
Quantum transitions take time – the news is its experimental demonstration - lisper
https://blog.rongarret.info/2019/06/quantum-transitions-take-time-this-is.html
======
jpmattia
The link brings us to an article focused on how another article in Quanta
Magazine did not summarize an article in Nature correctly.

Really, the right thing to do is go directly to the Nature article, and read
the abstract. If you've been reading things about QM, even as a non-physicist,
then you're probably pretty close to understanding the abstract.

Moreover, a quick reading of the Quanta Magazine article makes it seem as
though they shifted the focus of the original result from predictability and
determinism to instantaneous transitions. (Although I note I haven't had time
to read and fully digest the Nature paper.)

From the abstract in Nature:

> The times at which the discontinuous jump transitions occur are reputed to
> be fundamentally unpredictable. Despite the non-deterministic character of
> quantum physics, is it possible to know if a quantum jump is about to occur?
> Here we answer this question affirmatively: we experimentally demonstrate
> that the jump from the ground state to an excited state of a superconducting
> artificial three-level atom can be tracked as it follows a predictable
> ‘flight’, by monitoring the population of an auxiliary energy level coupled
> to the ground state. The experimental results demonstrate that the evolution
> of each completed jump is continuous, coherent and deterministic.

[https://www.nature.com/articles/s41586-019-1287-z](https://www.nature.com/articles/s41586-019-1287-z)

------
mcnamaratw
Sure, a quantum transition takes time. But demonstrating that point
experimentally is not new this year.

In The Feynman Lectures on Physics, vol II, p. 35-4, Feynman describes an
experiment by I.I. Rabi. I think Rabi did the work in the 1940s. The
reciprocal of the transition time is the 'Rabi Frequency'.
[http://mriquestions.com/who-discovered-nmr.html](http://mriquestions.com/who-
discovered-nmr.html)

[https://www.aps.org/programs/outreach/history/historicsites/...](https://www.aps.org/programs/outreach/history/historicsites/rabi.cfm)

Back to 2019, the dissertation abstract underlying the news story here doesn't
appear to claim that this is the first experimental demonstration that quantum
transitions take time. Which is probably good.

------
rubidium
Or more accurately: "The time it takes to make a quantum leap has been
experimentally measured for the first time".

There is experimental news. Very cool and newsworthy. Just not new theoretical
news.

~~~
NotSammyHagar
please tell us the amount of time it takes! It must be an incredibly small
time, but no one in the discussion or the brief abstract did I see it.

------
kgwgk
> To go smoothly from 0 to 1, the system transitions through a series of
> superpositions of both states, i.e. it starts out entirely in state 0, and
> then transitions smoothly to being mostly in state 0 and a little bit in
> state 1, to being half in each state, to being mostly in 1 and a little bit
> in 0, to being entirely in 1.

Not really. An atom can be described by its quantum state only if it's
isolated and in that case its energy is constant. If the states s_1 and s_2
have different energies E_1 and E_2 then the quantum state may be a
superposition of the states s_1 and s_2 with an expected value for the energy
between E_1 and E_2 but no "smooth transition" is possible.

For the transition to happen the atom has to be coupled with an external
field. One way to think of it is as the evolution of the system composed of
the atom plus a photon, where the transition between the states "excited atom
and no emitted photon" and "unexcited atom and emited photon" happens at
constant energy.

Edit: See
[https://en.m.wikipedia.org/wiki/Spontaneous_emission#Theory](https://en.m.wikipedia.org/wiki/Spontaneous_emission#Theory)

~~~
effie
You are right for spontaneous emission, there we need to connect the system to
something else, or to formulate a detailed-enough relativistic model with
retarded EM fields that is able to lose energy (ordinary Hamiltonian of
charged particle variables only cannot describe loss of energy to radiation).

But for spontaneous absorption/emission, you are not right. Describing atoms
by psi function is extremely successful, it is what Schroedinger did in his
papers, what got him the right results for positions and intensities of
spectral lines and what put his equation to the center of interest. It is true
this successful description has some problems, such as the question of
consistency of different world cuts (into system and environment), but the
ordinary way to do the split for atom-radiation interaction (time-independent
atom Hamiltonian and time-dependent external EM field) describes stimulated
emission and absorption very well.

Stimulated absorption/emission does require presence of external field, but
this field can be taken into account in the usual psi function formulation -
this is called semi-classical theory of radiation, it is what Schroedinger
proposed in 1920's for simple atoms and is still heavily used for description
of interaction of radiation with atoms and molecules.

~~~
lisper
> You are right for spontaneous emission

> But for spontaneous absorption/emission, you are not right

Was one of those instances of "spontaneous" supposed to be "stimulated"?

~~~
effie
Yes, the second one should have been "stimulated". Thanks for pointing that
out.

------
tlb
For a transition of an electron between energy levels that emits a photon,
should the time it takes for the transition be the same as the duration of the
probability envelope of the photon?

