Quantum Entanglement
http://www.energetic-medicine.net/bioenergetic-articles/articles/106/1/Quantum-Entanglement/Page1.html
Dr Mae-Wan Ho
Biophysicist, Geneticist, NGO Director from China
Dr. Mae-Wan Ho is a world renowned geneticist & biophysicist. She is Director of the Institute of Science in Society, she is co-founder of the International Science Panel on Genetic Modification and is scientific advisor to the Third World Network. She has written more than 300 publications and over a dozen books including ‘Genetic Engineering – Dream or Nightmare?’ and ‘The Case for a GM-free Sustainable World.” By Dr Mae-Wan Ho
Published on 10/21/2008
An simple explanation of Quantum Entanglement
the theory of quantum entanglement
The Theory
When a photon (usually polarized laser light) passes through matter, it
will be absorbed by an electron. Eventually, and spontaneously, the electron
will return to its ground state by emitting the photon. Certain crystal
structures increase the likelihood that the photon will split into two photons,
both of them with longer wavelengths than the original. Keep in mind that a
longer wavelength means a lower frequency, and thus less energy. The total
energy of the two photons must equal the energy of the photon originally fired
from the laser (conservation of energy).
When the original photon splits into two photons, the resulting photon pair is considered entangled.
The process of using certain crystals to split incoming photons into pairs of photons is called parametric down-conversion.
Normally the photons exit the crystal such that one is aligned in a horizontally polarized light cone, the other aligned vertically. By adjusting the experiment, the horizontal and vertical light cones can be made to overlap. Even though the polarization of the individual photons is unknown, the nature of quantum mechanics predicts they differ.
To illustrate, if an entangled photon meets a vertical polarizing filter, the photon may or may not pass through. If it does, then its entangled partner will not because the instant that the first photon's polarization is known, the second photon's polarization will be the exact opposite.
It is this instant communication between the entangled photons to indicate each other's polarization that lies at the heart of quantum entanglement. This is the "spooky action at a distance" that Einstein believed was theoretically implausible.
The Practice
Experiments have shown that Einstein may have been wrong: entangled photons seem to communicate instantaneously. illustration below shows how to create entangled photons.
An ultraviolet laser sends a single photon through Beta Barium Borate.
As the photon travels through the crystal, there is a chance it will split.
If it splits, the photon will exit from the Beta Barium Borate as two photons.
The resulting photon pair are entangled.
An ultraviolet laser is used because the laser light has a high frequency. A high frequency implies a greater chance of splitting into two entangled photons.
The Result
The image below is an enhanced photograph of a photon that has split into an entangled pair.
When a photon (usually polarized laser light) passes through matter, it
will be absorbed by an electron. Eventually, and spontaneously, the electron
will return to its ground state by emitting the photon. Certain crystal
structures increase the likelihood that the photon will split into two photons,
both of them with longer wavelengths than the original. Keep in mind that a
longer wavelength means a lower frequency, and thus less energy. The total
energy of the two photons must equal the energy of the photon originally fired
from the laser (conservation of energy).
When the original photon splits into two photons, the resulting photon pair is considered entangled.
The process of using certain crystals to split incoming photons into pairs of photons is called parametric down-conversion.
Normally the photons exit the crystal such that one is aligned in a horizontally polarized light cone, the other aligned vertically. By adjusting the experiment, the horizontal and vertical light cones can be made to overlap. Even though the polarization of the individual photons is unknown, the nature of quantum mechanics predicts they differ.
To illustrate, if an entangled photon meets a vertical polarizing filter, the photon may or may not pass through. If it does, then its entangled partner will not because the instant that the first photon's polarization is known, the second photon's polarization will be the exact opposite.
It is this instant communication between the entangled photons to indicate each other's polarization that lies at the heart of quantum entanglement. This is the "spooky action at a distance" that Einstein believed was theoretically implausible.
The Practice
Experiments have shown that Einstein may have been wrong: entangled photons seem to communicate instantaneously. illustration below shows how to create entangled photons.
An ultraviolet laser sends a single photon through Beta Barium Borate.
