In quantum physics, mathematically discontinuous changes – or jumps – between quantum states are popularly referred to as quantum leaps (remember the TV series). As to whether quantum leaps are instantaneous (zero time) or not – as well as random (without any harbinger) – is an open question in physics. This article below recaps additional research on the findings of Yale researchers noted in my April 2, 2020, comment on “Imaging a light pulse?”

• Scientific American > “New Views of Quantum Jumps Challenge Core Tenets of Physics” by Eleni Petrakou (December 29, 2020) – 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.

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 [at Yale University] 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.

“In the end, our experiment worked, and from it one can infer that quantum jumps are random and discrete,” Minev [a researcher at the IBM Thomas J. Watson Research Center and lead author of the earlier Yale study] says. “Yet on a finer timescale, their evolution is coherent and continuous. These two seemingly opposed viewpoints coexist.”

The mystery might not just be going away, though. As Snizhko [a postdoctoral researcher now at Karlsruhe Institute of Technology in Germany] 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.”

Update 1-25-2021

• YouTube > PBS Space Time > “What Happens During a Quantum Jump?” (January 12, 2021) [Hosted by Matt O’Dowd]

… one of the principal founders of quantum mechanics … Erwin Schrodinger himself never accepted the idea of the quantum jump – but could also never prove it wrong. That proof required precision measurements that didn’t exist in Schrodinger’s time. However they exist now – and the reality of the quantum jump has finally been tested.

In 1952, Schrödinger published a two-part essay titled “Are there quantum jumps?” wherein he compared the theory of quantum jumps to that of epicycles—the long dead theory about the motion of the planets in an Earth-centered solar system. He claimed that both epicycles and quantum jumps were “ingenious constructs of the human mind” that nevertheless were not true descriptions of nature.

[Schrödinger] believed it all came down to waves — and that nothing was particularly special about these waves compared to any other kind of classical wave. He argued that most “spooky” quantum phenomena could be explained by classical resonance phenomena. He rejected the idea of the “photon” as an irreducible energy packet, and even dismissed the notion that electrons transitioned between discrete energy levels. He argued that the same emission spectra could be got by thinking of these levels as fundamental vibrational modes, like on a drum or guitar string.

O’Dowd unpacks a seminal 1986 experiment, how it worked with a single atom and a laser beam. The beam’s frequency was tuned to the energy difference between two of an electron’s levels. That part demonstrated continuous jumping at a rate of “like 100 million times per second.” Characterizing a single jump required another laser beam tuned to a third level. The jump “appeared to occur at completely random times.”

Then ~30 years later, better technology permitted exploring the question of instantaneity. Exploring three “energy” levels of a “sort of artificial atom made of two superconducting circuits.” And making use of quantum trajectory theory.

They found that [a “jump”] was not instantaneous after all, but rather was a continuous transition over intermediate states that took a few microseconds.

More ongoing research has explored not just the random spacing between events but harbingers of those events. Being able to predict an onset not only permits some control but also further speculation about any “mechanism” (re evolution in quantum systems). And weak vs. strong measurement – “how strongly the system is coupled to the measurement apparatus.” The exploration of fundamental quantum continuity and determinism continues.

See also Quantum Zeno effect.

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Quantum trajectory theory?

Quantum mechanics math basics – this comment