> On 4 May 2018, at 01:03, Bruce Kellett <[email protected]> wrote: > > From: Bruno Marchal <[email protected] <mailto:[email protected]>> >>> On 3 May 2018, at 04:16, Bruce Kellett < >>> <mailto:[email protected]>[email protected] >>> <mailto:[email protected]>> wrote: >>>> From: smitra <[email protected] <mailto:[email protected]>> >>>> >>>> On 02-05-2018 03:21, Brent Meeker wrote: >>>> >>>> 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. >>>> >>>> Saibal >>> >>> You can prove anything in a simulation, because you get to choose the >>> physical laws that will be obeyed. How do you know that the simulation will >>> bear any relation to reality? A measurement is something that leaves a >>> permanent (un-erasable) record. >> >> But for Deutsch experiments, we don’t need to erase the information, only to >> discard it in the vanilla ways. That seems impossible because we are used to >> leaking environment, but if we can do a quantum computer, we can do that, >> and it means we did found ways to a sort of absolute isolation, measurement >> without interactions, (à-la Eiltzur Vaidman), etc. >> >> With a quantum brain, a human can do that experience, and come back >> remembering well opening the box, and saying to itself “I definiitly saw the >> definite result”, but after the coming back he said "now despite I remember >> all that, I get a blank when trying to remember that specific result, I >> can’t recall it (and this despite the quantum brain “knows the answer”, but >> is well blocked by some quantum Freudian algorithm (grin). >> >> I tend to think that the laws of physics are reversible. > > Actually, this is the basis of MWI -- everything in physics is based on > unitary transformations. The Schrödinger equation can be derived by assuming > time evolution is unitary. So, in the wider context, everything, even > decoherence into the wider universe, is reversible, in the sense that there > is a unitary transformation that, when applied to any final state, restores > the initial state -- just take the unitary operator that describes the time > evolution, say U, and then take its inverse, U^{-1}. > > The problem, of course, is that this unitary operator is formed in the > multiverse, so to form its inverse we have to have access to the other worlds > of the multiverse. And this is impossible because of the linearity of the SE. > So although the mathematics of unitary transformations is perfectly > reversible, measurements are not reversible in principle in the one world we > find ourselves to inhabit. > > So even Deutsch's quantum brain is likely to run into difficulties, since it > has to communicate with the real world.
OK. But that is the same with any quantum computer. Are you saying that quantum computing is not possible in practice? There are quantum algorithm capable of “fighting by quantum error procedures” the effect of decoherence. Imo, the Deutsch experiment is as much possible as a working quantum computer. I am pretty sure this is technologically possible, although plausibly not even in a near future, but soon after :) Bruno > > Bruce > > -- > You received this message because you are subscribed to the Google Groups > "Everything List" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected] > <mailto:[email protected]>. > To post to this group, send email to [email protected] > <mailto:[email protected]>. > Visit this group at https://groups.google.com/group/everything-list > <https://groups.google.com/group/everything-list>. > For more options, visit https://groups.google.com/d/optout > <https://groups.google.com/d/optout>. -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send email to [email protected]. Visit this group at https://groups.google.com/group/everything-list. For more options, visit https://groups.google.com/d/optout.
