DELAYED CHOICE
QUANTUM ERASER
(LAB VERSION)
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
DELAYED CHOICE QUANTUM ERASER
This is an actual experiment conducted invoking a "delayed choice
quantum eraser", one of the strangest and most thought provoking in
physics.
This experiment demonstrates a specific type of double slit experiment
where "which slit" information can be found and even erased by actions
AFTER the experiment has been completed.
Y-H. Kim et al., PRL, 1999
Scully, M., Phys. Rev A, 1982
Explanation help from B. Greene's book The Fabric of the Cosmos
SPONTANEOUS PARAMETRIC DOWNCONVERSION
SPDC: A process that converts a single photon into a pair of entangled
photons. One output photon is typically labeled the "signal" photon,
while the other the "idler" photon.
DELAYED CHOICE QUANTUM ERASER:
EXPERIMENTAL SETUP
DELAYED CHOICE QUANTUM ERASER:
DESCRIPTION
Overview: Light is emitted toward a double slit screen. Behind each slit
lies a SPDC that converts each entering photon into two entangled
photons. One set of photons, the "signal" photons, goes north
(up). The other set of photons, the "idler" photons go south (down).
DELAYED CHOICE QUANTUM ERASER:
EXPERIMENT I
Assume that the two possible photon streams heading south are kept
separate and strike separate detectors. Therefore a southern
experimenter would know which slit each southern photon went
through. The corresponding entangled two photon streams heading
north, however, are combined on a single image screen. What pattern
appears on this northern image screen?
1. An interference pattern.
2. No interference pattern.
3. Directions out of crazy-ville.
DELAYED CHOICE QUANTUM ERASER:
EXPERIMENT I
2. No interference pattern.
Since the southern set of photons can be traced back to specific slits,
and since the southern photons are entangled with the northern
photons, "which-slit" information for the northern photons can be
inferred, and so no interference pattern will result.
DELAYED CHOICE QUANTUM ERASER:
EXPERIMENT II
Same as Experiment I, except now some of the "which slit" information
is erased for the southern photons. In this experiment, two distinct
southern groups have their "which slit" information erased. What
happens to these distinct groups of southern photons?
1.
2.
3.
4.
Each group shows an interference pattern.
Neither groups shows an interference pattern.
One group shows an interference pattern, the other does not.
The photons move to an easier experiment.
DELAYED CHOICE QUANTUM ERASER:
EXPERIMENT II
1. Each group shows an interference pattern.
Since which slit information is lost, these southern photons could be
considered, again, to have gone through both slits and interfered.
A further oddity is that the two southern interference patterns are offset
compared to the other. Specifically, when one interference pattern
peaks, the other valleys. (This will turn out to be important.)
DELAYED CHOICE QUANTUM ERASER:
EXPERIMENT II
Using timing coincidence information, the northern photons can be
found that are entangled with the two sets of southern photons that
showed the interference patterns. Together on the northern
screen, what pattern do these northern photons show?
1.
2.
3.
4.
An interference pattern.
No interference pattern.
Two interference patterns superposed.
God does not play dice with the universe.
DELAYED CHOICE QUANTUM ERASER:
EXPERIMENT II
2. No interference pattern. AND
3. Two interference patterns superposed.
Since they were entangled with the southern photons, they show the
same patterns as the southern photons whose "which slit" information
had been erased. Therefore they show two interference patterns -- but
this time superposed on the same screen. Since each interference
pattern was offset from the other, the peaks and valleys filled in to
APPEAR like there was no interference at all.
DELAYED CHOICE QUANTUM ERASER:
CONCLUSIONS
Therefore
•
•
•
•
even though the northerner sees no interference pattern, she still
does not know which of his northern photons went through which
slits.
the northerner thinks that the southerner can use her entangled
photons to figure out "which slit" information for each
northern photon.
in Experiment I, the southerner would indeed infer which slit
information or each northern photon.
in Experiment II, the southerner would infer which slit information for
only some of the northern photons.
DELAYED CHOICE QUANTUM ERASER:
CONCLUSIONS
Furthermore
•
•
•
the southern half of the experiment could be chosen to be either
Experiment I or Experiment II YEARS AFTER the photons have
impacted the northerners' image screen.
Whether some northern photons take one path or both paths
depends on a southern experiment that can happen years in the
future.
the present depends on the future.
DELAYED CHOICE QUANTUM ERASER:
DESCRIPTION
Finally:
•
no matter which experiment the southerner does (I or II), the
northerner will see the same result: no interference pattern.
•
there is no way the future can send a signal
to the present.