As the photon travels through the crystal, there is a chance it will split.
If it splits, the photon will exit from the Beta Barium Borate as two photons.
The resulting photon pair are entangled.
An ultraviolet laser is used because the laser light has a high frequency. A high frequency implies a greater chance of splitting into two entangled photons.
The Result
The image below is an enhanced photograph of a photon that has split into an entangled pair.
Spooky Behaviour
Spooky Behaviour
This section describes some of the strange behaviours seen in experiments.
Double-slit Experiments
The image below shows what happens when a source of light shines through two tiny slits onto a screen. The detector screen illuminates a wave-like pattern caused by light interfering with itself. This is how waves are expected to behave.
Since photons are also particles, we can transmit them one at a time. The image below shows the result of many solitary photons being fired at the detector.
The interference pattern still appears; but if photons are fired alone, then with what do they interfere? Quantum theory tells us that each photon interferes with itself. If true, then it implies that we cannot know through which slit the photon travels; the photon seems to have travelled both slits simultaneously!
Trying to detect which slit the photons travel puts this quantum weirdness in the spotlight, so to speak.
For example, we can polarize the light before it goes through the slits. Like rippling a rope through a picket fence to permit only vertical waves, polarizing allows us to limit the type of light waves that make it through the slits to the detector.
When we put a polarizing filter around one of the two slits, the interference pattern disappears. The result is shown below.
Whenever we can detect, or deduce, through which slit a photon has travelled, the interference pattern instantly disappears. An interference pattern only appears when the photon's path is unknown.
It gets weirder.
Even if we examine the photon's trail after it passes the double-slits (but before it reaches the detector), the interference pattern disappears. And it disappears regardless of whether the examination uses a direct or indirect measurement of the photon.
But what if we used two photons that are inextricably linked (through entanglement), to perform the experiment?
Entanglement Experiments
I was born not knowing and have had only a little time to change that here and there.
Richard Feynman, Letter to Armando Garcia.
Parametric Down-conversion
We have already seen how to create entangled photons through a process called spontaneous parametric down-conversion:
To review, the laser in this image, fires a high-energy beam into a special type of crystal. Every once in a while one of the photons from the beam will split into two less energetic photons. These two entangled photons will have opposite polarizations and travel in two different directions, resulting in two streams of polarized light.
The previous double-slit experiments detected interference patterns by shining a single light source through two slits . The next experiment uses two streams of entangled photons.
Quantum Eraser
the image below shows a Bell-state quantum eraser, named after John Bell. It illustrates the application of the following steps:
a laser fires photons into a Beta Barium Borate (BBO) crystal;
the crystal entangles some of the photons; and then
entangled photons travel to two different detectors: A and B.
Placed between the crystal and detector B is a double-slit, like in the previous experiments. Immediately in front of detector A is a polarizing filter that can be rotated. The images before showed an experiment using sunglasses to see the effects of rotating a polarizer. Those same effects apply here.
The Bell-state quantum eraser has one more feature: each slit is covered by a substance that filters the polarization of a photon. Consequently, the left-hand slit will receive photons with a counter-clockwise polarization, and the right-hand slit will pass photons with a clockwise polarization.
Note: Polarization does not affect interference patterns.
Initially, neither detector shows an interference pattern. Since we control the polarization of photons passing through the slits and we know the polarization accepted by each slit, we can deduce which way the photons travelled (counter-clockwise through the left; clockwise through the right). Thus no interference patterns are detected.
However, if we rotate the polarizing filter in front of detector A so that the polarizations of the photons that hit the detector are the same (that is, we can no longer distinguish between clockwise and counter-clockwise polarizations), then the interference pattern appears at both detectors!
How do the photons arriving at detector B know that the polarizations have been "erased" at detector A?
Conclusion
Quantum Theory is continually being challenged and tested; physicists are finding new ways to explain the world of the tiny. Each passing year brilliant minds add to, or subtract from, the pool of knowledge about quantum behaviour.
