On 10/28/2022 4:33 AM, John Clark wrote:
On Fri, Oct 28, 2022 at 1:09 AM Brent Meeker <[email protected]>
wrote:
/> I was citing Carroll on the delayed choice quantum
erasure...not on anything Deutsch said. /
OK, then whatever Carroll was saying that you were referring to is
irrelevant to what we were discussing. And by the way, Carroll is just
as big an advocate of the Many World's idea as Deutsch is and wrote an
excellent book about that very subject.
/> The problem I see in this is that the computer must make a
measurement that does two things
1) it prints out a document that is causally dependent on a
distinct measured value existing (not just: there was an electron
so it prints "I measured it.") /
Yes, when the computer writes the document, writes it after the
electron passes through the slits but before it hits the photographic
plate, the machine knows which slit the electron went through but it
reframes from including that bit information in the document, it
retains that bit of information safely in its quantum memory and
doesn't quantum erase it until a nanosecond before the electron hits
the photographic plate.
>///(2) the measurement must be quantum erased.
I think this is impossible because (1) depends on the measurement,
being either LEFT or RIGHT/
No, the document is EXACTLY the same in both universes,
I know that. But it must be causally connected (entangled with) the
measurement value of either L xor R. It can't just be causally
connected with L or R, because that's the printer that just says, "I
measured it." whatever the value.
that's why I kept emphasizing that it does not contain any information
about which slot the electron went through, it just tells us if it was
able to successfully make such a measurement and if it knows which
slot the electron went through. So the only thing different about the
two universes is the computer's memory about which slot the electron
went through and if, unlike classical computers, that bit of
information is stored quantum mechanically, then that bit of
information can be erased quantum mechanically. At that point the two
universes are identical again, that is to say they would have exactly
the same quantum wave function, so it would be silly to pretend there
are still two distinct universes.
/> and that specific distinct value being amplified to a classical
variable/
That's what usually happens and the amplification typically happens at
enormous speed, that's why making any quantum experiment is difficult
and making a quantum computer is even more difficult, but that's just
an engineering difficulty caused by our limited technology, it is not
a limitation imposed by scientific fundamentals. I'm talking about
heroic engineering not impossible engineering, like a faster than
light rocket or a perpetual motion machine.
Well, I think your condition that it be erased before the particle hits
the screen is another misunderstanding your part. But it's not relevant
to what I'm questioning because in principle you can have the detector
screen a lightyear away.
>/if the distinct variable is amplified to a classical value, a
print command, it can't be quantum erased./
It makes no difference if the electron went left or right, the EXACT
same print command is issued in both universes in either case, and
that results in the EXACT same document being printed in both
universes. I agree that if you let the distinct knowledge of which
slit the electron went through get amplified and spread into the
classical realm then it's game over,
But how else is the computer going to "truthfully" report that a
measurement was made, i.e. a report that is entangled with either a
measurement of L xor with a measurement of R? I think this is
impossible. I'd like to the causal chain from a measurement of L that
avoids amplifying that into decoherence. In Carroll's exposition he
considers measuring the L in the Up/Dwn direction, which you might
consider amplifying to trigger the printer. But then he shows that this
doesn't actually erase the interference, it is just hidden:
============================================
When we measured the recording spin in the vertical direction, the
result we obtained was entangled with a definite path for the traveling
electron: [↑] was entangled with (*L*), and [↓] was entangled with
(*R*). So by performing that measurement, we knew that the electron had
traveled through one slit or the other. But now when we measure the
recording spin along the horizontal axis, that’s no longer true. After
we do each measurement, we are again in a branch of the wave function
where the traveling electron passes through both slits. If we measured
spin-left, the traveling electron passing through the right slit picks
up a minus sign in its contribution to the wave function, but that’s
just math.
By choosing to do our measurement in this way, we have erased the
information about which slit the electron went through. This is
therefore known as a “quantum eraser experiment.” This erasure doesn’t
affect the overall distribution of flashes on the detector screen. It
remains smooth and interference-free.
But we not only have the overall distribution of electrons hitting the
detector screen; for each impact we know whether the recording electron
was measured as spin-left or spin- right. So, instructs our professor
with a flourish, let’s go to our computers and separate the flashes on
the detector screen into these two groups — those that are associated
with spin- left recording electrons, and those that are associated with
spin-right. What do we see now?
Interestingly, the interference pattern reappears. The traveling
electrons associated with spin-left recording electrons form an
interference pattern, as do the ones associated with spin-right.
(Remember that we don’t see the pattern all at once, it appears
gradually as we detect many individual flashes.) But the two
interference patterns are slightly shifted from each other, so that the
peaks in one match up with the valleys in the other. There was secretly
interference hidden in what initially looked like a featureless smudge.
==========================================
Brent
you're never gonna be able to erase all of that, so you're going to
have to arrange things so that doesn't happen, or at least delay it
from happening for long enough to complete the experiment. To do all
this would be very difficult but it would not be impossible.
John K Clark See what's on my new list at Extropolis
<https://groups.google.com/g/extropolis>
7vv
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