Exciting channels

Jun Chen, Yu Ya-Fei, Zhi‐Ming Zhang

The transfer of quantum states between distant nodes is one of the most fundamental tasks in quantum-information processing. Recent studies show that the antiferromagnetic spin chain initially prepared in a multi-excitation state can provide suitable pathways for quantum state transfer. In this paper, we investigate the quality of state transfer through a uniformly coupled antiferromagnetic spin chain where the initial state of the channel varies with the number of spin excitations. Firstly, by analyzing the dynamics of observables for the output qubit using the information-flux approach, the explicit relation about how the average fidelity of state transfer depends on the initial state of the spin channel is obtained. The results show that the average fidelity of state transfer through a multi-excitation spin channel only relates to the parity of the number of spin excitations in the channel. Then we compare the maximum average fidelity of state transfer through the odd-excitation with those through the even-excitation spin channels, and provide a simple criterion to optimize the quality of state transfer by choosing appropriate channels from the odd-excitation and the even-excitation channels. Compared with the previous studies which initialize the chains into the ground state of the ferromagnetic medium or the Nel state, the maximum average fidelity of state transfer is evidently enhanced by using the optimized channel. Moreover, we analyze the entanglement distribution through the channel having different number of spin excitations via the information-flux approach. It is found that the quality of entanglement distribution not only relates to the number of initial spin excitations present in the channel, but also depends on the initial ordering of these excited spins. The numerical results suggest that the amount of distributed entanglement and duration of distribution in the channel where all spins are down or up are larger than those in other excited channels. Based on these results, we can choose appropriate quantum channels for state transfer and entanglement distribution in practice.

G. Giedke, Michael M. Wolf

Jihad S. Addasi

Saturation of complex molecules affects the nonlinear processes occurring in the medium. The medium can be described by two-, three-, and four-level configurations. The principal singlet-singlet and excited triplet-triplet channels of a four-level, N-type, configuration were used to study a nonlinear medium, which was excited by radiation with two frequencies. Under weak population of energy levels in the triplet-triplet channel (a small ratio of the total probability of spontaneous and irradiate transitions for both channels p23/p31), the principal (singlet-singlet) channel is similar to a two-level configuration. At the same time, the effective population of the energy levels in the triplet-triplet channel (the ratio p23/p31 is large and the radiation intensity in the principal channel is high) makes the excited (triplet-triplet) channel coincide with the two-level configuration. The saturation intensity of radiation in principal (excited) channel, as effective two-level configuration, and nonlinear processes in this channel can be controlled by external radiation acting on excited (principal) channel. Finally, the average excitation of molecules into the energy levels in the triplet-triplet channel (ratio p23/p31 ≈ 1) makes the four-level configuration similar to the three-level configuration with effective level as a result of combination between second singlet level S2 and first triplet level T1.

Jungsang Kim, Seema Somani, Y. Yamamoto

Xu Zhao, Xing-Dong Zhao, Lu Zhou +2 more

We investigate the quantum interference of spin wave excitations of a spin-1 atomic Bose condensate confined in an optical lattice. Single-channel and dual-channel interactions are employed in our system, and their induced excitations are compared. Also we consider the interplay of magneto-optical excitations, which leads to a constructive or destructive effect for the creation of magnons based on background excitations. The population distributions of excited magnons can be well controlled by steering the long-range dipole-dipole interactions. Such a scheme can be used to demonstrate conventional quantum-optical phenomena like dynamical Casimir effect at finite temperatures.

James M. McCracken