X-Message-Number: 18721
Date: Fri, 08 Mar 2002 12:02:19 +0100
From: Henri Kluytmans <>
Subject: Re: Simulation using qubits

I wrote :

>> And how do you read them out ??????
>> You would still need to perform more quantum logic, and you would 
>> still need 2^608 qubits to transfer result into, and then collapse 
>> those qubits, to be able to read out the bit content.
>> And how do you perform the simulation itself without more qubits?

Yvan Bozzonetti replied :

>Assume we have 3 qbits, mark them "x" when they are in 
>a superposition state,  so at start we have: xxx

<snip>

>digit is not the same, so the final state is: 0x1, here, one digit 
>remains in a superposition state. You don't read that state because 

The problem is : You want to know the answer. Therefore you will 
have to read it out somewhere. By reading out the qubits they will 
collapse. As soon as you read out the middle bit, then it will 
collapse into either a 1 or a 0. 

Furthermore, if you have only one qubit in superposition state 
than that qubit can be either 1 or 0. If you have two qubits 
there are 4 combinations possible. But maybe only three are 
allowed. How do you get to know all the allowed combinations ????

You will still have to transfer the answer to more qubits, and 
then let those qubits collapse, to get the answer(s) (i.e. to 
get to know all allowed combinations).

In principle you can store information in a superposition 
of entangled qubits, but to read out all the information 
you will need to transfer the result to more qubits first.


====

An example : 

We have 3 qubits again. But they are in an entangled state. 
None of the qubits has collapsed. (I.e. the state is XXX.)
However let's assume the allowed states are only 101 and 010,
but you don't know that. However I do know, because I have 
performed some quantum algorithms that resulted in the 
elimination of all other states. (Note that any of the 3 
qubits can be either 1 or 0, but only 2 total states of the 8 
possible ones are allowed.) Now I send these 3 qubits in 
entangled state to you.

How do you get to know all the combinations that are still 
stored in these 3 qubits ?



PS: And we didn't even discuss the problem performing the 
simulation itself yet, only about reading the result out 
of the qubits.

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