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RE: Legget's Problem
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> Would those be the boundary conditions that, at the beginning of
I don't know. I was only thinking of constraints on the behaviour or 2
> Shor's algorithm, the quantum computer must hold a representation of
> a large integer, and at the end, representations of its factors?
entangled photons. Sorry, I'm not sufficiently knowledgeable about the inner
workings of qcs to say. The whole process should in theory be
timereversible under the known laws of physics unless it involves something
like gravitational collapse (perhaps) or neutral kaon decay (I think). It
seems unlikely that macroscopic arrows of time (e.g. the thermodynamic one)
can be applied to a system of 2 entangled photons so that they "think"
something like "Hmm, this end of my trajectory is what macroscopic beings
would call 'earlier', so I can only be constrained by THIS boundary
condition, what they would call my emission, but not THAT one, which they
would call my absorption".
As it were!
Charles 0 Attachment
The cover article from New Scientist of 12th May claims that "decoherence
theory cannot be correct." Has anyone read it, and do they have any
comments?
The beginning of the article in question is on their website here:
http://www.newscientist.com/channel/fundamentals/mg19426031.400curiositydo
esnthavetokillthequantumcat.html
Charles 0 Attachment
 In FabricofReality@yahoogroups.com, "Charles Goodwin"
<charlesgoodwin@...> wrote:
> The cover article from New Scientist of 12th May claims that
"decoherence
> theory cannot be correct." Has anyone read it, and do they have any
Martinis has a web page:
> comments?
>
> The beginning of the article in question is on their website here:
>
>http://www.newscientist.com/channel/fundamentals/mg19426031.400curiositydo
> esnthavetokillthequantumcat.html
http://gabriel.physics.ucsb.edu/~martinisgroup/#home
I'm guessing the article refers to this paper:
http://www.sciencemag.org/cgi/content/abstract/312/5779/1498
Coherent State Evolution in a Superconducting Qubit from
PartialCollapse Measurement
"Measurement is one of the fundamental building blocks of
quantuminformation processing systems. Partial measurement, where
full wavefunction collapse is not the only outcome, provides a
detailed test of the measurement process. We introduce quantumstate
tomography in a superconducting qubit that exhibits highfidelity
singleshot measurement. For the two probabilistic outcomes of partial
measurement, we find either a full collapse or a coherent yet
nonunitary evolution of the state. This latter behavior explicitly
confirms modern quantummeasurement theory and may prove important for
errorcorrection algorithms in quantum computation."
So in other words by his own admission Martinis's work doesn't refute
decoherence theory regardless of the spin he might wish to put on it.
Alan 0 Attachment
No comments yet, but the following articles show the preceding work:
1) N. Katz et al., Coherent State Evolution in a Superconducting
Qubit from PartialCollapse Measurement, Science, vol 312, p 1498 (9
Jun 2006)
2) Alexander N. Korotkov and Andrew N. Jordan, Undoing a Weak Quantum
Measurement of a SolidState Qubit, Physical Review Letters, vol 97,
p 166805 (20 Oct 2006)
and a report on "weak measurements" in New Scientist , 10 May 2003, p
28.
The idea is explained as:
' Performing the experiment will involve manipulating the quantum
state of a loop of superconducting wire known as a phase qubit.
First, the researchers fire a finely tuned microwave pulse at the
loop. This puts the qubit into a "cat state", akin to the deadand
alive state of SchrÃ¶dinger's cat, in which it sits in an equal
superposition of both of the qubit's possible energy states.
Until, that is, the measurement begins. As soon as the researchers
begin to perform a measurement, the superposition slides towards one
state or the other. The question is, which one? Is the cat going to
live or die?
To answer that question without killing the cat, the researchers will
look to see whether or not the qubit performs a quantum trick called
tunnelling. Faced with an insurmountable barrier, there is nothing
that a classical particle can do. A quantum particle, on the other
hand, can take advantage of the uncertainty principle, which says you
can never precisely define all the particle's properties. That means
that in certain circumstances there is a small probability you will
find it on the far side of this apparently insurmountable barrier.
The more energy a particle has, the more likely it is to tunnel when
given the opportunity; if the researchers see the qubit has tunnelled
they will know it has collapsed to the higher energy state.
In itself, that is disastrous, of course: if the researchers see the
burst of magnetic energy that indicates the particle has tunnelled to
a higher energy state, it means the measurement was completed and the
cat is dead or alive. The trick is to catch the qubit before it
actually gets there.
To sneak a peek at the qubit's state midway through its collapse, the
researchers induce a steadily increasing voltage across the wire
ring. This is like teasing the qubit into "thinking" about tunnelling
by making it easier to cross the barrier. Then, at a certain
threshold, they drop the voltage back down again. It is equivalent to
opening the box and then quickly closing the lid again.
