
Quantum Leaps, Long Assumed to Be Instantaneous, Take Time - _Microft
https://www.quantamagazine.org/quantum-leaps-long-assumed-to-be-instantaneous-take-time-20190605/
======
lisper
This headline is deeply misleading. It is true that energy states are
quantized. It does NOT follow that the transition between them is
"instantaneous". The transition has always been known to be gradual, but the
"gradualness" is not a smooth transition in the energy of the system, it's a
smooth transition between being in one energy state to being in a
superposition of two energy states to being entirely in the second energy
state. That process plays out over (typically a very short but nonetheless
non-zero) time. This has been known since the very beginning of QM.

What's news here is that this transition, which has always been predicted by
theory, has been experimentally observed for the first time.

~~~
posix_compliant
I have questions: you describe the change of states of particle as

    
    
        "a smooth transition between being in one energy state to
        being in a superposition of two energy states to being
        entirely in the second energy state."
    

At the terminus of this transition, is the particle no longer in a
superposition? IE there is now a 100% chance it's in the second state and 0%
chance that it's in the original state?

If so, does that imply that the function of the particle's state (with respect
to time) is discontinuous? Since there's a point at which it goes from being a
superposition to exactly 0%.

~~~
lisper
That's a very good question. It has two answers.

The first answer is that a system is never 100% in any particular state
because that would violate the uncertainty principle. When we speak of a
system "definitely" being in a particular state that's an
approximation/simplification. It actually means that the system is in a
superposition of some range of states that for all intents and purposes we can
treat as being the same, and the probability of it being in a state that
cannot be treated the same for all intents and purposes is so close to zero we
can ignore that.

The second answer is that whether or not a system is in a superposition
depends on your point of view. A system can only ever be in a "single state"
(according to the above approximation) with respect to some observable, and if
it is in a single state with respect to that observable then it is necessarily
in a superposition with respect to the complementary observable, e.g. a
particle that is in a definite state with respect to position is necessarily
in a superposition with respect to velocity.

So the whole process is everywhere and always continuous.

------
molticrystal
While not a perfect analogy, if I understand this properly, essentially if
electrons jumping states were pitching in an unregulated game of baseball, the
chances a pitcher will toss the ball increases as time goes on and previously
we thought the ball just appeared over the plate instantaneously when that
random time occurred.

But instead, while you still don't know when exactly, you do know there will
be a wind up for the pitch before the ball is thrown, and we can see this
windup, and we can time things to hit the ball, or shoot the pitcher with a
blowdart and keep the pitch from happening.

So if there are systems in certain configurations to which this property is
applicable to, and if we needed to keep things in that configuration until a
time of our choosing, for instance if a state change occurs it would have
cascading effects, we can do so.

~~~
inflatableDodo
So god doesn't play dice, she plays poker.

~~~
khazhou
How do you get poker from that?

~~~
inflatableDodo
Both are games of chance, but dice has no tells.

------
trurl42
As a quantum physicist: This might be an interesting experiment, but as far as
the theory goes, this is exactly what is to be expected from standard quantum
mechanics.

Quantum leaps being instantaneous would be a (possibly common) misconception.

The leap in a quantum leap is describing the notion of a discrete jump in
measurement outcomes.

~~~
neaanopri
Okay, so you're a quantum physicist and you can answer my question!

My understanding of QM is that a quantum's system's state is suddenly and
discontinuously changed by a measurement.

My understanding of this article is a bit confused, and I think that there are
two possible things we could be seeing: 1\. Quantum collapses of
superpositions actually do take time ("This Changes Everything") 2\. This
particular quantum system is not actually being measured, but is oscillating
in superposition in some odd way. ("Just a Particular System")

Which case is it?

~~~
trurl42
That would be 2.

What's done in such an experiment is that we initialize a quantum state in a
particular state (that this is possible is not actually obvious, but let's
assume it's true) and then we make a lot of repeated measurements after
different amounts of times.

So we're not talking about a single system, but instead about statistics about
a set of measurements with systems that have been set up in the same initial
state. (Each system only being measured once after a certain time after it's
being set up).

~~~
neaanopri
Gotcha, so the ideas of "Shut Up and Calculate" QM about wavefunction collapse
for ideal systems aren't incomplete.

