Researchers suggest one can affect an atom's spin by adjusting the way it is measured

Mar 18, 2013
Causing collapse Enlarge
All spin directions (represented by the spheres) collapse on one or the opposite direction depending on the measured photon polarization

One of the most basic laws of quantum mechanics is that a system can be in more than one state – it can exist in multiple realities – at once. This phenomenon, known as the superposition principle, exists only so long as the system is not observed or measured in any way. As soon as such a system is measured, its superposition collapses into a single state. Thus, we, who are constantly observing and measuring, experience the world around us as existing in a single reality.

The principle of superposition was first demonstrated in 1922 by Otto Stern and Walther Gerlach, who observed the phenomenon in the spin of silver atoms. Spin is the intrinsic magnet in , and when a particle's spin is in superposition, it points in more than one direction at the same time. (Instead of the north and south of magnets, these are referred to as up and down.) Dr. Roee Ozeri and research students Yinnon Glickman, Shlomi Kotler and Nitzan Akerman, of the Physics of Complex Systems Department studied how the spin of a single atom collapsed from superposition to one state when it was observed with light. They "measured" the atom by shining on it. Just as our eyes observe the world by absorbing the photons – – scattered in our direction by objects, the researchers observed the process of spin collapse in the atoms by measuring the scattered photons. In results that appeared recently in Science, they showed that the direction that a photon takes as it leaves the atom is the direction that the spin adopts when superposition collapses.

Next, the team measured the polarization of the emitted photon and found that the observed polarization determines the effect of measurement on the spin. This suggests that an observer can influence the collapse of superposition just by adjusting the orientation of his photon- measurement apparatus.

The reason for this "action-at-a-distance" is that the spins of the measured and the emitted photons were entangled. That is, even after they were separated, a measurement of one of them instantaneously affected the other.

The experiment is an important step in understanding the measurement process in quantum systems.

Explore further: Researchers discover a way to avoid decoherence in a quantum system

More information: www.sciencemag.org/content/339/6124/1187.full

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almac
3.9 / 5 (10) Mar 18, 2013
PLEASE stop repeating a common error: "a system can be in more than one state". A superposition of two states is another SINGLE state. A single (nonentangled) atom doesn't "point in more than one direction at the same time"; it points in a SINGLE direction.
Tektrix
1.5 / 5 (2) Mar 18, 2013
Yes, thank you for pointing that out, Almac. "States" are unique configurations of a system and "superposed" is one of the possible system configurations.
vacuum-mechanics
1.4 / 5 (10) Mar 18, 2013
…. Spin is the intrinsic magnet in quantum particles, and when a particle's spin is in superposition, it points in more than one direction at the same time. (Instead of the north and south of magnets, these are referred to as up and down.) Dr. Roee Ozeri and research students Yinnon Glickman, Shlomi Kotler and Nitzan Akerman, of the Physics of Complex Systems Department studied how the spin of a single atom collapsed from superposition to one state when it was observed with light…. In results that appeared recently in Science, they showed that the direction that a photon takes as it leaves the atom is the direction that the spin adopts when superposition collapses.

This seems not explaining how the collapse occurs; it is just show what happen according to quantum mechanics concept! Maybe this could help to understand how the mechanism of quantum mechanics works.
http://www.vacuum...19〈=en
Static
not rated yet Mar 19, 2013
So is this anything different from the paper uploaded to Arxiv in June of last year?

http://arxiv.org/abs/1206.3847

Same title exact idea, pretty sure this is the same thing going on as back then. No doubt interesting material, maybe even useful for spintronics and Quantum computing, but...???
rubberman
2.3 / 5 (3) Mar 19, 2013
"Thus, we, who are constantly observing and measuring, experience the world around us as existing in a single reality."

"This seems not explaining how the collapse occurs;"-VM

The collapse occurs because photonic interaction with an atom causes the atom to exhibit a property that it wouldn't if left in it's normal quantum state. But the atom cannot be measured without an EM process. It's a catch 22.
Noumenon
4.2 / 5 (10) Mar 19, 2013
PLEASE stop repeating a common error: "a system can be in more than one state". A superposition of two states is another SINGLE state. A single (nonentangled) atom doesn't "point in more than one direction at the same time"; it points in a SINGLE direction.