Because quantum processes take a finite time, lowering the energy
barrier then raising it again acts as a "weak" form of measurement
(New Scientist , 10 May 2003, p 28). If we don't see the qubit
tunnel, we learn that there is some finite probability that it is in
the lower energy state. In other words, we have gained information
about a quantum system without destroying its delicate superposition.
The more time we risk leaving the barrier down without the qubit
tunnelling, the more certain we are of its low energy state.
Now it is time to undo any harm we have inflicted in the process. To
do this, the physicists fire another kind of microwave pulse, known
as a pipulse, at the qubit. This inverts the quantum states of the
qubit: the higher energy level is now the lower level and vice versa.
The voltage is then ramped up and dropped again. If the qubit doesn't
tunnel this time, it becomes more likely that it is in what is now
the lower energy level. Where the first weak measurement pushed the
superposition one way, the second pushes it by the same amount the
other way, which means we end up right where we started. It is as if
the qubit, or the cat, had never been disturbed at all. '
The decoherence theory is challenged, the article says:
' If confirmed by experiment, the researchers believe they will have
ruled out one of the most popular explanations for how quantum things
turn classical. "Decoherence theory" suggests that the superposition
never really collapses  it only appears to collapse. What actually
happens, according to this idea, is that all the information about
the system disperses into the environment: when a quantum system
interacts with a classical measuring device, it becomes irreversibly
entangled with all the particles that make up the measuring device
and its surroundings. All the information about the original state of
the system in superposition is then spread so thinly throughout the
massively bigger environment that it is, essentially, lost. The odds
of identifying the original state become far worse than the odds of
randomly shuffling a deck of cards back into perfect order.
According to Jordan, the weak measurement experiment should
demonstrate that decoherence theory cannot be correct. Weak
measurements make superpositions evolve towards one of the well
defined original states of the isolated system, not into an ever
bigger mess of entanglements with everything around it. "In our
analysis of continuous weak measurements, we see that the system gets
drawn to one state or another," Jordan says. "That rules out
decoherence theory." The reversibility of weak measurements also
stands against decoherence: if information does spread into the
environment, it shouldn't be possible to get it back." '
Babak
On 21May07, at 7:49 PM, Charles Goodwin wrote:
> The cover article from New Scientist of 12th May claims that
> "decoherence
> theory cannot be correct." Has anyone read it, and do they have any
> comments?
>
> The beginning of the article in question is on their website here:
>
> http://www.newscientist.com/channel/fundamentals/mg19426031.400
> curiositydo
> esnthavetokillthequantumcat.html
>
> Charles 0 Attachment
Hi again.
I have found the full article of this online!
Here is the link to this. http://postbiota.org/pipermail/tt/2007May/000515.html
Mark.
Charles Goodwin <charlesgoodwin@...> wrote:
The cover article from New Scientist of 12th May claims that "decoherence
theory cannot be correct." Has anyone read it, and do they have any
comments?
The beginning of the article in question is on their website here:
http://www.newscientist.com/channel/fundamentals/mg19426031.400curiositydo
esnthavetokillthequantumcat.html
Charles

The allnew Yahoo! Mail goes wherever you go  free your email address from your Internet provider.
[Nontext portions of this message have been removed] 0 Attachment
Hi Charles.
Although I couldn't get all the article obviously, in the link you
sent,
would I be right to think this implies that maybe the Copenhagen
Interpretation is correct after all? I ask as I know that decoherence
is used to
explain why the wave function only seems to collapse, when either
measured or
observed. Without decoherence, the wave collapse would follow the concept
of Occam's razor.
I know that the Copenhagen view is unpopular these days, but maybe this
will at least revive the debate surrounding it.
Cheers for the link.
Mark.
Charles Goodwin <charlesgoodwin@...> wrote:> The cover article from New Scientist of 12th May claims that
http://www.newscientist.com/channel/fundamentals/mg19426031.400curiositydo
> "decoherence theory cannot be correct." Has anyone read it, and do
> they have any comments?
>
> The beginning of the article in question is on their website here:
>
>
esnthavetokillthequantumcat.html>
> Charles 0 Attachment
 Babak Seradjeh <babaks@...> wrote:
> No comments yet, but the following articles show the preceding work:
Sigh. Decoherence theory states that *if* we do a measurement and don't
>
> 1) N. Katz et al., Coherent State Evolution in a Superconducting
> Qubit from PartialCollapse Measurement, Science, vol 312, p 1498 (9
> Jun 2006)
> 2) Alexander N. Korotkov and Andrew N. Jordan, Undoing a Weak Quantum
> Measurement of a SolidState Qubit, Physical Review Letters, vol 97,
> p 166805 (20 Oct 2006)
>
> and a report on "weak measurements" in New Scientist , 10 May 2003, p
> 28.
>
> The idea is explained as:
>
> ' Performing the experiment will involve manipulating the quantum
> state of a loop of superconducting wire known as a phase qubit.
> First, the researchers fire a finely tuned microwave pulse at the
> loop. This puts the qubit into a "cat state", akin to the deadand
> alive state of SchrÃ¶dinger's cat, in which it sits in an equal
> superposition of both of the qubit's possible energy states.
