Direct interaction

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NASSER METWALLY

In this contribution, we investigate the possibility that one member of a quantum network can steer the information that encoded in the state of other member. It is assumed that, these members have a direct or indirect connections. We show that, the steerability increases at small values of the channel' strength. Although, the degree of entanglement between the direct interacted nodes is smaller than that displayed for the non-interacted nodes, the possibilities of steering a member of the direct interacted nodes and the non-direct nodes are almost similar.

Noriaki Horiuchi

Giampaolo Co’

Franck Laloë

The process of measurement plays an important role in quantum theory. Measurements can be direct, if the physical system S interacts directly with the measurement apparatus M (as we have assumed until now), or indirect. In the latter case, the physical system first interacts with an ancillary system B , which may have a space of states that is very different from that of S , for instance much larger; after this interaction has finished, M is used to perform a measurement on B , without any direct interaction with A . Because S is then “protected” from any direct interaction with the measurement apparatus, the state of S is not necessarily strongly modified, and may even be only weakly affected. In both cases, the process implies entanglement between several physical systems. In this chapter, we study how this entanglement is created and used for measurements as well as the notion of weak and continuous measurements. These questions play an important role in several of the interpretations of quantum mechanics that we discuss in Chapter 10. Direct measurements The Von Neumann model of quantum measurement [4] provides a general frame for describing the process in terms of correlations appearing (or disappearing) in the state vector associated with the whole system S + M . In this model, the two systems S and M are initially described by a product state ∣ψ 0 〈 and interact during the time of measurement, so that they become entangled; they then reach a final state ∣ψ′〈 and do not interact anymore.

Shang-Yu Zhai, Rui Shi, Fu-Han Xie

In this work, a Y-type energy level ensemble of an alkali metal Rydberg rubidium cold atomic system under complex interactions is considered. To solve this nonlinear optical response, Newton’s iterative method is employed with random initial values for solving the multi-body density matrix equations and further observing the Rydberg excitation’s instability. The effects of self-interaction and direct and exchange interactions in the cross-interactions are comprehensively considered via three real cases: (I) only self-interaction; (II) self-interaction and direct interaction; and (III) self-interaction, direct interaction, and exchange interaction simultaneously. The results indicate that under the condition of I, since the system only has self-interaction, it exhibits only a typical bistable phenomenon. With the introduction of direct interaction, the Rydberg population in II shows a bifurcated bistable phenomenon. Finally, a more complex exchange interaction is further introduced. Under the condition of III, the system exhibits a complex multi-stable phenomenon in the same detuning direction. More importantly, such bistable and multi-stable phenomena can be well regulated by changing the cooperative nonlinearity modulated via the pump field. In addition, the bistable and multi-stable atomic excitation responses of the system on one transition branch can also be controlled by the coupling field Rabi frequency on another transition branch. This work applies numerical iterative algorithms to achieve a detailed analysis of the new phenomena with bistable and multi-stable excitation responses for nonlinear characteristics and provides theoretical support for the preparation of optical multistable materials.