------
Xcelerate
I guess I’m kind of confused on what the discovery is; we have a quantum
system evolving unitarily from one state at t_0 to another state at t, and the
probability of measuring the system in one of two discrete states changes
continuously as well (despite the actual measurement outcome being discrete).
I thought this has been known for a long time, implied by the time dependent
Schrodinger equation, so I didn’t quite catch from the Quanta article what is
new, mathematically. Can anyone clarify?

~~~
pdonis
_> I didn’t quite catch from the Quanta article what is new, mathematically_

Nothing is new. This experiment is simply confirming a prediction of standard
QM. All the talk about "quantum jumps" and how something has supposedly
changed about the way we understand them is just pop science reporters
misunderstanding the actual science.

------
maxharris
If you're a lay person like me and you want to make sense of this, read
"Einstein’s Unfinished Revolution: The Search for What Lies Beyond the
Quantum" by Lee Smolin. It was just published recently, and it anticipates
this result!

[https://www.penguinrandomhouse.com/books/316818/einsteins-
un...](https://www.penguinrandomhouse.com/books/316818/einsteins-unfinished-
revolution-by-lee-smolin/9781594206191/)

~~~
outlace
I just it but don’t recall where he predicted this. Do you remember what he
said?

------
joycian
Uhm...forgive me for asking, but a continuous quantum jump is...what exactly?

How is this in any way consistent with the rest of quantum mechanics?

Edit: I don't mean to sound snide, I am genuinely confused about what this
experiment means.

~~~
trevyn
It’s a process of transition between two distinct states. Say a bit flips from
0 to 1; there is a short time period during which that bit is physically “in
transition”, but it would be incorrect to say that the bit has a value of,
say, 0.5 during the transition.

Also remember that quantum mechanics is a way to make statistical predictions
of outcomes of certain experiments; it does not claim to explain what is
_actually happening_ underneath.

~~~
DiogenesKynikos
I strongly disagree with your second paragraph. Quantum theories do, we think,
correspond to what is _actually happening_ underneath. They might not be
perfect correspondences, in that our theories are incomplete or approximate,
in the same way that Newtonian gravity is an approximation of General
Relativity in the weak-gravity regime.

Bell's inequality places a strong constraint on what sort of physical theories
can explain quantum phenomena. If you believe in locality (the universe has no
global variables, and information propagates outwards through local
interactions), then the wavefunction is a real thing, and the actual state of
the universe.

~~~
trevyn
The formalisms of quantum mechanics are widely accepted, but as you know,
there is considerable disagreement about what may actually be happening
underneath:

[https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_m...](https://en.m.wikipedia.org/wiki/Interpretations_of_quantum_mechanics)

(Aside: Bell’s theorem additionally requires one to dismiss the possibility of
superdeterminism.)

~~~
joycian
So this is what I do not understand about Bell's Theorem: it seems to dismiss
in the assumptions the possibility of determinism (I don't understand where
the super comes from, as there is no distinction) and then goes on to conclude
"Hey, there must be fundamental randomness". Didn't we just assume the
conclusion?

~~~
DiogenesKynikos
It does not assume from the outset that there is no determinism. It sets
bounds on the types of results that you can get with deterministic local
variables. Those bounds are violated in the real world. Hence, the universe is
either quantum mechanical, or there exist hidden nonlocal variables that give
the illusion of quantum behavior.

~~~
trevyn
Bell:

 _There is a way to escape the inference of superluminal speeds and spooky
action at a distance. But it involves absolute determinism in the universe,
the complete absence of free will. Suppose the world is super-deterministic,
with not just inanimate nature running on behind-the-scenes clockwork, but
with our behavior, including our belief that we are free to choose to do one
experiment rather than another, absolutely predetermined, including the
"decision" by the experimenter to carry out one set of measurements rather
than another, the difficulty disappears. There is no need for a faster than
light signal to tell particle A what measurement has been carried out on
particle B, because the universe, including particle A, already "knows" what
that measurement, and its outcome, will be._

[https://en.m.wikipedia.org/wiki/Superdeterminism](https://en.m.wikipedia.org/wiki/Superdeterminism)

I think it’s useful to be clear about the dividing line between demonstrated
scientific results and our scientific intuitions, even when those intuitions
are driven by observed patterns that have been reliable in the past.
Intuitions are excellent drivers for formulating new theories and experiments,
but it is epistemically dangerous to conflate beliefs and knowledge in our
minds.

~~~
joycian
> Suppose the world is super-deterministic, with not just inanimate nature
> running on behind-the-scenes clockwork, but with our behavior, including our
> belief that we are free to choose to do one experiment rather than another,
> absolutely predetermined, including the "decision" by the experimenter to
> carry out one set of measurements rather than another, the difficulty
> disappears.

You see, I think Bell is kind of obfuscating here: this is just normal
determinism and it seems to me like Lagrange would have been perfectly fine
with this. Causality only, even for the atoms that happen to reside in a human
brain.

Edit: what do you mean with your last paragraph? Because I interpret it like
this: the belief that interferes is that we have free will and can choose and
basically change the past (counterfactuals). But I have a hunch that you mean
it like: QM is weird, be careful.

~~~
trevyn
My understanding is that the “super” in superdeterminism is just making clear
that we are considering the experimenter completely determined as well, but
yes, they are essentially the same thing.

