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Presently, how much information can a quantum computer store, in how many qubits? What restrictions are there and how does it vary across realizations (efficiency of data storage, ease of reading and writing, etc)?

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  • $\begingroup$ It's already explained here. Basically, with x amount of qubits, you can have 2^x different values at the same time. $\endgroup$ – Juan González Salinas Mar 19 '18 at 10:12
  • $\begingroup$ I understand this is theoretically possible but I’m interested in what has been demonstrated. And potentially more interesting which realization (optical, ion, superconducting) currently allows the greatest ease of writing the data to a significant number of qbits. $\endgroup$ – Young Mar 19 '18 at 14:52
  • $\begingroup$ Maybe the question doesn’t even make sense because I’m having trouble visualizing the writing of 2^N data. Is data storage only inherent to the algorithm implementation (coefficients generated by coupling and operations)? $\endgroup$ – Young Mar 19 '18 at 15:04
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    $\begingroup$ @JuanGonzálezSalinas I don't believe it has been asked; you're not looking at the OP's comments carefully. The OP is asking about the current state of quantum memory in experiment, and about the ease of reading and writing in different implementations, and what a circuit to store memory might look like, if that's possible. $\endgroup$ – heather Mar 19 '18 at 15:54
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    $\begingroup$ Note that the question in the title and the one in the question are two entirely different things. The latter is a basic question the answer of which you can find in many places online (here is an answer of mine on physics.SE answering basically that same question). On the other hand, the question in the title is interesting and worth asking, and refers to the experimental state of the art in implementing quantum memories (it may still be a bit broad though). Could you edit the post accordingly? $\endgroup$ – glS Mar 19 '18 at 16:05
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Unfortunately the state of the technology regarding memories is not as developed as you seem to expect. When we talk about a memory, we think of a device that can store information for an infinite amount of time (for all practical purposes). So before we can think about the size of the memory in a quantum computer, we should look at whether a single quantum memory has been built. There is a lot of progress in this direction, but to my knowledge the currently best "memory" achieved a coherence time of about 6 hours (which is amazing, but still not what we are used from classical computers). Although the fidelity of the retrieved state is in the high nineties, the success probability for storage and readout is very low.

There is also work on using error correction codes to built a memory, but those approaches do not give better results so far.

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