Transcript here - Starship Asterisk

DOUBLE SLIT III:
DELAYED CHOICE
by
Robert Nemiroff
Michigan Tech
Physics X: About This Course
• Officially "Extraordinary Concepts in Physics"
• Being taught for credit at Michigan Tech
o Light on math, heavy on concepts
o Anyone anywhere is welcome
• No textbook required
o Wikipedia, web links, and lectures only
o Find all the lectures with Google at:
 "Starship Asterisk" then "Physics X"
o
http://bb.nightskylive.net/asterisk/viewforum.php?f=39
DOUBLE SLIT EXPERIMENT:
SLIT SCREEN TILTED
•
The usual double-slit experiment is done except now the screen
containing both slits is tilted so that one slit is significantly closer to
the source than the other slit.
What happens?
1.
2.
3.
4.
No interference pattern appears.
A normal interference pattern appears.
The image screen remains blank.
The slits slide off the slit screen.
DOUBLE SLIT EXPERIMENT:
SLIT SCREEN TILTED
2. A normal interference pattern appears.
Comment: An interference pattern will appear that is normal but shifted
from the pattern that would appear for a non-tilted slit screen. The
single slit peaks would appear in their non-tilted locations as they are
still the projection of the light onto the image screen, however.
DOUBLE SLIT EXPERIMENT:
QUANTUM EAVESDROPPING
•
The usual double-slit experiment is done except now using
electrons and illuminating the slits with photons of sufficient energy
to discern which slit each electron traversed.
What does the electron image screen show?
1.
2.
3.
No interference pattern appears.
A normal interference pattern appears.
An image of a single electron fluctuating.
DOUBLE SLIT EXPERIMENT:
QUANTUM EAVESDROPPING
1. No interference pattern appears.
Since an observer knows which slit each electron went through, the
interference pattern will be destroyed for all to see. This is one basis
for knowing if someone is "eavesdropping" on your quantum
experiment.
DOUBLE SLIT EXPERIMENT:
BAD EYE BEFORE IMAGE SCREEN
•
The usual double-slit experiment is done except now a human places one eye
in front of part of the image screen. The eye is small compared to the
(previous) interference fringes and too blurry to see each slit
independently. The person is therefore not able to say which slit each photon
went through.
What happens?
1.
2.
3.
4.
The person sees part of an interference pattern.
The person sees part of a "no-interference" pattern.
The image screen remains blank.
You need glasses -- go to an optometrist..
DOUBLE SLIT EXPERIMENT:
BAD EYE BEFORE IMAGE SCREEN
1. The person sees part of an interference pattern.
Comment: Specifically, the person sees the part of the interference
pattern that would have been projected on the image screen just behind
him. Since the person cannot resolve the slits, the person cannot gain
"which way" information for the photons, and so the interference pattern
at their location persists.
DOUBLE SLIT EXPERIMENT:
GOOD EYE BEFORE IMAGE SCREEN
•
The usual double-slit experiment is done except now, again, a human places
one eye in front of part of the image screen. This eye, however, is large
enough and good enough to see each slit independently. The person is
therefore able to say which slit each photon went through.
What happens?
1.
2.
3.
4.
The person sees part of an interference pattern.
The person sees part of a "no-interference" pattern.
The image screen remains blank.
You go blind just like your mother warned.
DOUBLE SLIT EXPERIMENT:
GOOD EYE BEFORE IMAGE SCREEN
2. The person sees part of a "no-interference" pattern.
Comment: The person sees part of the pattern that would be created if
the slits did not interfere. Being able to tell which slit each photon goes
through destroys the part of the interference pattern the person would
see.
DOUBLE SLIT EXPERIMENT:
DELAYED CHOICE
•
The usual double-slit experiment is done except now a person with a large but
blurry eye stands in front of the image screen. The observer cannot resolve
the two slits. With glasses, however, the observer CAN resolve the two
slits. A bunch of photons are emitted from the source. The observer is
allowed a "delayed choice" about whether to put on the glasses AFTER the
group of photons has passed the slit screen. What happens?
1.
2.
3.
4.
The person will always see part of an interference pattern.
The person will always see part of a "no-interference" pattern.
What the person sees depends on the observer's choice.
Saying "E=mc2" makes me feel smart regardless.
DOUBLE SLIT EXPERIMENT:
DELAYED CHOICE
3. The person always sees part of a "no-interference" pattern.
Comment: For an observer to resolve the slits, the observer's
eye must be so large that both maxima and minima of the interference pattern fall
onto the eye. Without glasses, these brightness peaks and valleys blur into a "nointerference" pattern. With good glasses, the slits are resolved but the total flux
into the eye is still the same.
DOUBLE SLIT EXPERIMENT:
SLITS POLARIZED
•
The usual double-slit experiment is done except now a vertical
polarizer is placed directly after one slit, while a horizontal polarizer
is placed directly after the other slit.
What happens?
1.
2.
3.
No interference pattern appears.
A normal interference pattern appears.
A polaroid picture of the observer appears.
DOUBLE SLIT EXPERIMENT:
SLITS POLARIZED
1. No interference pattern appears.
Comment: The polarizers allows an observer at the screen to have
"which path" information just by observing what direction polarization
each arriving photon has. Once an observer knows which slit a photon
went though, that photon can no longer appear to go through both slits,
cannot interfere with itself, and so cannot contribute to an interference
pattern on the image screen.
DOUBLE SLIT EXPERIMENT:
QUANTUM ERASER
•
The usual double-slit experiment is done again with horizontal and
vertical polarizers placed after individual slits. Now, however, a
large third polarizer is placed after both slits that rotates the
polarization of all photons by 45 degrees, giving all photons that
strike the image screen a 45 degree polarization.
What pattern appears on the image screen?
1.
2.
3.
No interference pattern appears.
A normal interference pattern appears.
The electrons now appear wearing sunglasses.
DOUBLE SLIT EXPERIMENT:
QUANTUM ERASER
1. A normal interference pattern appears.
Comment: The third polarizer erases the "which way" information so
that it is no longer possible to discern which slit which photon went
through. Therefore this information is effectively "erased" and an
interference pattern (re-)emerges.
DOUBLE SLIT EXPERIMENT:
DELAYED CHOICE QUANTUM ERASER
•
The usual double-slit experiment is done again with single photons and
horizontal and vertical polarizers placed after individual slits. A bunch of
photons are released. An observer just in front of the image screen is given
the option of deploying a third polarizer that would rotate all of the photons by
45 degrees and hide which slit each photon went through. The observer is
allowed to choose to deploy this polarizer AFTER the photon bunch has
crossed the slit screen. What pattern appears on the image screen?
1.
2.
3.
No interference pattern appears.
A normal interference pattern appears.
The pattern depends on the observer's choice.
DOUBLE SLIT EXPERIMENT:
DELAYED CHOICE QUANTUM ERASER
3. What the person sees depends on the observer's choice.
Comment: A classic version of this experiment was initially proposed by
Wheeler. It demonstrates that photon interference is actually dependent on the
observer. Even though the photons (given their speed) are computed to have
passed the slit screen, the observer can determine which type of pattern they see
by deploying the third polarizer or not. Their own choice determines the perceived
pattern, even though the photons seem to have passed the slit screen
already. Yes, this has been confirmed experimentally.
WOW!
EXTRAORDINARY DOUBLE SLIT
EXPERIMENTS
"If you think you understand quantum physics then you don’t
understand enough to understand that you don’t understand
it!" - Feynman