If so, that's easily observable by looking at the width of the spectral line.
If the transition is fast, that implies a short duration and therefore the
spectral lines should be smeared out.

~~~
lisper
Yes, that is exactly right. All of this just boils down to the energy-time
version of the uncertainty principle: the more precisely you can measure
_when_ a transition happened, the less certain you can be about the change in
energy of that transition.

Here is another example of this principle at work:

[http://blog.rongarret.info/2018/05/a-quantum-mechanics-
puzzl...](http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle.html)

[http://blog.rongarret.info/2018/05/a-quantum-mechanics-
puzzl...](http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle-part-
deux.html)

[http://blog.rongarret.info/2018/05/a-quantum-mechanics-
puzzl...](http://blog.rongarret.info/2018/05/a-quantum-mechanics-puzzle-part-
drei.html)

~~~
effie
What you're saying about uncertainty principle is strange. The principle says
the longer the transition takes, the sharper the spectral line can be (but
does not have to be, hence the inequality). There is nothing about uncertainty
of measurement of time of some point event. The transition is not a point
event. If the two states involved in the transition are known, then change of
energy is known. How long the transition will take depends on strength and
frequency characteristics of the external field, but the energy difference of
energy of the molecule after the transition is over does not depend on that.

~~~
lisper
Yes, everything you say is correct. That's exactly what makes this experiment
so interesting. They didn't just measure the energy state at one time and then
again at another time. That wouldn't work (because of the quantum Zeno
effect).

------
JumpCrisscross
The paper’s abstract [1] states:

“certain classical phenomena, like tsunamis, while unpredictable in the long
term, may possess a degree of predictability in the short term, and in some
cases it may be possible to prevent a disaster by detecting an advance warning
signal.”

Is there a name for such class of phenomena?

For the opposite, where small-scale unpredictability ( _e.g._ particle motion)
because large-scale decipherable (fluid mechanics), we have the law of large
numbers and “emergent phenomena.”

[1] [http://qulab.eng.yale.edu/documents/theses/Minev_ZK-
Disserta...](http://qulab.eng.yale.edu/documents/theses/Minev_ZK-
Dissertation_Jumps.pdf)

------
cubano
> To get around this problem, Devoret and colleagues employ a clever trick
> involving a second excited state. The system can reach this second state
> from the ground state by absorbing a photon of a different energy. The
> researchers probe the system in a way that only ever tells them whether the
> system is in this second “bright” state, so named because it’s the one that
> can be seen. The state to and from which the researchers are actually
> looking for quantum jumps is, meanwhile, the “dark” state — because it
> remains hidden from direct view.

This is absolutely brilliant work. I had no idea that experimenters had found
a way around the quantum measurement problem.

I have always wondered "how in hell can people experiment with something that
is actually affected by simply LOOKING at it?" This gives me my answer.

Even though this idea is only one step in the findings of this article, I am
so glad I came across it.

------
astazangasta
I'd like to point out that this sort of story vitiates the Popperian tale of
how scientific knowledge is created, i.e. it was "known" well in advance of
experimental evidence that something is true, and the confirmation is seen as
banal.

~~~
qtplatypus
Which is ironic since popper had a real problem with QM.

------
Anon84
Discussion on the Quant magazine article from yesterday:
[https://news.ycombinator.com/item?id=20105091](https://news.ycombinator.com/item?id=20105091)

------
RootKitBeerCat
I love Hacker News

------
will_brown
I have a elementary question about the Heisenberg Uncertainty Principle (my
understanding is Quatum Leap/transitions are just a description of the
uncertainty principle wave function)...

As we know from “flatland” as a sphere passes through the flatland plane, the
sphere can be measured as a dot/circle at any given time.

[https://demonstrations.wolfram.com/ASphereVisitsFlatland/](https://demonstrations.wolfram.com/ASphereVisitsFlatland/)

Wouldn’t that measurement of a higher dimensional object passing through the
lower dimensional world sort of begin to appear and fit the definition of the
uncertainty principle for particles in our own 3D world?

In other words as the 3D sphere is measured passing through the 2d flatland
flatlanders can either know the position of the sphere or momentum, but as
they measure one more accurately they can’t measure the other as accurately?

I guess what I’m trying to ask is it accurate to use the uncertainty principle
as a flatlander describing a 3D sphere passing through flatland and if so,
Would that give any credence to the bizarre possibility that particles in our
world are actually 4D objects?

~~~
lisper
> Quatum Leap/transitions are just a description of the uncertainty principle
> wave function

That is almost right. But it's not a _description_ of the uncertainty
principle, it's a _manifestation_ of the uncertainty principle, specifically
the time-energy uncertainty principle.

But none of this has anything to do with flatland. It has to do with Fourier
analysis: the more localized a signal is in the time domain the more spread
out it is in the frequency domain, and vice versa.

