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Register a new userQuantum and Classical Data Transmission Through Completely Depolarising Channels in a Superposition of Cyclic Orders
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Completely depolarising channels, which transform every input state into white noise, are often regarded as the prototype of physical processes that are useless for communication. Here we show that the ability to combine N completely depolarising channels in a superposition of N alternative causal orders related by cyclic permutations enables a high-fidelity, heralded transmission of quantum information in the large N limit, and a deterministic transmission of quantum data assisted by classical communication for every N > 13. These phenomena highlight a fundamental difference with the N = 2 case, where the superposition of causal orders does not enable transmission of quantum data through completely depolarising channels. In addition, the ability to control N > 2 causal orders leads to an increase of the classical communication capacity, for which we derive an exact single-letter expression.
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We consider the transfer of classical and quantum information through a memory amplitude damping channel. Such a quantum channel is modeled as a damped harmonic oscillator, the interaction between the information carriers - a train of qubits - and the oscillator being of the Jaynes-Cummings kind. We prove that this memory channel is forgetful, so that quantum coding theorems hold for its capacities. We analyze entropic quantities relative to two uses of this channel. We show that memory effects improve the channel aptitude to transmit both classical and quantum information, and we investigate the mechanism by which memory acts in changing the channel transmission properties.
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