[ExI] for the fermi paradox fans

[Anders Sandberg]

Ah, forgot to add to the previous post:
http://arxiv.org/abs/1301.1995 shows that depending on the kind of noise error correcting quantum computations can go on for different length of time before noise derails them. For depolarizing noise the limit is logarithmic in the number of bits, but for dephasing noise there is a tradeoff where it is possible to compute on O(n^a)) qubits for O(n^b) time stepsif a+b < 1, and impossible even to store a single (unknown) qubit for more than O(n^3) time steps. For amplitude dampening noise computations can go on for exponential time. They show this using a very fun "quantum refrigerator" trick where "dirty" qubits are left to be further randomized by the noise: since some of them get put into useful states they can be re-used as if they were clean, "cold" qubits. 
These results imply again that future cold intelligences may have fairly frequent retracing loops, especially since there may be limits to how long a working superposed qubit can be kept in memory. 

Anders Sandberg, Future of Humanity Institute Philosophy Faculty of Oxford University
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