From xxxxxx <[email protected]>
Subject New Views of Quantum Jumps Challenge Core Tenets of Physics
Date January 4, 2021 6:35 AM
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[One of the most basic processes in all of nature—a subatomic
particle’s transition between discrete energy states—is
surprisingly complex and sometimes predictable, recent work shows]
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NEW VIEWS OF QUANTUM JUMPS CHALLENGE CORE TENETS OF PHYSICS  
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Eleni Petrakou
December 29, 2020
Scientific American
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_ One of the most basic processes in all of nature—a subatomic
particle’s transition between discrete energy states—is
surprisingly complex and sometimes predictable, recent work shows _

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Quantum mechanics, the theory that describes the physics of the
universe at very small scales, is notorious for defying common sense.
Consider, for instance, the way that standard interpretations of the
theory suggest change occurs in the quantum turf: shifts from one
state to another supposedly happen unpredictably and instantaneously.
Put another way, if events in our familiar world unfolded similarly to
those within atoms, we would expect to routinely see batter becoming a
fully baked cake without passing through any intermediate steps.
Everyday experience, of course, tells us this is not the case, but for
the less accessible microscopic realm, the true nature of such
“quantum jumps” has been a major unsolved problem in physics.

In recent decades, however, technological advancements have allowed
physicists to probe the issue more closely in carefully arranged
laboratory settings. The most fundamental breakthrough arguably came
in 1986, when researchers for the first time experimentally verified
that quantum jumps are actual physical events that can be observed and
studied. Ever since, steady technical progress has opened deeper
vistas upon the mysterious phenomenon. Notably, an experiment
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overturned the traditional view of quantum jumps by demonstrating that
they move predictably and gradually once they start—and can even be
stopped midway.

That experiment, performed at Yale University, used a setup that let
the researchers monitor the transitions with minimal intrusion. Each
jump took place between two energy values of a superconducting qubit,
a tiny circuit built to mimic the properties of atoms. The research
team used measurements of “side activity” taking place in the
circuit when the system had the lower energy. This is a bit like
knowing which show is playing on a television in another room by only
listening for certain key words. This indirect probe evaded one of the
top concerns in quantum experiments—namely, how to avoid influencing
the very system that one is observing. Known as “clicks” (from the
sound that old Geiger counters made when detecting radioactivity),
these measurements revealed an important property: jumps to the higher
energy were always preceded by a halt in the “key words,” a pause
in the side activity. This eventually permitted the team to predict
the jumps’ unfolding and even to stop them at will.

Now a new theoretical study delves deeper into what can be said about
the jumps and when. And it finds that this seemingly simple and
fundamental phenomenon is actually quite complex.

CATCH ME IF YOU CAN

The new study
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published in _Physical Review Research,_ models the step-by-step,
cradle-to-grave evolution of quantum jumps—from the initial
lower-energy state of the system, known as the ground state, then a
second one where it has higher energy, called the excited state, and
finally the transition back to the ground state. This modeling shows
that the predictable, “catchable” quantum jumps must have a
noncatchable counterpart, says author Kyrylo Snizhko, a postdoctoral
researcher now at Karlsruhe Institute of Technology in Germany, who
was formerly at the Weizmann Institute of Science in Israel, where the
study was performed.

Specifically, by “noncatchable” the researchers mean that the jump
back to the ground state will not always be smooth and predictable.
Instead the study’s results show that such an event’s evolution
depends on how “connected” the measuring device is to the system
(another peculiarity of the quantum realm, which, in this case,
relates to the timescale of the measurements, compared with that of
the transitions). The connection can be weak, in which case a quantum
jump can also be predictable through the pause in clicks from the
qubit’s side activity, in the way used by the Yale experiment.

The system transitions by passing through a mixture of the excited
state and ground state, a quantum phenomenon known as superposition.
But sometimes, when the connection exceeds a certain threshold, this
superposition will shift toward a specific value of the mixture and
tend to stay at that state until it moves to the ground unannounced.
In that special case, “this probabilistic quantum jump cannot be
predicted and reversed midflight,” explains Parveen Kumar, a
postdoctoral researcher at the Weizmann Institute and co-author of the
most recent study. In other words, even jumps for which timing was
initially predictable would be followed by inherently unpredictable
ones.