Unlike the static nature of the web pages presented here, quantum physics is ever changing. Physicists are confronted with problems that will take many iterations, many years, to solve. Scores of theories will be presented, some of them merely tweaking, while others radically alter, our perceptions of quantum nature.
Whatever we observe in the future, it promises to be exciting!
This section describes some of the strange behaviours seen in experiments.
Double-slit Experiments
The image below shows what happens when a source of light shines through two tiny slits onto a screen. The detector screen illuminates a wave-like pattern caused by light interfering with itself. This is how waves are expected to behave.
Since photons are also particles, we can transmit them one at a time. The image below shows the result of many solitary photons being fired at the detector.
The interference pattern still appears; but if photons are fired alone, then with what do they interfere? Quantum theory tells us that each photon interferes with itself. If true, then it implies that we cannot know through which slit the photon travels; the photon seems to have travelled both slits simultaneously!
Trying to detect which slit the photons travel puts this quantum weirdness in the spotlight, so to speak.
For example, we can polarize the light before it goes through the slits. Like rippling a rope through a picket fence to permit only vertical waves, polarizing allows us to limit the type of light waves that make it through the slits to the detector.
When we put a polarizing filter around one of the two slits, the interference pattern disappears. The result is shown below.
Whenever we can detect, or deduce, through which slit a photon has travelled, the interference pattern instantly disappears. An interference pattern only appears when the photon's path is unknown.
It gets weirder.
Even if we examine the photon's trail after it passes the double-slits (but before it reaches the detector), the interference pattern disappears. And it disappears regardless of whether the examination uses a direct or indirect measurement of the photon.
But what if we used two photons that are inextricably linked (through entanglement), to perform the experiment?
Entanglement Experiments
I was born not knowing and have had only a little time to change that here and there.
Richard Feynman, Letter to Armando Garcia.
Parametric Down-conversion
We have already seen how to create entangled photons through a process called spontaneous parametric down-conversion:
To review, the laser in this image, fires a high-energy beam into a special type of crystal. Every once in a while one of the photons from the beam will split into two less energetic photons. These two entangled photons will have opposite polarizations and travel in two different directions, resulting in two streams of polarized light.
The previous double-slit experiments detected interference patterns by shining a single light source through two slits . The next experiment uses two streams of entangled photons.
Quantum Eraser
the image below shows a Bell-state quantum eraser, named after John Bell. It illustrates the application of the following steps:
a laser fires photons into a Beta Barium Borate (BBO) crystal;
the crystal entangles some of the photons; and then
entangled photons travel to two different detectors: A and B.
Placed between the crystal and detector B is a double-slit, like in the previous experiments. Immediately in front of detector A is a polarizing filter that can be rotated. The images before showed an experiment using sunglasses to see the effects of rotating a polarizer. Those same effects apply here.
The Bell-state quantum eraser has one more feature: each slit is covered by a substance that filters the polarization of a photon. Consequently, the left-hand slit will receive photons with a counter-clockwise polarization, and the right-hand slit will pass photons with a clockwise polarization.
Note: Polarization does not affect interference patterns.
Initially, neither detector shows an interference pattern. Since we control the polarization of photons passing through the slits and we know the polarization accepted by each slit, we can deduce which way the photons travelled (counter-clockwise through the left; clockwise through the right). Thus no interference patterns are detected.
However, if we rotate the polarizing filter in front of detector A so that the polarizations of the photons that hit the detector are the same (that is, we can no longer distinguish between clockwise and counter-clockwise polarizations), then the interference pattern appears at both detectors!
How do the photons arriving at detector B know that the polarizations have been "erased" at detector A?
Conclusion
Quantum Theory is continually being challenged and tested; physicists are finding new ways to explain the world of the tiny. Each passing year brilliant minds add to, or subtract from, the pool of knowledge about quantum behaviour.
Unlike the static nature of the web pages presented here, quantum physics is ever changing. Physicists are confronted with problems that will take many iterations, many years, to solve. Scores of theories will be presented, some of them merely tweaking, while others radically alter, our perceptions of quantum nature.
Whatever we observe in the future, it promises to be exciting!