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TravisM
2005-Mar-20, 01:55 PM
I ran across this article and I was floored.

http://physicsweb.org/articles/news/9/3/1/1

Instead of a double slit in space, they have a double slit in time... And it still works!!! :o

Quantum is so 8)

The Supreme Canuck
2005-Mar-20, 03:50 PM
:o

Okay, that is strange.

Jpax2003
2005-Mar-20, 09:22 PM
New look for classic experiment
2 March 2005

Physicists in Europe and the US have performed a novel version of the double-slit quantum-interference experiment with single electrons. In the classic version of the experiment, electrons pass through a mask containing two parallel slits and produce a pattern of bright and dark interference fringes on a screen. Now, Gerhard Paulus of Texas A&M University and co-workers in Berlin, Munich, Sarajevo and Vienna have observed an interference pattern with electrons that pass through a double slit in time, not space, as a result of being ejected from an atom at one of two possible times by a laser pulse.

We know from watching Stargate-SG1 that passing through a slit in time leads to nothing but interference. In fact, it reminds me of something he said to the Asgard: "What is with you people?!? Time machines are nothing but problems. Even we know that."

The Supreme Canuck
2005-Mar-20, 09:23 PM
I'm guessing O'Neill said that. :)

Nick
2005-Mar-20, 09:30 PM
I'm guessing O'Neill said that. :)

It sure wasn't Sam :o

Nick

Jpax2003
2005-Mar-20, 09:31 PM
I'm guessing O'Neill said that. :)Yeah, you know Jack.

Chip
2005-Mar-21, 07:15 AM
Some related discussion here (http://www.badastronomy.com/phpBB/viewtopic.php?p=292895).

Double slits and times also discussed here (http://www.badastronomy.com/phpBB/viewtopic.php?p=22864), though differently than this thread. (Tim Thompson provided an interesting link off of this one.)

kg034
2005-Mar-21, 07:40 AM
I ran across this article and I was floored.

http://physicsweb.org/articles/news/9/3/1/1

Instead of a double slit in space, they have a double slit in time... And it still works!!! :o

Quantum is so 8)

I looked up the web-article above. As usual with "popular explanations", I found it lacking. Most likely my lack of knowledge has a large degree on forming such a perception, but can someone please describe to me the goals of this experiment....In particular, I was confused by:


The team registered the arrival times of the electrons at both detectors and then plotted the number of electrons as a function of energy. The researchers observed interference fringes at the first detector because it was impossible to know if an electron counted by the detector was produced during the first or second maximum.


Now, I know that delta_p * delta_x > h, as well as delta_E * delta_t >h.
But, from the above explanation I could not deduce how either the E or the t was attempted to be measured, and hence incited an increase in the uncertainty of the other. In particular, won't the electrons that are emitted at the maximum always have the same energy? I could understand if they had an energy proportional to the e-m wave amplitude, since then a measure of energy would give a measure of when they were emitted.....but somehow I don't think thats true. Help, experts? :-? :-? :-?

papageno
2005-Mar-21, 05:09 PM
The team registered the arrival times of the electrons at both detectors and then plotted the number of electrons as a function of energy. The researchers observed interference fringes at the first detector because it was impossible to know if an electron counted by the detector was produced during the first or second maximum.


Now, I know that delta_p * delta_x > h, as well as delta_E * delta_t >h.
But, from the above explanation I could not deduce how either the E or the t was attempted to be measured, and hence incited an increase in the uncertainty of the other. In particular, won't the electrons that are emitted at the maximum always have the same energy? I could understand if they had an energy proportional to the e-m wave amplitude, since then a measure of energy would give a measure of when they were emitted.....but somehow I don't think thats true. Help, experts? :-? :-? :-?

I had a quick look at the pre-print of the main author (following the link to his homepage).
What they see in energy is interference fringes of small amplitude, within the same energy range.