But there is yet more nuance when examining the originally catchable
jumps. Snizhko says that even these possess an unpredictable element.
A catchable quantum jump will always proceed on a “trajectory”
through the superposition of the excited and ground states, but there
can be no guarantee that the jump will ever finish. “At each point
in the trajectory, there is a probability that the jump continues and
a probability that it is projected back to the ground state,”
Snizhko says. “So the jump may start happening and then abruptly get
canceled. The trajectory is totally deterministic—but whether the
system will complete the trajectory or not is unpredictable.”

This behavior appeared in the Yale experiment’s results. The
scientists behind that work called such catchable jumps “islands of
predictability in a sea of uncertainty.” Ricardo
Gutiérrez-Jáuregui, a postdoctoral researcher at Columbia University
and one of the authors of the corresponding study, notes that “the
beauty of that work was to show that in the absence of clicks, the
system followed a predetermined path to reach the excited state in a
short but nonzero time. The device, however, still has a chance to
‘click’ as the system transitions through this path, thus
interrupting its transition.”

“QUANTUM PHYSICS IS BROKEN!”

Zlatko Minev, a researcher at the IBM Thomas J. Watson Research Center
and lead author of the earlier Yale study, notes that the new
theoretical paper “derives a very nice, simple model and explanation
of the quantum jump phenomenon in the context of a qubit as a function
of the parameters of the experiment.” Taken together with the
experiment at Yale, the results “show that there is more to the
story of discreteness, randomness and predictability in quantum
mechanics than commonly thought.” Specifically, the surprisingly
nuanced behavior of quantum jumps—the way a leap from the ground
state to the excited state can be foretold—suggests a degree of
predictability inherent to the quantum world that has never before
been observed. Some would even consider it forbidden, had it not
already been validated by experiment. When Minev first discussed the
possibility of predictable quantum jumps with others in his group, a
colleague responded by shouting back, “If this is true, then quantum
physics is broken!”

“In the end, our experiment worked, and from it one can infer that
quantum jumps are random and discrete,” Minev says. “Yet on a
finer timescale, their evolution is coherent and continuous. These two
seemingly opposed viewpoints coexist.”

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As to whether such processes can apply to the material world at
large—for instance, to atoms outside a quantum lab—Kumar is
undecided, in large part because of how carefully specific the
study’s conditions were. “It would be interesting to generalize
our results,” he says. If the results turn out similar for different
measurement setups, then this behavior—events that are in some sense
both random and predictable, discrete yet continuous—could reflect
more general properties of the quantum world.

Meanwhile the predictions of the study could get checked soon.
According to Serge Rosenblum, a researcher at the Weizmann Institute
who did not participate in either study, these effects can be observed
with today’s state-of-the-art superconducting quantum systems and
are high on the list of experiments for the institute’s new qubits
lab [[link removed]]. “It was quite
amazing to me that a deceptively simple system such as a single qubit
can still hide such surprises when we measure it,” he adds.

For a long time, quantum jumps—the most basic processes underlying
everything in nature—were considered nearly impossible to probe. But
technological progress is changing that. Kater Murch, an associate
professor at Washington University in St. Louis, who did not
participate in the two studies, remarks, “I like how the Yale
experiment seems to have motivated this theory paper, which is
uncovering new aspects of a physics problem that has been studied for
decades. In my mind, experiments really help drive the ways that
theorists think about things, and this leads to new discoveries.”

The mystery might not just be going away, though. As Snizhko says,
“I do not think that the quantum jumps problem will be resolved
completely any time soon; it is too deeply ingrained in quantum
theory. But by playing with different measurements and jumps, we might
stumble upon something practically useful.”

_ELENI PETRAKOU
[[link removed]]  is an
experimental particle physicist turned freelancer. Previous work in
axion searches and collider physics. Currently spending more time on
astrophysics and data analysis._

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