Observatory in California.
If a photon had a wavelength less than a certain threshold value, the random number generator spit out a 0, otherwise a 1. In principle, this bit can be used to randomly choose the experimental settings.
If the results continue to support Wheeler's original argument, then,
For now, the dragon's
body, which for a brief few weeks had come into focus, has gone
back to being smoky and indistinct.