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
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 published in 2019 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.
The new study, 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.”
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.”
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.
“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.”