It is well known that quantum switch is an example of indefinite causal order. Recently, application of quantum switch on quantum channels, became a hot topic of discussion. It is possible to achieve significant improvement in communication, when a q
uantum switch is applied on quantum channels. Though above-said improvement is not possible for all quantum channels. For some quantum channels, improvement can be very high. One such example has been discussed in [New J. of Phys. 23, 033039 (2021)] where perfect communication can be achieved. But incidentally that example of channel is unique up to unitary transformations. Therefore, it is important to study the application of quantum switch on other quantum channels where improvement may not be ultimate but significant. Here, we study the application of quantum switch on various quantum channels. In particular we show that if it is not possible to achieve improvement deterministically, it may be possible to achieve improvement probabilistically. It is known that if a quantum channel is useless for some information theoretic task, concatenation of quantum channels generally does not provide any advantage whenever that channel is used. But we show that if a quantum channel is useless even after use of quantum switch, concatenation of quantum channel can make it useful. We also show that quantum switch can help to get quantum advantage in quantum random access code when only useless channels are available for communication. Then we show that quantum switch can be useful to prevent the loss of coherence in a quantum system. We also discuss the fact that if noise is introduced in the switch, then improvement can significantly be decreased.
Quantum walk is a synonym for multi-path interference and faster spread of a particle in a superposition of position space. We study the effects of a quantum mechanical interaction modeled to mimic quantum mechanical gravitational interaction between
the two states of the walkers. The study has been carried out to investigate the entanglement generation between the two quantum walkers that do not otherwise interact. We see that the states do in fact get entangled more and more as the quantum walks unfold, and there is an interesting dependence of entanglement generation on the mass of the two particles performing the walks. We also show the sensitivity of entanglement between the two walkers on the noise introduced in one of the walks. The signature of quantum effects due to gravitational interactions highlights the potential role of quantum systems in probing the nature of gravity.
