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Suppose that fast quantum random number generator (QRNG, https://qrng.physik.hu-berlin.de/) is placed in a subsystem which has limited interaction with the rest of the world. What would happen if decoherence rate for this subsystem were not fast enough to extract all the random numbers being created by QRNG?

It seems that then the subsystem has to evolve into a state with macroscopic coherent properties (Schrödinger cat state). At least this occurs if one simulate QRNG using measurement of a rotated qubit and copying a result in a vector of qubits representing a (partly) closed subsystem.

If so, why the question about feasibility of macroscopic coherent states is still not settled?

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    $\begingroup$ Why do you think the question about feasibility of macroscopic coherent states is still not settled? References? $\endgroup$ – Peter Shor May 21 '13 at 12:50
  • $\begingroup$ @PeterShor Sorry, I have no references. I thought opponents of the idea of scalable quantum computer argue that macroscopic coherent states rapidly decay. I just wanted to show an example where coherent states exist not due to total absence of decoherence but because decoherence rate is not large enough. $\endgroup$ – dvs May 21 '13 at 13:31

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