[MUSIC PLAYING]
The quantum eraser experiment tantalizes us
with the apparent instantaneous flow of information
between entangled photon pairs.
In fact, the delayed choice quantum eraser
seems to show that information can travel backwards in time.
For today's challenge question, I'm
going to propose a way to use it to create
a time-traveling communication device
and use it to cheat on the lottery.
I want your honest opinion.
First, though, you really should go back
and watch that quantum eraser episode, unless you memory
is incredible.
Good?
Let me review the important bits, anyway.
We start with a standard double-slit apparatus.
Photons are fired one at a time through the two slits.
On the opposite side, each photon
is split into an entangled pair of photons.
One of these is sent off to a screen where its location is
recorded, while the other is used
to identify which slit the original photon passed through.
The setup looks like this.
In the interest of accuracy, let me add a little detail that I
skipped in the previous video.
When the researchers conducted this experiment,
they didn't bother with detector B. There were really only two
sets of possible outcomes.
One-- a photon finds its way to detector A. In that case,
we know the original photon must have passed through slit A.
Or two, either detector C or D fire, in which case
we have no idea which slit was traversed.
And the result is the photons whose entangled twins land at A
produce no interference pattern.
They just land in a single pile as
though they had traveled as particles
through the entire experiment.
But photos landing at C or D have
twins that do produce an interference
pattern at the screen.
One important extra piece of information--
that interference pattern is slightly different depending
on whether the C or D fire.
The pattern for C has peaks that line up with the valleys
of the interference pattern from D.
But you see the clear patterns when you separate those C and D
photons.
So the patent that forms at the screen
depends on whether we gain knowledge
of the path of the original photon.
But crazily, all of this detecting or erasing
of path information happens after each photon lands
at the screen.
After those photons land according to an interference
distribution, or a single pile distribution,
the information regarding whether the observer knows
the path appears to travel back in time,
even if was only a tiny fraction of a second in the experiment
that was performed.
So with a couple of slight modifications,
I think we can turn this into a communication device that
can not only send information instantaneously between two
points, but can also send information back in time.
So my evil genius plan-- I'm going
to build a version of this experiment that
will let me send tomorrow's winning lottery numbers back
to myself.
Here's how it works.
First, let's bring back detector B. We don't really need it,
but it makes the explanation simpler.
As it was executed, photons can travel
to the "which way" section-- so detectors A and B--
or to the "eraser" section-- so C and D. That choice
is made by these beam splitters, which
randomly reflect 50% of photons and let 50% pass through.
What if we could make this choice?
Let's replace these beam splitters with mirrors
attached to a switch.
With the mirrors in place, photons
are reflected to the which way detectors, and no interference
pattern is formed.
But activate the switch, and the mirrors move out of the way.
Photons travel through to the eraser section,
resulting in an interference pattern at the screen.
We make the decision of whether to know
the path of the original photon or whether to erase
that knowledge.
This conscious choice should be no
different to the random decision of the beam splitter.
Now we have a way to send a binary code
from the switch, which is located at the which way/eraser
end of the experiment, to the screen, which
could theoretically be very distant in both space and time.
We need one more alteration.
Before the photons get to the which way end,
we freeze them for a day.
That's long enough to buy lots of lottery tickets
before the draw.
Actually, you can't really freeze photons.
So maybe we bounce them between Earth and the moon, like,
8,000 times.
Let's just assume we can do that part perfectly.
So we turn on our device.
Photons start hitting the screen,
building up some pattern.
Once the lottery is drawn, we use our switch
to tap out the lottery numbers in binary code.
Those numbers will emerge in the interference patterns
that were recorded a day ago.
Here's my question-- why am I going to be sadly disappointed
after I turn on my invention?
Oh, I do get a signal-- nice work, future me.
And that signal does contain the interference patterns
from my future choices with perfect fidelity.
But for some reason, I can't read the numbers.
What went wrong?
Think carefully about this, and submit
your detailed explanation to PBSSpaceTime@gmail.com
with the subject line "Quantum Eraser Challenge Question."
Be sure to use exactly these words because we
filter by subject line.
We'll choose five correct answers
to receive "Space Time" t-shirts.
Of course, if you figure out a way
to make this invention work, then you
should certainly submit your explanation.
You'll get a t-shirt and half of any lottery winnings.
See next week for a fresh new episode of "Space Time."
[MUSIC PLAYING]