Weird quantum effects stretch across hundreds of miles
July 19, 2016
Source:Phys.org
In the world of quantum,
infinitesimally small particles, weird and often logic-defying behaviors
abound. Perhaps the strangest of these is the idea of superposition, in which
objects can exist simultaneously in two or more seemingly counterintuitive states.
For example, according to the laws of quantum mechanics, electrons may spin
both clockwise and counter-clockwise, or be both at rest and excited, at the
same time.
The physicist Erwin
Schrödinger highlighted some strange consequences of the idea of superposition
more than 80 years ago, with a thought experiment that posed that a cat trapped
in a box with a radioactive source could be in a superposition state, considered
both alive and dead, according to the laws of quantum mechanics. Since then,
scientists have proven that particles can indeed be in superposition, at
quantum, subatomic scales. But whether such weird phenomena can be observed in
our larger, everyday world is an open, actively pursued question.
Now, MIT physicists have
found that subatomic particles called neutrinos can be in superposition, without individual identities, when
traveling hundreds of miles. Their results, to be published later this month in Physical
Review Letters, represent the longest distance over which quantum mechanics
has been tested to date.
A subatomic journey across
state lines
The team analyzed data on
the oscillations of neutrinos—subatomic particles that interact extremely
weakly with matter, passing through our bodies by the billions per second
without any effect. Neutrinos can oscillate, or change between several distinct
"flavors," as they travel through the universe at close to the speed
of light.
The researchers obtained
data from Fermilab's Main Injector Neutrino Oscillation Search, or MINOS, an
experiment in which neutrinos are produced from the scattering of other
accelerated, high-energy particles in a facility near Chicago and beamed to a
detector in Soudan, Minnesota, 735 kilometers (456 miles) away. Although the
neutrinos leave Illinois as one flavor, they may oscillate along their journey,
arriving in Minnesota as a completely different flavor.
The MIT team studied the
distribution of neutrino flavors generated in Illinois, versus those detected
in Minnesota, and found that these distributions can be explained most readily
by quantum phenomena: As neutrinos sped between the reactor and detector, they
were statistically most likely to be in a state of superposition, with no
definite flavor or identity.
What's more, the
researchers found that the data was "in high tension" with more
classical descriptions of how matter should behave. In particular, it was
statistically unlikely that the data could be explained by any model of the
sort that Einstein sought, in which objects would always embody definite
properties rather than exist in superpositions.
"What's fascinating
is, many of us tend to think of quantum mechanics applying on small
scales," says David Kaiser, the Germeshausen Professor of the History of
Science and professor of physics at MIT. "But it turns out that we can't
escape quantum mechanics, even when we describe processes that happen over
large distances. We can't stop our quantum mechanical description even when
these things leave one state and enter another, traveling hundreds of miles. I
think that's breathtaking."
Kaiser is a co-author on
the paper, which includes MIT physics professor Joseph Formaggio, junior Talia
Weiss, and former graduate student Mykola Murskyj
A flipped inequality
The team analyzed the MINOS
data by applying a slightly altered version of the Leggett-Garg inequality, a
mathematical expression named after physicists Anthony Leggett and Anupam Garg,
who derived the expression to test whether a system with two or more distinct
states acts in a quantum or classical fashion.
Leggett and Garg realized
that the measurements of such a system, and the statistical correlations
between those measurements, should be different if the system behaves according
to classical versus quantum mechanical laws.
"They realized you get
different predictions for correlations of measurements of a single system over
time, if you assume superposition versus realism," Kaiser explains, where
"realism" refers to models of the Einstein type, in which particles
should always exist in some definite state.
Formaggio had the idea to
flip the expression slightly, to apply not to repeated measurements over time
but to measurements at a range of neutrino energies. In the MINOS experiment,
huge numbers of neutrinos are created at various energies, where Kaiser says
they then "careen through the Earth, through solid rock, and a tiny
drizzle of them will be detected" 735 kilometers away.
According to Formaggio's
reworking of the Leggett-Garg inequality, the distribution of neutrino
flavors—the type of neutrino that finally arrives at the detector—should depend
on the energies at which the neutrinos were created. Furthermore, those flavor
distributions should look very different if the neutrinos assumed a definite
identity throughout their journey, versus if they were in superposition, with
no distinct flavor.
"The big world we live
in"
Applying their modified
version of the Leggett-Garg expression to neutrino oscillations, the group
predicted the distribution of neutrino flavors arriving at the detector, both
if the neutrinos were behaving classically, according to an Einstein-like
theory, and if they were acting in a quantum state, in superposition. When they compared both predicted distributions, they found there was
virtually no overlap.
More importantly, when they
compared these predictions with the actual distribution of neutrino flavors
observed from the MINOS experiment, they found that the data fit squarely
within the predicted distribution for a quantum system, meaning that the
neutrinos very likely did not have individual identities while traveling over
hundreds of miles between detectors.
But what if these particles
truly embodied distinct flavors at each moment in time, rather than being some
ghostly, neither-here-nor-there phantoms of quantum physics? What if these
neutrinos behaved according to Einstein's realism-based view of the world?
After all, there could be statistical flukes due to defects in instrumentation,
that might still generate a distribution of neutrinos that the researchers
observed. Kaiser says if that were the case and "the world truly obeyed
Einstein's intuitions," the chances of such a model accounting for the
observed data would be "something like one in a billion."
"What gives people
pause is, quantum mechanics is quantitatively precise and yet it comes with all
this conceptual baggage," Kaiser says. "That's why I like tests like
this: Let's let these things travel further than most people will drive on a
family road trip, and watch them zoom through the big world we live in, not
just the strange world of quantum mechanics, for hundreds of miles. And even
then, we can't stop using quantum mechanics. We really see quantum effects persist across macroscopic
distances."
SOURCE: PHYS .ORG
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