>
> Until, that is, the measurement begins. As soon as the researchers
> begin to perform a measurement, the superposition slides towards one
> state or the other. The question is, which one? Is the cat going to
> live or die?
>
> To answer that question without killing the cat, the researchers will
> look to see whether or not the qubit performs a quantum trick called
> tunnelling. Faced with an insurmountable barrier, there is nothing
> that a classical particle can do. A quantum particle, on the other
> hand, can take advantage of the uncertainty principle, which says you
> can never precisely define all the particle's properties. That means
> that in certain circumstances there is a small probability you will
> find it on the far side of this apparently insurmountable barrier.
> The more energy a particle has, the more likely it is to tunnel when
> given the opportunity; if the researchers see the qubit has tunnelled
> they will know it has collapsed to the higher energy state.
>
> In itself, that is disastrous, of course: if the researchers see the
> burst of magnetic energy that indicates the particle has tunnelled to
> a higher energy state, it means the measurement was completed and the
> cat is dead or alive. The trick is to catch the qubit before it
> actually gets there.
>
> To sneak a peek at the qubit's state midway through its collapse, the
> researchers induce a steadily increasing voltage across the wire
> ring. This is like teasing the qubit into "thinking" about tunnelling
> by making it easier to cross the barrier. Then, at a certain
> threshold, they drop the voltage back down again. It is equivalent to
> opening the box and then quickly closing the lid again.
>
> Because quantum processes take a finite time, lowering the energy
> barrier then raising it again acts as a "weak" form of measurement
> (New Scientist , 10 May 2003, p 28). If we don't see the qubit
> tunnel, we learn that there is some finite probability that it is in
> the lower energy state. In other words, we have gained information
> about a quantum system without destroying its delicate superposition.
> The more time we risk leaving the barrier down without the qubit
> tunnelling, the more certain we are of its low energy state.
>
> Now it is time to undo any harm we have inflicted in the process. To
> do this, the physicists fire another kind of microwave pulse, known
> as a pipulse, at the qubit. This inverts the quantum states of the
> qubit: the higher energy level is now the lower level and vice versa.
> The voltage is then ramped up and dropped again. If the qubit doesn't
> tunnel this time, it becomes more likely that it is in what is now
> the lower energy level. Where the first weak measurement pushed the
> superposition one way, the second pushes it by the same amount the
> other way, which means we end up right where we started. It is as if
> the qubit, or the cat, had never been disturbed at all. '
>
> The decoherence theory is challenged, the article says:
>
> ' If confirmed by experiment, the researchers believe they will have
> ruled out one of the most popular explanations for how quantum things
> turn classical. "Decoherence theory" suggests that the superposition
> never really collapses  it only appears to collapse. What actually
> happens, according to this idea, is that all the information about
> the system disperses into the environment: when a quantum system
> interacts with a classical measuring device, it becomes irreversibly
> entangled with all the particles that make up the measuring device
> and its surroundings. All the information about the original state of
> the system in superposition is then spread so thinly throughout the
> massively bigger environment that it is, essentially, lost. The odds
> of identifying the original state become far worse than the odds of
> randomly shuffling a deck of cards back into perfect order.
>
> According to Jordan, the weak measurement experiment should
> demonstrate that decoherence theory cannot be correct. Weak
> measurements make superpositions evolve towards one of the well
> defined original states of the isolated system, not into an ever
> bigger mess of entanglements with everything around it. "In our
> analysis of continuous weak measurements, we see that the system gets
> drawn to one state or another," Jordan says. "That rules out
> decoherence theory." The reversibility of weak measurements also
> stands against decoherence: if information does spread into the
> environment, it shouldn't be possible to get it back." '
undo it *then* we can't get back to the original state. This is the usual
situation in everyday life partly because we don't know exactly what
measurments are being done at any given time and partly because some
measurements cannot be undone (mostly the former). For example, I don't
know exactly what air currents are hitting me right now and so I can't undo
the interactions that occur as a result. Second, if a photon were to hit me
and zoom off into space and it was never reflected or absorbed it would be
physically impossible for me to reverse the measurement because I couldn't
catch up to the photon and the information it carries. That's why
decoherence is a suitable explanation for the fact that the multiverse is
divided up into parallel universes to a good approximation.
If we know a lot about a measurement and we carefully arrange it then we
can undo it to a good approximation, which is what Jordan and Korotkov did:
http://arxiv.org/abs/condmat/0606713
So this experiment doesn't refute decoherence theory.
Alan
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> From: FabricofReality@yahoogroups.com [mailto:Fabricof
Sounds correct to me. Surely if this experiment DID disprove decoherence
> Reality@yahoogroups.com] On Behalf Of Alan Forrester
> (SNIP)
> So this experiment doesn't refute decoherence theory.
>
> Alan
theory, then so would quantum erasure?
Charles
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