verified—atoms won't move while you watch

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dragon
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verified—atoms won't move while you watch

Post by dragon »

hey look tiny weeping angels
'Zeno effect' verified—atoms won't move while you watch
October 23, 2015 by Bill Steele
'Zeno effect' verified—atoms won't move while you watch
Graduate students Airlia Shaffer, Yogesh Patil and Harry Cheung work in the Ultracold Lab of Mukund Vengalattore, assistant professor of physics.

One of the oddest predictions of quantum theory – that a system can't change while you're watching it – has been confirmed in an experiment by Cornell physicists. Their work opens the door to a fundamentally new method to control and manipulate the quantum states of atoms and could lead to new kinds of sensors.
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The experiments were performed in the Utracold Lab of Mukund Vengalattore, assistant professor of physics, who has established Cornell's first program to study the physics of materials cooled to temperatures as low as .000000001 degree above absolute zero. The work is described in the Oct. 2 issue of the journal Physical Review Letters

Graduate students Yogesh Patil and Srivatsan K. Chakram created and cooled a gas of about a billion Rubidium atoms inside a vacuum chamber and suspended the mass between laser beams. In that state the atoms arrange in an orderly lattice just as they would in a crystalline solid.,But at such low temperatures, the atoms can "tunnel" from place to place in the lattice. The famous Heisenberg uncertainty principle says that the position and velocity of a particle interact. Temperature is a measure of a particle's motion. Under extreme cold velocity is almost zero, so there is a lot of flexibility in position; when you observe them, atoms are as likely to be in one place in the lattice as another.

The researchers demonstrated that they were able to suppress quantum tunneling merely by observing the atoms. This so-called "Quantum Zeno effect", named for a Greek philosopher, derives from a proposal in 1977 by E. C. George Sudarshan and Baidyanath Misra at the University of Texas, Austin,, who pointed out that the weird nature of quantum measurements allows, in principle, for a quantum system to be "frozen" by repeated measurements.

Previous experiments have demonstrated the Zeno Effect with the "spins" of subatomic particles. "This is the first observation of the Quantum Zeno effect by real space measurement of atomic motion," Vengalattore said. "Also, due to the high degree of control we've been able to demonstrate in our experiments, we can gradually 'tune' the manner in which we observe these atoms. Using this tuning, we've also been able to demonstrate an effect called 'emergent classicality' in this quantum system." Quantum effects fade, and atoms begin to behave as expected under classical physics.

The researchers observed the atoms under a microscope by illuminating them with a separate imaging laser. A light microscope can't see individual atoms, but the imaging laser causes them to fluoresce, and the microscope captured the flashes of light. When the imaging laser was off, or turned on only dimly, the atoms tunneled freely. But as the imaging beam was made brighter and measurements made more frequently, the tunneling reduced dramatically.

"This gives us an unprecedented tool to control a quantum system, perhaps even atom by atom," said Patil, lead author of the paper. Atoms in this state are extremely sensitive to outside forces,l he noted, so this work could lead to the development of new kinds of sensors.

The experiments were made possible by the group's invention of a novel imaging technique that made it possible to observe ultracold atoms while leaving them in the same quantum state. "It took a lot of dedication from these students and it has been amazing to see these experiments be so successful," Vengalattore said. "We now have the unique ability to control quantum dynamics purely by observation."

The popular press has drawn a parallel of this work with the "weeping angels" depicted in the Dr. Who television series – alien creatures who look like statues and can't move as long as you're looking at them. There may be some sense to that. In the quantum world, the folk wisdom really is true: "A watched pot never boils."
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Re: verified—atoms won't move while you watch

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Wait...we know the particles energy, and we can stop them moving, so we know both, so the Uncertainty Principle doesn't apply, so classical physics reasserts itself. Thats awesome and scary and now my brain hurts.
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Re: verified—atoms won't move while you watch

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Wait, atoms? I though tunneling and the so called Zeno effect was sub atomic particles. And isn't the 'watching' thing a bit overplayed, shouldn't it really be introducing energy into the experiment by way of the laser? Quantum physics makes my head hurt.
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Re: verified—atoms won't move while you watch

Post by jwl »

