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RE: Legget's Problem

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  • Charles Goodwin
    ... I don t know. I was only thinking of constraints on the behaviour or 2 entangled photons. Sorry, I m not sufficiently knowledgeable about the inner
    Message 1 of 27 , Apr 30, 2007
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      > Would those be the boundary conditions that, at the beginning of
      > Shor's algorithm, the quantum computer must hold a representation of
      > a large integer, and at the end, representations of its factors?

      I don't know. I was only thinking of constraints on the behaviour or 2
      entangled photons. Sorry, I'm not sufficiently knowledgeable about the inner
      workings of qcs to say. The whole process should in theory be
      time-reversible 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
    • Charles Goodwin
      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
      Message 2 of 27 , May 21 7:49 PM
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        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-curiosity-do
        esnt-have-to-kill-the-quantum-cat.html

        Charles
      • Alan Forrester
        ... decoherence ... Martinis has a web page: http://gabriel.physics.ucsb.edu/~martinisgroup/#home I m guessing the article refers to this paper:
        Message 3 of 27 , May 22 10:13 AM
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          --- In Fabric-of-Reality@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
          > comments?
          >
          > The beginning of the article in question is on their website here:
          >
          >http://www.newscientist.com/channel/fundamentals/mg19426031.400-curiosity-do
          > esnt-have-to-kill-the-quantum-cat.html

          Martinis has a web page:

          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
          Partial-Collapse Measurement

          "Measurement is one of the fundamental building blocks of
          quantum-information processing systems. Partial measurement, where
          full wavefunction collapse is not the only outcome, provides a
          detailed test of the measurement process. We introduce quantum-state
          tomography in a superconducting qubit that exhibits high-fidelity
          single-shot 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 quantum-measurement theory and may prove important for
          error-correction 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
        • Babak Seradjeh
          No comments yet, but the following articles show the preceding work: 1) N. Katz et al., Coherent State Evolution in a Superconducting Qubit from
          Message 4 of 27 , May 22 12:15 PM
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            No comments yet, but the following articles show the preceding work:

            1) N. Katz et al., Coherent State Evolution in a Superconducting
            Qubit from Partial-Collapse Measurement, Science, vol 312, p 1498 (9
            Jun 2006)
            2) Alexander N. Korotkov and Andrew N. Jordan, Undoing a Weak Quantum
            Measurement of a Solid-State 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 dead-and-
            alive state of Schrödinger'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 pi-pulse, 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 21-May-07, 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-
            > curiosity-do
            > esnt-have-to-kill-the-quantum-cat.html
            >
            > Charles
          • Mark Smith
            Hi again. I have found the full article of this online! Here is the link to this. http://postbiota.org/pipermail/tt/2007-May/000515.html Mark. Charles Goodwin
            Message 5 of 27 , May 22 6:22 PM
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              Hi again.

              I have found the full article of this online!

              Here is the link to this. http://postbiota.org/pipermail/tt/2007-May/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/mg19426031.400-curiosity-do
              esnt-have-to-kill-the-quantum-cat.html

              Charles






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            • Mark Smith
              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
              Message 6 of 27 , May 23 9:52 AM
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                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
                > "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-curiosity-do
                esnt-have-to-kill-the-quantum-cat.html
                >
                > Charles
              • Alan Forrester
                ... Sigh. Decoherence theory states that *if* we do a measurement and don t undo it *then* we can t get back to the original state. This is the usual situation
                Message 7 of 27 , May 23 11:54 AM
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                  --- Babak Seradjeh <babaks@...> wrote:

                  > No comments yet, but the following articles show the preceding work:
                  >
                  > 1) N. Katz et al., Coherent State Evolution in a Superconducting
                  > Qubit from Partial-Collapse Measurement, Science, vol 312, p 1498 (9
                  > Jun 2006)
                  > 2) Alexander N. Korotkov and Andrew N. Jordan, Undoing a Weak Quantum
                  > Measurement of a Solid-State 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 dead-and-
                  > alive state of Schrödinger'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 pi-pulse, 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." '

                  Sigh. Decoherence theory states that *if* we do a measurement and don't
                  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/cond-mat/0606713

                  So this experiment doesn't refute decoherence theory.

                  Alan



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                • Charles Goodwin
                  ... Sounds correct to me. Surely if this experiment DID disprove decoherence theory, then so would quantum erasure? Charles
                  Message 8 of 27 , May 23 12:26 PM
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                    > From: Fabric-of-Reality@yahoogroups.com [mailto:Fabric-of-
                    > Reality@yahoogroups.com] On Behalf Of Alan Forrester
                    > (SNIP)
                    > So this experiment doesn't refute decoherence theory.
                    >
                    > Alan

                    Sounds correct to me. Surely if this experiment DID disprove decoherence
                    theory, then so would quantum erasure?

                    Charles
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