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Passive states of quantum systems are states from which no system energy can be extracted by any cyclic (unitary) process. Gibbs states of all temperatures are passive. Strong local (SL) passive states are defined to allow any general quantum operation, but the operation is required to be local, being applied only to a specific subsystem. Any mixture of eigenstates in a system-dependent neighborhood of a nondegenerate entangled ground state is found to be SL passive. In particular, Gibbs states are SL passive with respect to a subsystem only at or below a critical system-dependent temperature. SL passivity is associated in many-body systems with the presence of ground state entanglement in a way suggestive of collective quantum phenomena such as quantum phase transitions, superconductivity, and the quantum Hall effect. The presence of SL passivity is detailed for some simple spin systems where it is found that SL passivity is neither confined to systems of only a few particles nor limited to the near vicinity of the ground state.

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An upper bound B 1 is given for the information capacity of the Poisson channel with marginally stationary random noise intensity and causal feedback. The capacity is shown to converge to B 1 in the limit of longer and longer communication intervals for a class of random noise intensities including the case in which the noise intensity is nontime-varying. An upper bound B 1 on the capacity is also established for the Poisson channel with marginally stationary noise and no feedback. In this case also, for a class of random noise intensities including nontime-varying noise intensity, B. 1 is found to be the capacity of the channel without feedback in the limit of longer and longer communication intervals. The fractional difference between B 1 and B 1 is considered as a means to quantify the improvement afforded by feedback. Also, certain nonstandard encoder constraints are addressed and the importance of the encoder intensity peak constraint to the channel capacity problem is explored.

For the quantum depolarizing channel with any finite dimension, we compare three schemes for channel identification: unentangled probes, probes maximally entangled with an external ancilla, and maximally entangled probe pairs. This comparison includes cases where the ancilla is itself depolarizing and where the probe is circulated back through the channel before measurement. Compared on the basis of (quantum Fisher) information gained per channel use, we find broadly that entanglement with an ancilla dominates the other two schemes, but only if entanglement is cheap relative to the cost per channel use and only if the external ancilla is well shielded from depolarization. We arrive at these results by a relatively simple analytical means. A separate, more complicated analysis for partially entangled probes shows for the qudit depolarizing channel that any amount of probe entanglement is advantageous and that the greatest advantage comes with maximal entanglement.