This is at nanokelvin. At that kind of temperature, atoms undergoing tunnelling is on the cards. The reason this works, to my understanding, is that you keep collapsing the wavefunction of the atom before it has much of a chance to extend across the barrier, meaning there is a lower probability of the atom being found on the other side than if you had waited. The wavefunction, in this case, needs to change sufficiently slowly that the effect is larger than that of multiplying the probabilities because you are sampling multiple times. In the other case, you would have an anti-zeno effect instead.
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Re: verified—atoms won't move while you watch

Post by dragon »

Knife wrote:Wait, atoms? I though tunneling and the so called Zeno effect was sub atomic particles. And isn't the 'watching' thing a bit overplayed, shouldn't it really be introducing energy into the experiment by way of the laser? Quantum physics makes my head hurt.
Tell me about it even after taking my into to quantum mechanics class it made my head hurt and the next level didn't help matters. Good thing I only needed the two course.
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Re: verified—atoms won't move while you watch

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jwl wrote:This is at nanokelvin. At that kind of temperature, atoms undergoing tunnelling is on the cards. The reason this works, to my understanding, is that you keep collapsing the wavefunction of the atom before it has much of a chance to extend across the barrier, meaning there is a lower probability of the atom being found on the other side than if you had waited. The wavefunction, in this case, needs to change sufficiently slowly that the effect is larger than that of multiplying the probabilities because you are sampling multiple times. In the other case, you would have an anti-zeno effect instead.

Ok, I stand corrected.
They say, "the tree of liberty must be watered with the blood of tyrants and patriots." I suppose it never occurred to them that they are the tyrants, not the patriots. Those weapons are not being used to fight some kind of tyranny; they are bringing them to an event where people are getting together to talk. -Mike Wong

But as far as board culture in general, I do think that young male overaggression is a contributing factor to the general atmosphere of hostility. It's not SOS and the Mess throwing hand grenades all over the forum- Red
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Re: verified—atoms won't move while you watch

Post by SpottedKitty »

That settles another long-argued question — the basic design specs for this universe really were written by Douglas Adams. Image
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Re: verified—atoms won't move while you watch

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SpottedKitty wrote:That settles another long-argued question — the basic design specs for this universe really were written by Douglas Adams. Image
I was unaware there was even a debate in that area.

Hehe, I'm using humour to hide how fucking terrified of the universe I am right now...
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Re: verified—atoms won't move while you watch

Post by Eternal_Freedom »

Me too. I'd just about wrapped my head around the idea that at small scales the Universe is really fucking weird and then THIS comes along. I'm an astronomer damnit, I like my science to have clear answers and as little uncertainty as possible.
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Re: verified—atoms won't move while you watch

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Under extreme cold velocity is almost zero, so there is a lot of flexibility in position; when you observe them, atoms are as likely to be in one place in the lattice as another.
What is this bullshit? That is the exact opposite of the truth - it is heat that produces a situation where there is a lot of flexibility in position, not cold. Without energy, atoms are less capable of escaping their local minimums of potential energy, so they move less.
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Re: verified—atoms won't move while you watch

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I think that since the energy of the particles is so low, the uncertainty of that measurement is also very low, and if the Uncertainty Principle holds, the uncertainty in the atom's position should therefore be very high, hence the quantum tunneling to different points in the lattice.
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Corrax Entry 7:17: So you walk eternally through the shadow realms, standing against evil where all others falter. May your thirst for retribution never quench, may the blood on your sword never dry, and may we never need you again.
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Re: verified—atoms won't move while you watch

Post by Purple »

So a watched pot really does not boil? :)
It has become clear to me in the previous days that any attempts at reconciliation and explanation with the community here has failed. I have tried my best. I really have. I pored my heart out trying. But it was all for nothing.

You win. There, I have said it.

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Re: verified—atoms won't move while you watch

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Eternal_Freedom wrote:I think that since the energy of the particles is so low, the uncertainty of that measurement is also very low, and if the Uncertainty Principle holds, the uncertainty in the atom's position should therefore be very high, hence the quantum tunneling to different points in the lattice.
But the uncertainty isn't high. The only reason why there was uncertainty in the position of the atoms during this experiment was literally because they turned off the light that they needed to see the atoms. "It's not working because you're looking at it" is the reasoning of a conman, not a scientist.
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Re: verified—atoms won't move while you watch

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Yes, the position uncertainty is low while they're being observed...that's the whole damn point of the article. It is a result we would not expect if the Uncertainty Principle is as correct as we thought it was.
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Re: verified—atoms won't move while you watch

Post by Feil »

Jeeze. Calm down. Nobody's suggesting that the uncertainty principle isn't accurate. Atoms aren't Weeping Angels. The sky is not falling.

