Transmitting quantum information by superposing causal order of mutually unbiased measurements

Two quantum measurements sequentially acting one after the other, if they are mutually unbiased, will lead to a complete removal of information encoded in the input quantum state. We find that if the order of the two sequential measurements can be superposed, with a quantum switch, then the information encoded in the input can still be retained in the output state.Comment: 8 Pages, 1 figur

A no-go theorem for superpositions of causal orders

The causal order of events need not be fixed: whether a bus arrives before or after another at a certain stop can depend on other variables---like traffic. Coherent quantum control of causal order is also possible and is a useful resource for several tasks. However, quantum control implies that a controlling system carries the which-order information---if the control is traced out, the order of events remains in a probabilistic mixture. Can the order of two events be in a pure superposition, uncorrelated with any other system? Here a negative result is presented, showing that it is not possible to have a superposition of two Markovian, unitary quantum processes with different causal orders and equal local dimensions. The result imposes constraints on novel resources for quantum information processing and on possible processes in a theory of quantum gravity.Comment: 4+3 pages. A wall of text with no figure