Since the very beginning of quantum mechanics, a debate has raged about how to interpret its bizarre predictions.

And at the heart and origin of that debate is the quantum jump or quantum leap - the seemingly miraculous and instantaneous transitions of quantum systems that have always defied observation or prediction.

At least, until now.

The notion of a quantum jump or quantum leap is one of the founding concepts of quantum mechanics.

It’s really the OG of quantum weirdness - so much so that it’s become part of common lexicon, with very loose fidelity to the original meaning.

It comes from the idea that electrons in atoms jump randomly and instantaneously from one orbit or energy level to another, without ever occupying the intervening space.

The idea has become so ingrained into how we think about atoms that few think to question the notion.

But one of the principal founders of quantum mechanics thought otherwise.

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.

It all started back in 1913, when the great Danish physicist Niels Bohr set about to explain emission spectra - the sharp bands of color produced when a simple tube of gas is energized.

He took a lead from the recent success of Max Planck, who in 1901 had managed to explain the colors of light produced by a heated object - the so-called blackbody spectrum - by assuming that light is made up of irreducible packets of energy that we now call photons.

Bohr placed a similar restriction on atoms - he required that electron energy levels were quantized - could only have very specific energies that depended on the element.

Electrons would then jump between energy levels by emitting or absorbing a photon that corresponded to the difference in energy.

The result was the Bohr model of the atom - the very first attempt at a quantum theory, and it very neatly explained the specific frequencies of light observed in emission spectra of hydrogen - although it failed for more complex elements Bohr’s work inspired Werner Heisenberg and Erwin Schrodinger to develop the first complete formulations of quantum mechanics in 1925.

Ultimately, these successfully predicted the spectra of elements of any complexity - and much more besides.

But one thing remained mysterious - what was actually happening during the quantum jump, and what determined when a quantum jump would occur?

To address questions like this, Bohr and Heisenberg teamed up to develop the “Copenhagen interpretation” of quantum mechanics.

Copenhagen describes transitions in quantum states as fundamentally random - the dice are rolled, and the system transitions instantaneously.

The electron goes from one energy level to the other without moving in between.

Copenhagen says that “measurement collapses the wavefunction”, which these days is more often taken to mean that interaction with the environment causes the state transition - causes the quantum jump.

But one guy was not impressed by this idea - the inventor of the wavefunction himself, Erwin Schrödinger.

When he visited Bohr’ home in 1926, Schrödinger said “If we are still going to have to put up with these damn quantum jumps, I am sorry that I ever had anything to do with quantum theory.” Schrödinger became bed-ridden with an illness during that visit - perhaps he was that sick of the idea.

But it didn’t help - supposedly Bohr continued haranguing the poor sick man with the Copenhagen world-view.

Bohr’s efforts were to no avail.

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.

So why did Schrodinger hate quantum jumps so much?

Simply put, they seemed unnatural and unphysical - a hack added to cover up a phenomenon that quantum theory could not yet properly explain.

The debate over quantum jumps was just one part of the larger discussion about the quantum nature of reality.

It was the Copenhagen interpretation versus - well “not-Copenhagen”.

The most famous quote on the not-Copenhagen side was from Albert Einstein - “God does not play dice with nature”.

It was a reaction against one of the central tenants of Copenhagen - that subatomic phenomena are fundamentally random, or probabilistic.

But Schrodinger had his arguments too.

He 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.

An atomic electron could then be considered a superposition of multiple vibrational modes.

And that meant the electron could transition smoothly through a series of intermediate states during each transition, rather than undergoing instantaneous quantum jumps.

To Schrodinger, a big part of the problem was that Bohr and others were using the behavior of systems of many, many individual particles to infer the behavior of individual particles.

He believed that it was completely nonsensical to even think about single particles.

As he put it, “we never experiment with just one electron or atom.

In thought-experiments we sometimes assume that we do; this invariably entails ridiculous consequences…” That was in 1952 - and in 1952 we had never seen a single photon produced by a single quantum jump in a single atom.

But in time we figured out how to do exactly that.

Fast forward 30 years.

By the 80s we’d learned how to trap and cool a single atom with lasers.

And in 1986, almost simultaneously, three different teams observed quantum jumps in such an atom.

Here’s how it worked.

The single atom - in this case mercury or barium - is bathed in a laser beam with a frequency exactly tuned to the energy difference between two of its electron levels - call them 1 and 2.

If the electron is in level 1, it should jump to level 2 by absorbing a photon from the laser light.

