Transcript Quantum Tunneling - Santa Rosa Junior College

Quantum Tunneling
Ashley Gnoss & Kyle Kucker
Physics 43/ May 10th, 2011
Santa Rosa Junior College Spring ‘11
Image: http://www.google.com/imgres?imgurl=http://www.colorado.edu/physics/phys3220/phys3220
Introduction to the concept
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Quantum tunneling is one of the
phenomena displaying the odd nature
of Quantum Mechanics.
It concerns particles in their wave
states, where each observable can be
described most acutely by a distribution
of probability.
This is the main point, that there is no
point.
Classically, it is assumed that particles
that we can observe to be point-like
should behave as if under the rules that
govern the more massive objects that
we have encountered, that is, they
should behave deterministically.
But due to the nature of Quantum
Mechanics, there is a magnitude of size
and complexity for which the rules
seem to change.
Image:http://www.lightandmatter.com/html_books/6mr/ch05/figs/big-hydrogen-wavefunction.jpg
Quantum Mechanics: Introduction to the Concept
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A few wave distributions of a
hydrogen atom, describing varying
positions (or potential positions) of
the electron.
Image: http://upload.wikimedia.org/wikipedia/
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Particles that we had assumed to be
definite in their properties, (position,
energy, time, momentum…) can only
be described as distributions of
probability.
These distributions have another
limitation. Due to our methods of
detection, we are restricted to never
knowing two properties of a particle
simultaneously. We can never
understand fully (with our current
knowledge) what the particle/
distribution is doing, nor predict what
it will do.
This is the reason why we do not yet
know what to call these objects. Waves
or particles.
Quantum mechanics emphasizes the
“wave-nature”, because it is through
wave mechanics that we can describe
these probability distributions and
how they could be an adequate
description of the particle, though an
unintuitive one.
Heisenberg’s Uncertainty Principle
• The inability to know certain multiple
properties of an object at the same
time is due to the uncertainty
principle.
• This principle is the concept behind
quantum tunneling.
Equations of the
• It is due to this lack of knowledge Uncertainty Principle
about the particle that we call the
“If the properties of the atom
action of tunneling an odd
and particle could be known
phenomena.
with certainty, then they
• Indeed, it makes describing the rules would decohere and their
wave behavior and their
and properties of nature a much more ability to interfere would be
complex activity than once thought. destroyed”- An interpretation
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of Quantum Mechanics
The Tunneling of a Particle
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Tunneling refers to the ability of a
particle to overcome and cross a potential
energy barrier that it would not be able
do based on classical understanding.
It is only the particle’s wave-nature that
allows for this phenomenon.
The probability wave describing the
particle’s position is an integral that
overlaps into the energy barrier, allowing
for some finite probability that the
particle might actually “tunnel” through.
Images:http://4.bp.blogspot.com/,
abyss.uoregon.edu/.../ quantum_tunneling.gif
Tunneling in Nature
• Our sun persists in keeping us
warm (and alive) through the
process of fusion. Fusion is the
creation of larger atomic
nuclei/elements from smaller
ones.
• Fusion occurs only at extremely
high temperatures, where the
Kinetic Energy is high enough to
overcome the potential barrier
due to the repelling of the
nucleons.
• The only thing is that our sun is
not hot enough to produce these
high energies.
• Tunneling is the explanation.
Only due to the overlap of the
probability waves of the nuclei
can they overcome the barrier
separating them and fuse.
Radioactive decay is another example
of how tunneling circumvents the
tendencies of nature.
Particles such as alpha particles are held
in the nucleus by the strong force,
which means that they shouldn’t be able
to escape. Tunneling again explains
how the particles are able to overcome
the strongest known force in nature.
In the case of the alpha particle for
instance, the location of the particle
within the nucleus is fairly accurately
known, and due to the uncertainty
principle, the probability wave for the
velocity (and therefore kinetic energy)
is greater. The property unable to be
defined and the wave description gives
it a finite probability to arrive outside of
the nucleus.
Technological Applications
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Once of the most important uses of electron
tunneling is the production of current.
A new and interesting use is within the new
touch screen phones.
By applying a force (pressure with the
finger), the layers particles of polymer
within the screen come closer together.
Before they are moved together there is a
“large enough” vacuum between the layers
and so that the probability of electrons
tunneling from one layer to another is very
low.
By exerting force on the layers, we are able
to bring the probability waves of the
electrons closer together, increasing the rate
of tunneling, enough to produce current ondemand, changing the material from an
insulator to a “metal”.
The change in tunneling is extreme. For
every Angstrom closer (1/10th of a
nanometer) the rate of tunneling increases
by ten times. And the resistance changes
from 10^12 ohms to 1 ohm!
Another strange fact…
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Fruit flies have been shown to be
able to distinguish between a
particular molecule (such as
acetophenone) and its deuterated
version through the process of
quantum tunneling via the receptors
within the fruit fly’s nose. Within
some molecules, deuterium-carbon
bond vibrations are similar to the
vibrations found in the bonds
between carbon and nitrogen, which
will result in a similar sent. A
majority of fruit flies tend to avoid
nitrogen molecules (nitriles), and so
too with the deuterated versions of
molecules that tend to attract them.
Further studies in the nose’s abilities
to employ quantum tunneling could
lead to artificial noses and more
sensitive instruments in the field.
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Sources
(All researched May 5, 2011)
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New Ideas in Quantum
Tunneling.http://www.technologyreview.com/blog/a
rxiv/23409/
Explanation of simple
concept.http://abyss.uoregon.edu/~js/glossary/quant
um_tunneling.html
Lecture
onQuantumTunneling.http://www.youtube.com/wat
ch?v=vMndTqowzqU
http://www-istp.gsfc.nasa.gov/stargaze/Q8.htm
Molecular vibration-sensing component in
Drosophila melanogaster
olfaction<http://www.popsci.com/science/article/20
11-02/your-nose-could-leverage-quantum-toolsdistinguish-between-similar-molecule>).