From: *smitra* <smi...@zonnet.nl <mailto:smi...@zonnet.nl>>
On 03-05-2018 03:22, Brent Meeker wrote:
On 5/2/2018 6:02 PM, smitra wrote:
On 02-05-2018 03:21, Brent Meeker wrote:
On 5/1/2018 4:43 PM, smitra wrote:
On 01-05-2018 20:47, Brent Meeker wrote:
On 5/1/2018 9:01 AM, Bruno Marchal wrote:
On 29 Apr 2018, at 19:59, Brent Meeker
<meeke...@verizon.net
<mailto:meeke...@verizon.net>> wrote:
On 4/29/2018 8:53 AM, Bruno Marchal wrote:
But that's my question: Why isn't it the same?
And even if it's not
how would be know? The "conscious" quantum
computer assures us that
it not only detected that there was a welcher
weg photon but that
it's weg was known to the "consciousness" of
the quantum computer,
before it was erased. But why would we believe
it? We already have
these experiments in which we know the weg was
available and could
have been recorded, but was erased. So what is
the "consciousness"
that adds a secret-sauce to the experiment?
Good question. I doubt that you can fool
quantum mechanics by
calling it "consciousness". I think in this
case the interaction
with the welcher weg photon would amount to
sufficient decoherence
-- basically information was extracted that
was not restored. Also,
of course, if the QC "forgets" what it did,
how can it report on the
fact that it did anything. How can we believe
that it actually knew
which slit at some point?
Because in Deutsch experiment, not everything has
been erased, notably
the memory that he has known the result. He would
say something like:
I remember doing the measurement and writing it in
the enveloppe. Now
the envelop has been erased, and I can’t remember
its content, but I
definitely remember having known the content.
But two questions remain. First, the empirical
question of whether
this erasure is enough to restore interference.
I do not see why it would not been enough … in
theory. You need only
a computer able to forget a memory, but not some
meta-memory that it
has recorded a definite result. It isa bit like
remembering we have
done a dream, without being able to remember any
of its content.
In practice, that might be very difficult, if not
impossible. I am not
sure.
Second, why should we believe the quantum
computer.
In Deutsch proposal, it is a human.
No, it's a conscious quantum computer. It it
were a human or other
(quasi-) classical instrument decoherence would
happen when there was
a detection of welcher weg and erasure would be
impossible.
Brent
Yes, but note that you can make that quantum computing
simulation of the observer in that thought experiment
as precise as you like. You can in principle include a
simulation of the entire Earth
And the outgoing EM and neutrino waves and their
interaction with
interstellar atoms. I'm suspicious of these fantasy thought
experiments. But however detailed it may be doesn't answer my
question as to what it would mean to erase the welcher weg
but not the
memory that the weg was detected. I noted that this is
not like a
classical erasure of a memory because in this case the
coherence is
maintained, so when the welcher weg is erased there is no
long any
fact-of-the-matter as to which way it went. There is no
fact-of-the-matter that it was detected to go left or
right. So the
"memory" if it exists, is a false memory.
with billions of other people and a lot of decoherence
implemented by qubits that simulate e.g. soft photons
and other environmental degrees of freedom (and all
that decoherence will end up getting reversed by the
way the computation is set up ) The point is that if
computation generates consciousness, you can in
principle let any given person do the experimental
verification of the existence of multiple branches by
uploading the brain to a quantum computer and letting
it be subject to such a computation.
How will the person verify it? Reversing the computation
will reverse
the person and erase their memory.
Brent
It's a simple two step measurement process where you (as a
virtual person simulated by the QC) perform a measurement that
tells you that the spin (represented by a qubit) has been
measured without giving you the result. And then you perform
the next measurement where you actually measure the value of
the spin component. It can then be shown that there exists a
unitary transform that will restore the original spin state
that will preserve the record of the first measurement.
But you're speaking poetically. I, as a classical being cannot
perform such measurements. First, how can the simulated QC person
perform a measurement that tells you that the spin has been measured
without giving anyone the result? In what sense is this a measurement
of the spin, not merely a measurement of some proxy that is
independent of the spin value? Second, what is the point of the
second measurement "where you actually measure the spin component";
are you saying the first measurement did not actually measure the spin
component even though it is supposed to tell us that it was measured?
Third, all the techniques I've heard of for quantum erasing a
measurement and restoring the WF are like making it so it never
happened. You seem implicitly to take this view since you're
concerned to preserve the record of the first measurement (which
didn't actually measure the spin value) but not the second (which
makes no record).
Brent
Yes, it's a measurement of a proxy, analogous to letting someone else
measure the spin and then that person reporting to you that the spin
has been successfully measured, without disclosing the result to you.
It's not difficult to write down a QC program to see how this works in
detail. You can take a CNOT gate a very simple observer, the control
qubit is the spin that is going to be measured (using a Hadamrd
transform it can be put in a superposition of |0> and |1>), the other
bit is initialized to be in the |0> state. We then add another qubit
that changes from |0> to |1> when the gate is applied. One can then
return qubits of the CNOT gate to the original state while leaving
that extra qubit in the state it was after the measurement.
So, the record of the measurement having taking place will be kept,
while the original spin state of, say, |0> has been restored, and that
can be verified by repeatedly carrying out this process and then also
measuring the spin in the final restored state. That final measurement
always yields the same result, proving that the qubit is indeed always
restored to the |0> state. But if the measurement at the time of the
superposition were to collapse the wavefunction, eliminating one of
the two branches then the original state would not be restored.
What all of this proves, is that an observer implemented by a quantum
computer can experimentally falsify the Copenhagen Interpretation.
Unfortunately, it is not the case that you can implement absolutely any
unitary transformation in this way. For instance, you cannot implement
the unitary transformation that would reverse a totally decohered event.
Your quantum computer ceases to function if there is any decoherence!
For example, you cannot implement a unitary transformation that would
resurrect my dead grandfather, even though his life and death were
entirely unitary. So you cannot reverse a recorded measurement.
Bruce
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