Conversation

1/ We all love quantum mechanics (amiright?) But if you wait a long enough time, quantum mechanics usually becomes irrelevant for the dynamics of a many-body system. That's because quantum systems tend to interact with their environment, which messes them up.
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2/ In technical terms, interaction with the environment causes "decoherence", in which a quantum system loses its phase information and essentially becomes an inert classical lump that governed by (usually stochastic) classical laws of motion.
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3/ But in a just-published paper in (open access), my colleage Adam Nahum and I showed that this doesn't have to be the case. We studied a model system for which the dynamics at arbitrarily late times is fundamentally non-classical.
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4/ The key is to encode some of the quantum information non-locally ("topologically"), so that it is accessible only by making simultaneous measurements on two distant particles. This way the environment can't mess it up, because there is no local quantity that couples to it.
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5/ So even as other variables, like the particle positions, get scrambled by the environment, certain aspects remain quantum-entangled forever, and can play a key role in driving the dynamics.
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6/ In this type of system, answering even basic questions like "how many particles are left in the system after a long time?" requires reference to quantum entanglement, and there is no classical dynamical procedure that produces the same behavior.
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7/ One of the weirdest features is that, instead of destroying quantum entanglement, random buffeting from the environment tends to make the entanglement _grow_: producing ever more pairs of distantly-spaced entangled pairs. (it produces a "critically entangled state")
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