**Next message:**Colin Hales: "RE: quantum computing and the brain"**Previous message:**Samantha Atkins: "Re: Chomsky (was: Christopher Hitchens' Column)"**Maybe in reply to:**Miriam English: "quantum computing and the brain"**Next in thread:**Colin Hales: "RE: quantum computing and the brain"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

Miriam writes:

*> I don't understand how a quantum computer can work, but I read a little
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*> time back that it can do multiple calculations simulataneously. That is, if
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*> you have 2 qubits they can perform 4 calculations at once by simultaneously
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*> experiencing (in some way I can't begin to understand) all possible state
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*> combinations at once. Similarly that 3 qubits "feel out" 8 states, 4 qubits
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*> 16 states, and so on.
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Don't forget, with an ordinary computer 3 bits can hold any one

of 8 states, four bits can hold any one of 16 states, and so on.

The difference with the QC is that it can operate on multiple states

simultaneously. You can put your 3 bits into all 8 possible states

at the same time, and then perform some operation on each of these 8

possible states, ending up in a superposition of all 8 possible results.

*> This would be a pretty impressive computer if I have that right. Would that
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*> make it possible to handle all the computations of all the neurons (10^12)
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*> and synapses (perhaps 10^3 per neuron) of the brain even taking into
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*> account about 1,000 levels of exciteability for each synapse... with just
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*> 60 qubits!!?
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*>
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*> That can't be right... can it?
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I don't think it is quite right. Accepting your figures, you don't just

multiple 10^12 times 10^3 times 10^3 to get the number of possible brain

states, and then take the log base 2 to get the number of qubits needed.

Rather, to represent a single synapse state you would need 10 bits for

10^3 possible states. Then you would need to replicate those 10 bits,

10^15 times. So the total number of bits needed to represent a single

brain state is 10^16, not 60.

If you had a QC with 10^16 qubits, it could represent all possible

brain states at once. So it could in principle simulate all possible

brain experiences with one calculation.

*> It must require at least one qubit to store an item of information, even if
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*> calculations can be made in different ways simultaneously on that one bit
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*> of info, it is still just one bit.
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Yes, this is a better instinct. You can have quantum parallelism but

you still need enough qubits to represent the individual information

units that you are operating on in parallel.

*> Even if each bit can be a single atom (one day in the future) that still
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*> makes a tiny human mind -- using 10^18 atoms for storage. The entire mind
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*> would not be that small of course because you still need some way to
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*> shuttle the info around inside it, but even if the the means of moving data
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*> around required 100,000 atoms for each storage atom the entire thing is
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*> still just a sixth of a mole of material! If the "brain" used
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*> nanostructures of pure carbon for instance then that would be a device
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*> weighing just 2 grams (Avogadro's constant puts 6*10^23 atoms of carbon at
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*> 12 grams).
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This reminds me of Robin Hanson's "Lilliputian Uploads". Robin used

similar reasoning to compute that the brain could be miniaturized to just

about fit someone who was 1/4 inch tall. Also, the degree of speedup

due to the nanotech brain could be matched to the mechanical speedups

appropriate to a being of this size. I can't find the full article but

Robin has a short essay at http://hanson.gmu.edu/lilliput.html.

Hal

**Next message:**Colin Hales: "RE: quantum computing and the brain"**Previous message:**Samantha Atkins: "Re: Chomsky (was: Christopher Hitchens' Column)"**Maybe in reply to:**Miriam English: "quantum computing and the brain"**Next in thread:**Colin Hales: "RE: quantum computing and the brain"**Messages sorted by:**[ date ] [ thread ] [ subject ] [ author ]

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