Your objection is correct, especially when they made reference to "multiple realities".

However, a superposition of stationary states may not be observable,.. so its not even valid to say that it "points in a single direction", unless it is observed to.

The 'act of observation' places constraints upon the underlying reality, by conforming it within concepts. The collapse, is a collapse into conceptual forms suplied by the observer and his apparatus,... i.e. the idea of a localized "particle", or a "direction".
natello
1 / 5 (2) Mar 19, 2013
The quantum observation is something like the attempt for detection of path of water droplets falling from dripping faucet with bare finger (just suppose that our eyes are closed, so we can detect the position of droplet in mechanical way only). It's evident, every such an "observation" will affect the path of falling droplets accordingly, so we cannot evaluate the exact path of droplets with single observation - this is basically, what the uncertainty principle says at the qualitative level (every (attempt for) observation of particle location affects its momentum and vice-versa).

But if the droplets are falling regularly, then we can repeat the detection of their position in different moments from the start of their fall in mechanical analogy of stroboscope: each droplet will be still indeed affected with our finger during this, but from sequence of consecutive measurements we can still construct the exact path of droplets, as if they weren't affected with finger at all.
natello
1 / 5 (2) Mar 19, 2013
This is basically what the principle of so-called weak measurements and quantum tomography is all about. Weak measurement is nothing less, nothing more than the application of classical physics across the time scale. Repetitive measurements maintained across time dimensions make the quantum system classical and temporal in similar way, like the increase of number of qbits during one measurement makes it more spatial and delocalized. With repetition of measurements in time or space frames (stroboscopic or tomographic measurement) the measured system becomes gradually entangled not only with observer, but with it expanding memory and it becomes classical in this way. BTW this concept has been proposed with Hugh Everett before sixty years already (in 1957)!
natello
1 / 5 (2) Mar 19, 2013
BTW Instead of "direct measurement" I'd rather talk about "stroboscopic measurement", because 1) this denomination gives intuitive insight, how the "direct measurement" actually proceeds 2) it explains, why the "direct measurement" represents the dual time-scale counterpart to the space-scale "tomographic measurement" 3) it's better related semantically to its dual counterpart (tomography is a instrumentation method in similar way, like the stroboscopic technique).
Noumenon
3.7 / 5 (6) Mar 23, 2013
....this is basically, what the uncertainty principle says at the qualitative level (every (attempt for) observation of particle location affects its momentum and vice-versa).


That was merely a 'classical' analogy made by Heisenberg to explain to 'classical' minded physicists of the time.
kochevnik
2.3 / 5 (3) Mar 23, 2013
I did this experiment with three polarized filters at the Berkely science museum to demonstrate QM to my date. I didn't know it was so profound as to be worthy of publication

@almac "A superposition of two states is another SINGLE state." Yes, but there are SUPERPOSITIONS meaning the atom has multiple states. Otherwise quantum computers wouldn't be faster than classical. People voting you up don't understand QM
RealScience
not rated yet Mar 24, 2013
@Almac - No, the spin does not point in a single direction. That would be true in a classical combination of two directions (states), but a superposition of states is different.
A better example is the double-slit experiment - a photon DOESN'T go through a non-existent slit between the two slits, it goes through BOTH slits unless someone measures which slit it goes through.
(Or in the simplest interpretation, the observer's consciousness is in a superposition of both left-slit and right slight universes unless the observer determines which slit it went through).
Bryan_Sanctuary
1 / 5 (1) Apr 08, 2013
Enjoyed the comments. Wave function collapse is something many do not like about QM. For me QM is not complete and therefore superposition is a property of qm but not of Nature.

True a quantum state can be in a superposition of states, but that is because the wave function is a statistical ensemble of deeper states. All those deeper states are pure and not superposed.

However we do not measure one particle, but an ensemble of both.

I talk quite a lot on my blog:
quantummechanics.mchmultimedia.com

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