Quantum transitions take time – the news is its experimental demonstration

https://blog.rongarret.info/2019/06/quantum-transitions-take-time-this-is.html

by lisper • 7 years ago

107 44 7 years ago

blog.rongarret.info

rubidium• 7 years ago

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• 7 years ago

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.

tlb• 7 years ago

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.

lisper7• 7 years ago

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...

effie3• 7 years ago

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.

will_brown• 7 years ago

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/

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?

zitterbewegung• 7 years ago

lisper7• 7 years ago

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.

RootKitBeerCat• 7 years ago

I love Hacker News

jpmattia• 7 years ago

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

kgwgk10• 7 years ago

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

lisper7• 7 years ago

That's picking a pretty fine nit. Yes, obviously if an atom undergoes an energy transition then that energy has to come from (or go to) somewhere. That doesn't change the fundamental point, which is that the process doesn't happen instantaneously or discontinuously, and the continuity is provided by transition through a series of superposition states. Yes, to be strictly correct you'd have to say that the superposition is a joint state of the atom and something else and not the atom in isolation. But this post was targeted to a lay audience, and that sort of detail would be more of a distraction than a clarification IMHO.

ConcernedCoder• 7 years ago

Not if we're really living in a simulation: e.g. if you initiate the state change of an atom ( the example was the addition of one quanta of energy ), then eventually the state will change from quanta 0 to quanta 1, with possibly many superpositions of both states during the described finite lifetime of the transition.

This, to me, sounds so much like eventual consistency ( for observers ), that it's a bit scary.

ttctciyf• 7 years ago

The whole business of superposition and measurement so reeks of lazy initialization that you might want to worry about what performance constraints the simulation is trying to beat too.

duality2• 7 years ago

"An atom can be described by its quantum state only if it's isolated and in that case its energy is constant."

How do you figure? As a contradiction, take your atom+electromagnetic field system, describe the transition from excited atom to unexcited atom + photon state, and project out the E&M field. Voila, now you have a quantum description of an atom transitioning between different energy states. Its dynamics may look funny, i.e. they may appear nonlocal, they may not conserve energy, etc. but that's different from saying "there is not a quantum description of these dynamics" which is what you're claiming.

kgwgk10• 7 years ago

https://en.m.wikipedia.org/wiki/Quantum_state

project out the E&M field. Voila, now you have a quantum description

There is no wavefunction describing the state of the subsystem because the system is not separable.

Its dynamics may look funny, i.e. they may appear nonlocal, they may not conserve energy, etc. but that's different from saying "there is not a quantum description of these dynamics" which is what you're claiming.

What is the “quantum description of these dynamics”?

Quantum mechanics is usually based on something like the following postulate: “The state of a physical system is described by a well-behaved function of the coordinates and time, Ψ(q, t). The function contains all the information that can be known about the system.”

wwarner• 7 years ago

What puzzles me is where the photon goes when it bumps up the energy of the electron, and what does it look like when the electron shell drops back down. I realize too much "realism" in QM is just a waste of effort, but I keep seeing the field sort of pinching off a photon.

qtplatypus2• 7 years ago

The photon goes into the energy of the electron. Because mass and energy are the same thing the mass of the photon is converted into the energy of the electrons new state.

effie3• 7 years ago

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.

kgwgk10• 7 years ago

You’re right, I was thinking of a time-independent Hamiltonian. I may be mistaken but I thought that there are no stationary states otherwise (so it wouldn’t make sense either to say that the state is a superposition of the states 0 and 1).

lisper7• 7 years ago

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"?

gaze5• 7 years ago

This is incorrect. See the stochastic schroedinger equation. A quantum system will stay pure under measurement as long as the measurement process is completely efficient.

kgwgk10• 7 years ago

I’m not familiar with this extension of QM. What is the “system” here? Would the atom stay in a pure state or would a larger system stay in a pure state?

Anon84• 7 years ago

Discussion on the Quant magazine article from yesterday: https://news.ycombinator.com/item?id=20105091

JumpCrisscross• 7 years ago

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...

cubano• 7 years ago

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.

mcnamaratw• 7 years ago

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

https://www.aps.org/programs/outreach/history/historicsites/...

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.

astazangasta• 7 years ago

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.

qtplatypus2• 7 years ago

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

Quantum transitions take time – the news is its experimental demonstration

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