Last paragraph I mean what you interpret — our everyday experience of “free
will” may blind us to certain possibilities that we have no strong evidence
for or against, just as our everyday experience of a classical world makes QM
seem “weird” and unintuitive, even when the experimental evidence is well-
established.

There’s a particular curiosity when the possibility of a world without free
will calls into question the extent of the power of science itself. Are there
other techniques that can provide satisfying arguments about the potential
nature of reality in a world where we cannot rely on the power of experiment
to reveal this nature?

~~~
joycian
I think it does call into question the power of science, but also: assuming
non-determinism does not give us this power back! If materials (atoms /
humans) can arbitrarily disregard the laws of nature by making choices
(however that would be implemented), what does an experiment even mean?

Edit:

This is a relevant theorem, that nicely complements Bell:

[https://en.m.wikipedia.org/wiki/Free_will_theorem](https://en.m.wikipedia.org/wiki/Free_will_theorem)

------
elliekelly
Tangentially related request: I’m frequently very impressed with how many HN
users seem to have a pretty solid grasp of physics (or maybe the right term is
quantum mechanics?) and I’d love to be able to follow along but my science
education more or else ended with high school. Could anyone recommend a good
resource for someone with zero knowledge of this space to get very basic
foundation?

~~~
orbifold
You might want to check out
[http://www.staff.science.uu.nl/~hooft101/theorist.html](http://www.staff.science.uu.nl/~hooft101/theorist.html),
[http://www.staff.science.uu.nl/~gadda001/goodtheorist/index....](http://www.staff.science.uu.nl/~gadda001/goodtheorist/index.html)

~~~
elliekelly
Wow! This is quite a gem, thanks! I can pinpoint exactly where on this list my
physics knowledge drops off.

You should submit this to the main page.

------
sova
"Natura Facit Saltus or No" was something Schrodinger wrote in 1952! (does
nature jump or no?) Of course it's a continuous process, how else would water
flow? As they mention in the article, here's the link to Schrodinger's paper
"ARE THERE QUANTUM LEAPS?" [https://academic.oup.com/bjps/article-
abstract/III/11/233/14...](https://academic.oup.com/bjps/article-
abstract/III/11/233/1456020)

------
amatus
Preprint: [https://arxiv.org/abs/1803.00545](https://arxiv.org/abs/1803.00545)

------
rotrux
Here's an interesting passage:

> "The strategy reveals that quantum measurement is not about the physical
> perturbation induced by the probe but about what you know (and what you
> leave unknown) as a result. “Absence of an event can bring as much
> information as its presence,” said Devoret."

------
AnimalMuppet
If a quantum system is going to transition between two states that have
different energy, would we not expect it to take a time specified by the
Heisenberg Uncertainty Principle? If the change in energy is delta E, would we
not expect the transition to take delta T = h bar/delta E?

~~~
pdonis
_> would we not expect it to take a time specified by the Heisenberg
Uncertainty Principle?_

There isn't actually an energy-time version of the uncertainty principle, at
least not the simple one you're assuming here, although many pop science
presentations talk as if there is. A good article discussing this is here:

[http://www.math.ucr.edu/home/baez/uncertainty.html](http://www.math.ucr.edu/home/baez/uncertainty.html)

For a quantum system transitioning between states, the probability of
transition in general will vary as a function of time; how it varies depends
on the specific state of the system. There is no general rule that relates the
expected transition time to the change in energy. (Note also that not all
transitions are between energy eigenstates.)

------
xwdv
I feel like this could have profound implications on a macro level, but I’m
not sure what.

------
pontifier
I like the way they circumvented observation of the dark state. Very clever,
and a very interesting result.

There seems to be some sort of "hidden variable" there... can anyone explain
it?

~~~
woodandsteel
If I am understanding the article correctly, there is no hidden variable
implied. Quantum physics claims changes are random, with nothing precise and
determinate underneath that causes this. Think of the example of flipping a
coin. It's random, but for each flip if you had exact data for the movement of
the coin as it left your hand, you would presumably be able to predict if it
would land heads or tails. Quantum physics claims that for events at the
quantum level, there is nothing definite underneath that determines what will
happen, it is just fundamental randomness. The hidden variable idea is that
there is something definite underneath, we just haven't discovered it yet.

What the experiment seems to have found is only that the probability of an
event occurring changes smoothly over time from 0 to 1, not that there is some
underlying exact cause for what the probability is at a given point in time.

~~~
pontifier
They detected a quiet period of slower transitions before the spontaneous drop
to the middle state... this implies that "something" happens prior to the
transition.

They were able to detect this "something", and even prevent the state change.

