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The all-inorganic lead-free vacancy-ordered perovskite offers a promising avenue toward nontoxic, stable optoelectronic materials. Herein, we present a first-principles study of the structural stability, optical absorption, electronic structure, and some mechanical behavior of Cs2BCl6, a class of materials with B-site substitutions (B = Ge, Sn, Pb, Cr, Mo, W, Ti, Zr, and Hf). The structural analysis shows that the Cs2BCl6perovskite with face-centered cubes has a stable chemical environment, especially those with high-energy B-site ions (Cs2HfCl6, Cs2WCl6, and Cs2PbCl6). Hf4+and W4+with high-energy d-state external electron configurations can further lower the valence band maximum position of the Cs2BCl6structures and thus increase the band gap, assisting in tuning the optical absorption and emission properties of these structures in the optoelectronic application. For the light absorption properties of Cs2BCl6materials, the best light absorption properties have been concluded for Ti4+, Cr4+, and Pb4+-based perovskite in the visible range due to a suitable band gap. Therefore, their excellent option absorption and electronic properties make these vacancy-ordered perovskites promising candidates for optoelectronic applications.

Polarization transfers from an electron spin to a nuclear spin are essential for various physical tasks, such as dynamic nuclear polarization in nuclear magnetic resonance and quantum information processing on hybrid electron-nuclear spin systems. We present time-optimal schemes for electron-nuclear polarization transfers which improve on conventional approaches, and we thereby establish an important class of faster controls. We highlight how time-optimal polarization transfers and their optimality are related to the time optimality of unitary transformations. Moreover, our work develops generally applicable analytic methods for analyzing the limits in controlling quantum systems. [ABSTRACT FROM AUTHOR]

Experiments in coherent nuclear and electron magnetic resonance and quantum computing in general correspond to control of quantum-mechanical systems, guiding them from initial to final target states by unitary transformations. The control inputs (pulse sequences) that accomplish these unitary transformations should take as little time as possible so as to minimize the effects of relaxation and decoherence and to optimize the sensitivity of the experiments. Here, we derive a time-optimal sequences as fundamental building blocks to synthesize unitary transformations. Such sequences can be widely implemented on various physical systems, including the simulation of effective Hamiltonians for topological quantum computing on spin lattices. Experimental demonstrations are provided for a system consisting of three nuclear spins. [ABSTRACT FROM AUTHOR]

We consider the optimal control problem of transferring population between states of a quantum system where the coupling proceeds only via intermediate states that are subject to decay. We pose the question whether it is generally possible to carry out this transfer. For a single intermediate decaying state, we recover the stimulated Raman adiabatic passage process, which we identify as the global optimum in the limit of infinite control time. We also present analytical solutions for the case of transfer that has to proceed via two consecutive intermediate decaying states. We show that in this case, for.finite power the optimal control does not approach perfect state transfer even in the infinite time limit. We generalize our findings to characterize the topologies of paths that can be achieved by coherent control under the assumption of finite power. If two or more consecutive states in an AMevel chain are subject to decay, complete population transfer with finite-power controls is not possible. [ABSTRACT FROM AUTHOR]

In this technical note, the author studies the dynamics of open quantum system in Markovian environment. The author gives necessary and sufficient conditions for such dynamics to be majorization monotone, which are those dynamics always mixing the states. [ABSTRACT FROM AUTHOR]

Various strategies exist in quantum metrology, such as with or without ancillary system, with a fixed or optimized measurement, with or without monitoring the environment, etc. Different set of tools are usually needed for different strategies. In this article, we provide a unified framework for these different settings, in particular we introduce a quantum metrology matrix and show that the precision limits of different settings can all be obtained from the trace or the trace norm of the quantum metrology matrix. Furthermore, the probe state enters into the quantum metrology matrix linearly, which makes the identification of the optimal probe states, one of the main quests in quantum metrology, much more efficient than conventional methods. [ABSTRACT FROM AUTHOR]

A pivotal task in quantum metrology, and quantum parameter estimation in general, is to design schemes that achieve the highest precision with the given resources. Standard models of quantum metrology usually assume that the dynamics is fixed and that the highest precision is achieved by preparing the optimal probe states and performing optimal measurements. However, in many practical experimental settings, additional controls are usually available to alter the dynamics. Here we propose to use optimal control methods for further improvement of the precision limit of quantum parameter estimation. We show that, by exploring the additional degree of freedom offered by the controls, a higher-precision limit can be achieved. In particular, we show that the precision limit under the controlled schemes can go beyond the constraints put by the coherent time, which is in contrast with the standard scheme where the precision limit is always bounded by the coherent time. [ABSTRACT FROM AUTHOR]

We present observable lower bounds for several bipartite entanglement measures including entanglement of formation, geometric measure of entanglement, concurrence, convex-roof extended negativity, and G concurrence. The lower bounds facilitate estimates of these entanglement measures for arbitrary finite-dimensional bipartite states. Moreover, these lower bounds can be calculated analytically from the expectation value of a single observable. Based on our results, we use several real experimental measurement data to get lower bounds of entanglement measures for these experimentally realized states. In addition, we also study the relations between entanglement measures. [ABSTRACT FROM AUTHOR]

We propose a hierarchy of Bell-type inequalities for arbitrary n-partite systems that identifies the different degrees of nonlocality ranging from standard to genuine multipartite nonlocality. After introducing the definition of nonsignaling m locality, we show that the observed joint probabilities in any nonsignaling m-local realistic models should satisfy the (m-1)th Bell-type inequality. When m=2, the corresponding inequality reduces to the one shown earlier [Q. Chen et al., Phys. Rev. Lett. 112, 140404 (2014)] whose violation indicates genuine multipartite nonlocality, and when m=n, the corresponding inequality is just Hardy's inequality whose violation indicates standard multipartite nonlocality. Furthermore, several examples are provided to demonstrate their hierarchy of multipartite nonlocality. [ABSTRACT FROM AUTHOR]