Science Journalism:
"SCIENTISTS REVOLUTIONIZE REVOLUTIONS BY REVOLUTIONARY REVELATION, PROMPTING REVOLT"

Real Science:
"Yep, this works pretty much like everyone has been predicting it would for decades."


The position uncertainty is low. Frequent, accurate measurements of position cause the position uncertainty to remain low. This suppresses tunneling, which is a manifestation of high position uncertainty.

The momentum uncertainty is high. However, the momentum is so low to begin with that even with a high momentum uncertainty, the particles can still be treated as stationary for the purpose of the experiment. Anyway, momentum uncertainty doesn't give rise to tunneling, and the suppression of tunneling is the point of the experiment.

The journalists, as usual, don't know what they're talking about (Grumman already pointed out the most egregious error they make). And, as usual, journalists trying to transform an experimental confirmation of one a theoretical prediction into a revolutionary refutation of some fundamental principle of physics. The only useful part of the news article is the link to the journal article on Physical Review Letters.
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Re: verified—atoms won't move while you watch

Post by the atom »

Well, I only heard it on SB, so take this with a grain of salt, but I heard that to observe a quantum system you have to interact with it in some way, which causes some kind of interference that causes this effect. So, at least as far as this goes, atoms are not weeping angels, or so my brain hopes at least.

EDIT: ninja'd. That's what I get for not reading one fucking thread page lol.
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Re: verified—atoms won't move while you watch

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Feil wrote:Jeeze. Calm down. Nobody's suggesting that the uncertainty principle isn't accurate. Atoms aren't Weeping Angels. The sky is not falling.

Science Journalism:
"SCIENTISTS REVOLUTIONIZE REVOLUTIONS BY REVOLUTIONARY REVELATION, PROMPTING REVOLT"

Real Science:
"Yep, this works pretty much like everyone has been predicting it would for decades."


The position uncertainty is low. Frequent, accurate measurements of position cause the position uncertainty to remain low. This suppresses tunneling, which is a manifestation of high position uncertainty.

The momentum uncertainty is high. However, the momentum is so low to begin with that even with a high momentum uncertainty, the particles can still be treated as stationary for the purpose of the experiment. Anyway, momentum uncertainty doesn't give rise to tunneling, and the suppression of tunneling is the point of the experiment.

The journalists, as usual, don't know what they're talking about (Grumman already pointed out the most egregious error they make). And, as usual, journalists trying to transform an experimental confirmation of one a theoretical prediction into a revolutionary refutation of some fundamental principle of physics. The only useful part of the news article is the link to the journal article on Physical Review Letters.
The journalists literally put the word "verified" in the title of the article. That's another way of saying: yes, this has been predicted for decades, but hasn't been experimentally verified. In the body of the text is says "This so-called "Quantum Zeno effect", named for a Greek philosopher, derives from a proposal in 1977 by E. C. George Sudarshan and Baidyanath Misra at the University of Texas, Austin,, who pointed out that the weird nature of quantum measurements allows, in principle, for a quantum system to be "frozen" by repeated measurements." So the article is fine.

What they say that Grunmman takes an issue with is also fine, but rather irrelevant to the situation: high temperatures indeed mean lower position uncertainty, in the sense that if the relative momentum uncertainty is kept the same, the higher momentum means you can get lower uncertainty in position. However, this is only tangentially relevant to why this experiment is done at low temperatures. The reason this is done is that the tunnelling only happens when you can't classically smash through the barrier, and the tunnelling probability is proportional to the square root of the difference between barrier energy and particle energy, sqrt(V-E). So you need a particle energy that is lower than the barrier energy but not by a lot, which is easier to do when you have low barrier and particle energies to start with, which is what happens at low temperatures.

And the reason the Quantum Zeno effect is reminiscent of weeping angels is that weeping angels were based on the Quantum Zeno effect in the first place. Being surprised that atoms can undergo the Quantum Zeno effect like weeping angels is like being surprised black holes generate gravitational time dilation like in Interstellar.
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Re: verified—atoms won't move while you watch

Post by Feil »

Fair enough, and maybe I was too harsh on the author. But the thing that they said was verified (atoms won't move while you watch) not only wasn't verified, but wasn't predicted in the first place (tunneling is a kind of movement, but most movement isn't tunneling), so I think I'm still justified in thinking badly of them at least a little.
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