In my mind, that "something" represents a hidden state or something like it
within the synthetic atom.

~~~
woodandsteel
The trouble is there are some strong arguments against that idea, so quantum
physicists tend to assume that such "somethings" just don't exist. Yes, that
is very counter-intuitive, but that is one reason quantum physics is so weird.

------
coldcode
In physics if you wait long enough, everything you know will have changed:
often it happens on many scales at once.

------
wwarner
Amazing. What is the energy of the system while it's in transition? I guess
they're arguing superposition of 1 and 0, continuously sliding to a higher
likelihood with time. I mean, eventually a photon is emitted, and that can't
be continuous. I guess I'll have to read the preprint.

------
bladedtoys
>..to be instantaneous...

By which frame of reference? I never understood the language around
"instantaneous". Isn't simultaneity relative? So that where one frame of
reference says two events are simultaneous, there or others that say they are
not?

~~~
effie
The old quantum theory was developed in non-relativistic setting, so this was
not a concern. But you are right, relativity complicates lots of things in
quantum theory, including the idea of "instantaneous" quantum jumps. In
relativity, if some event is to be universally instantaneous, then it has to
happen at a single point of space. Which is possible with point particles, but
then you get the problem how those point particles can find each other to
interact at a single point so often as measured cross sections indicate...
perhaps they are not exactly points, but waves, but then we can't have
instantaneous events, the event has to happen to the wave in big region of
space where simultaneity is relative.

------
kgwgk
“Another text saying that the founding fathers of quantum mechanics were not
only wrong but idiots, as some current geniuses revealed. In reality, it's the
other way around, of course. [...] I don't think it makes sense for me to
discuss the paper and Ball's summary more deeply. One would have to correct
every sentence that is wrong or at least misleading – which is basically every
sentence both in Ball's text as well as the text in Nature.“

[https://motls.blogspot.com/2019/06/experimenters-and-
especia...](https://motls.blogspot.com/2019/06/experimenters-and-
especially.html)

~~~
deepspace
Any statement by Motl regarding the work of other physicists should be taken
with a heap of salt.

[https://rationalwiki.org/wiki/Lubo%C5%A1_Motl](https://rationalwiki.org/wiki/Lubo%C5%A1_Motl)

------
drenvuk
My undereducated guess is that there is nothing random about it. We just don't
understand the underlying workings of the system enough yet.

~~~
Simon_says
Your view was the consensus view among physicists until Bell's Inequality was
verified to be violated. Now it's the minority view.

------
songeater
Does this imply anything about the Many Worlds Interpretation [1]? I am not a
physicist (IANAP?), but if states change continuously vs discretely... should
that not discount MWI?

[1]: [https://en.wikipedia.org/wiki/Many-
worlds_interpretation](https://en.wikipedia.org/wiki/Many-
worlds_interpretation)

~~~
gaze
It does not. The could be a continuum of universes... or something.

------
Ono-Sendai
well, obviously. This was always pretty clear to anyone who put some serious
thought into it. Did anyone really think electrons just magically jump from
state to state? There is a real physical process there that takes time.

~~~
coldtea
> _well, obviously. This was always pretty clear to anyone who put some
> serious thought into it. Did anyone really think electrons just magically
> jump from state to state?_

Only prominent QM scientists. Amateur pundits always knew that they don't
magically jump.

~~~
effie
There is a famous story about a guy trying to convince Niels Bohr in 60's that
quantum transitions in different molecules close to each other can coherently
influence each other and result in amplified emission of coherent radiation.
Bohr utters something in the sense that is impossible, quantum transitions are
random, photons are not correlated, so the molecules can never cooperate. The
guy didn't take the prominent QM scientist too seriously, went on and built
the device. He was the famous Charles Townes, one of discoverers of the
maser/laser effect. Of course, he knew the transitions aren't instantaneous
but take time and mutual interaction of molecules and mirrors can synchronize
them.

~~~
coldtea
So is the moral of the story that everybody (or close) that doubts some
consensus in physics is a misunderstood genius?

There's also 1000 times more stories of crackpots "knowing" everything, from
how to do cold fusion, to perpetual machines, to why Relativity or QM is
wrong, etc. They even have the diagrams and math to show you they're right.

And yes, some of them even "knew" things verified later - if you have random
unsupported opinions some of those will also be legit.

Unless the parent had some actual proof for their insight before the
verification, the phrase "This was always pretty clear to anyone who put some
serious thought into it" (as if physicists who didn't regard this didn't) is
as good as someone saying the same about a coin toss ("hey, it turned out to
be heads, anyone could see that").

~~~
effie
No, the story is interesting because it shows the authorities on the subject
do get things wrong, and because Townes and many other important scientists
knew that the simple idea of instantaneous quantum jumps due to Bohr and
Heisenberg and maybe Pauli (I think these three were one the most prominent
proponents) wasn't that well secured by the general quantum theory and by the
experiments.

