Applications of the quantum switch on quantum channels have recently become a topic of intense discussion. In the present work, we show that some useless (for communication) channels may provide useful communication under the action of quantum switch for several information-theoretic tasks: quantum random access codes, quantum steering, etc. We demonstrate that the quantum switch can also be useful in preventing the loss of coherence in a system when only coherence-breaking channels are the available channels for communication. We also show that if a useless quantum channel does not provide useful communication even after using a quantum switch, concatenating the channel with another suitable quantum channel, and subsequently using the switch, one may achieve useful communication. Finally, we discuss how the introduction of noise in the quantum switch can reduce the advantage that the switch provides.
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. With the introduction of noise into the dynamics, 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.
Discrete versions of quantum walks, just like its classical counterpart, comprise of the external position space and the internal coin space. The interactions between the two Hilbert spaces due to quantum dynamics greatly influence the properties of the walk and have important consequences for the quantum algorithms and quantum simulations protocols they are used for. In this work, we study the effect of non-Hermitian evolution on the interactions between the coin and the position space. Such an evolution mimics a quantum walk which is interacting with an external environment. To understand this interaction, we study the non-Markovianity of the reduced dynamics and also the entanglement between the two spaces. The non-Hermitian evolution is studied from two perspectives: the normalised state method and the more recently proposed metric formalism. The results suggest that the metric formulation provides a more accurate description of the non-Hermitian and PTsymmetric evolution, describing trace preserving maps. We also show that the non-unitary walks preserve the purity under the metric formulations, due to which one can study entanglement under non-Hermiticity using entanglement entropy.
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