# Can you read and write a quantum state to a particle without collapsing its probability field?

I know the main problem with a quantum computer is that you can't read and write data from it until after it has run though all of its calculations. But would reading and writing quantum data to particles without collapsing their probability fields make this possible?

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The many worlds interpretation of quantum mechanics by Hugh Everett does not collapse the wave function. Maybe there is a way to apply this to quantum computing. – Heisenberg Feb 6 '14 at 19:23
Writing information to it before a computation is no problem. After it has started, you cannot touch it until it is finished, at which time you can read it. You may not get the right answer. You have to arrange the computation so that the right answer has an acceptably high probability. – Mike Dunlavey Feb 6 '14 at 21:05

With constant conditions - No. Because it fails energy/information conservation laws.

But you are able to changle temperature to travel between "macro" and "micro". Write information to macro - freeze - get micro particle with quantum condition.

But the quantum state is so unstable - so developers want to find something more stable but still quantum mechanical.

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You can write any quantum state to a (sub-)system as you like. The thing you need is to set an deterministic initial state and use a known unitary operator to let the system evolve to the state you want. But note that this operation itself is effectively performing a quantum computation.

In contrast, you cannot read out the state of the system in the middle of computation. Since the act itself destroy the quantum state, so you cannot know the system state.

You should know that there is no individual state in quantum computation, the whole system itself is a single state. So it doesn't make sense to read or write individual qubit during computation, in most case.

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Reading (or writing) in the middle of a computation is exactly what I am seeking to do. If we discovered a way to read the particle, without disrupting the quantum state, could we do this to determine intermediate values? (by using advanced technology). Or is this theoretically impossible? – Aaron Klap Feb 6 '14 at 22:40
@AaronKlap With your assumption, yes. But in reality, it is impossible theoretically. The quantum mechanics does not allow it to happen – hwlau Feb 7 '14 at 0:03
So, what I am hearing is that if my impossible assumption is true, then my scenario is possible. Yessss! ;) – Aaron Klap Feb 7 '14 at 0:29
@AaronKlap It is physical world, please don't have too much imaginary :) – hwlau Feb 7 '14 at 0:43