"Usual" double slit: with two slits, there are two possible directions (momenta) for the moving particle, which results in interference fringes in position.
New double-slit: two slits in time (of emission by ionization), result in interference fringes in energy.

This is how I figure it.
The electric field in the laser pulse, has two peaks that are more than enough to ionize atoms. The ionization occurs within one these two peaks, and an electron is emitted. By measuring the energy of the emitted electrons, it is not known during which of the two peaks the electron has been emitted.
The two peaks give a well defined delta t, which "fixes" delta E, giving coherence in the electron wave-packet in energy*, and therefore gives an interference pattern.


* The emitted electron can be described in terms of a wave-packet, composed of several "simple" waves of fixed energy. The resulting wave-packet has a spread delta E about the mean value of the energy (otherwise, the wave-packet is made of waves that have an energy within delta E of the mean value).

Normandy6644
2005-Mar-22, 05:50 AM
Wow. Quantum physics just keeps getting cooler and cooler.

TravisM
2005-Mar-22, 12:31 PM
Wow. Quantum physics just keeps getting cooler and cooler.

That is my point. Absolutely 8)

[edit to add]

I mighta oughta have put this in the BABBling section...
It seems that the realm of astronomy is increasingly becoming a quantum diciplin though, through the use of sophisticated sensory devices.
I hesitate to ponder the implications here, but cannot wait until someone comes up with a simple 'how' explination for such a complicated experimental result.
Still, 8) :o 8)

kg034
2005-Mar-29, 05:42 AM
So, since we are talking about QM, here's an interesting article in the current (18thMarch) issue of Science. From the summary page:


Although atoms are often depicted with discrete electrons orbiting the nucleus, electrons are more properly described as delocalized clouds. However, under the right excitation conditions, the classical model can pertain. When electrons are excited sufficiently that the level spacing is much smaller than the total energy, they can occupy several levels at once. This delocalization in energy leads to a corresponding localization in space, and temporarily the electrons resemble classical orbiting particles......[snip]...They further show that by adjusting the microwave frequency, they can fine-tune the period and radius of the electron orbit, along with the corresponding binding energy.

Paper: Maeda, Norum, Gallagher, Science, vol307, p1757-1760

ps. I hope posting this tidbit from Science does not violate the copyright rules. If so, please advise and remove the post. Thanks!

papageno
2005-Mar-29, 12:23 PM
Although atoms are often depicted with discrete electrons orbiting the nucleus, electrons are more properly described as delocalized clouds. However, under the right excitation conditions, the classical model can pertain. When electrons are excited sufficiently that the level spacing is much smaller than the total energy, they can occupy several levels at once.
But this would require that the electron is relatively distant from the nucleus. I am not surpised that the nucleus and the elctron would then interact more like classical particles.



This delocalization in energy leads to a corresponding localization in space,...
Usually, the uncertainty relations link energy with time, and momentum with position.
The link between energy and position is not that obvious.
However, I have a picture relative to a quantum oscillator which shows exactly what they are saying (since I cannot post a drawing of this, I do not know how to explain).


and temporarily the electrons resemble classical orbiting particles......[snip]...They further show that by adjusting the microwave frequency, they can fine-tune the period and radius of the electron orbit, along with the corresponding binding energy.

TravisM
2005-Mar-29, 01:43 PM
To further the discussion I submit this, interferrence of molecules!

http://www.physicsweb.org/articles/world/18/3/5

papageno
2005-Mar-29, 02:30 PM
To further the discussion I submit this, interferrence of molecules!

http://www.physicsweb.org/articles/world/18/3/5

What about macroscopic (as in 10^23 particles) systems exhibiting quantum behaviour, such as superconductors?

TravisM
2005-Mar-29, 03:17 PM
I wasn't aware that was a quantum effect, but it makes sense. Ahh, the world after quanta, totally :o
Strangeness never dreamt by man, embraced by few, understood by less. It's really amazing. 8)