If the electron then falls back to level 1 it should emit an identical photon in a random direction.

For the right choice of energy levels, this should happen extremely quickly - the electron should become locked between the two levels.

In the 1986 experiments, the electron in the trapped atom jumped between levels something like 100 million times per second.

The individual photons emitted in this process couldn’t be seen - instead the single atom just glowed, or fluoresced.

This wasn’t a direct observation of individual quantum jumps - that required an extra level of cleverness, as well as an extra energy level - we’ll call this level 3.

Level 3 is far more stable than level 2 - an electron that finds itself there may take many seconds to drop back down.

So, we have our atom happily fluorescing in the original laser beam.

Now flash a second laser with frequency tuned to take the electron from level 1 to level 3.

Suddenly the atom goes dark - the fluorescence stops, because the electron is stuck in level 3 and no longer available to cycle between 1 and 2.

Then, after a period of time, the electron decays and fluorescence starts again.

In this way, physicists were able to gain fairly direct evidence of a single quantum jump.

And the downward jumps when the electron decayed out of level 3 appeared to occur at completely random times.

Just as Bohr had predicted.

So, that settled it.

Schrödinger was wrong and Bohr was right.

Right?

Not so fast.

Although the jump appeared random, there was no way to tell whether it was instantaneous, or whether the electron passed through some intermediate states during the jump Fast forward another 30+ years to, well, a year ago.

Technology has advanced to the point that we can not only see individual quantum jumps - we can monitor their progress, and even interrupt them mid-jump.

Now this isn’t with an actual atom, but rather a sort of “artificial atom made of two superconducting circuits.

The 3 different energy levels of this artificial atom corresponded to the number of electromagnetic quanta of energy stored in the circuits.

The ground state (or state 1 using the notation from the previous experiment) corresponded to zero quanta in either circuit, states 2 and 3 corresponded to 1 quantum in either respective circuit.

They then placed these artificial atoms inside a microwave cavity - analogous to the laser, which could cause the “atom” to transition between states.

But this also allowed the researchers to monitor the state of the system with far greater resolution than in the 1986 experiment.

They could actually zoom in on a quantum jump and finally figure out whether it truly was an instantaneous transition.

They found that … no, it was not instantaneous after all, but rather was a continuous transition over intermediate states that took a few microseconds.

And that transition appeared to be perfectly described by theory - in this case quantum trajectory theory.

But what about the randomness of the event?

Well, the spacing between events did appear to be random, as Bohr thought.

But just prior to each jump the system started to shift in a way that enabled the researchers to predict the oncoming jump.

And that ability to predict also allowed them to reverse the quantum jumps midflight by adjusting the microwave field during the process.

Given that the quantum jump onset was predictable, and that its trajectory was extremely well described by theoretical calculations, it’s tempting to wonder if the whole process is driven by an underlying deterministic mechanism, rather than fundamental randomness.

I suspect both Schrodinger and Einstein would agree.

But what’s really going on here?

Well, just a couple of months ago, a group of theorists claim to have taken a major step towards figuring it out.

They explain this non-instantaneous quantum transition in terms of something called the Quantum Zeno Effect.

We'll need a full episode to explain this phenomenon properly, but in short, the act of measuring a system will, in Copenhagen terms, collapse the wavefunction, which drastically changes how the system behaves - for example, trapping the system in one state.

The theorists showed how quantum states can transition very predictably through a series of superposition states - much as Schrodinger proposed - but in addition to these predictable quantum jumps there are fundamentally unpredictable ones--truly unpredictable and sudden quantum jumps that would've made Bohr proud.

And the difference?

It's to do with how strongly the system is coupled to the measurement apparatus.

The weaker the measurement, the less likely a true quantum jump is to occur.

But more on that another time.

For now, both the Bohr and Schrodinger camps have new evidence in their favour.

For the longest time, physicists have shied away from asking which interpretation of quantum mechanics is correct.

It’s considered by many to be a point of philosophical preference whether you roll dice with Bohr and Heisenberg in Copenhagen, or ride the continuous and deterministic wave of Schrodinger and others.

But experimentalists are proving far cleverer than the naysayers imagined.

Nearly a century after Bohr and Schrodinger started the argument, we may be on the verge of the next quantum leap - to learning whether quantum jumps are instantaneous or continuous, and perhaps even whether the quantum world is built upon fundamentally random processes, or is driven by rigidly deterministic mechanic of space time.