Yu-Ran Zhang

Crystal structure and magnetic properties of SmCo7-xHfx compounds

The crystal structure and magnetic properties of SmCo7-xHfx (x=0.05,0.1,0.2,0.3) compounds were studied by means of x-ray powder diffraction and magnetic measurements. The as-cast compounds SmCo7-xHfx with x=0.1 and 0.2 crystallize in the TbCu7-type structure with the space group P6/mmm. According to the structure refinement, the doping element Hf prefers to occupy the 2e site. The compounds have high Curie temperatures (1080 K for x=0.1, 1075 K for x=0.2), large saturation magnetization (104 emu/g for x=0.1, 102 emu/g for x=0.2 at 5 K), and large magnetic anisotropy fields (244 kOe for x=0.1, 282 kOe for x=0.2 at 5 K) with strong uniaxial magnetocrystalline anisotropy

Time-correspondence differential ghost imaging

Experimental data with digital masks and a theoretical analysis are presented for an imaging scheme that we call time-correspondence differential ghost imaging (TCDGI). It is shown that by conditional averaging of the information from the reference detector but with the negative signals inverted, the quality of the reconstructed images is in general superior to all other ghost imaging (GI) methods to date. The advantages of both differential GI and time-correspondence GI are combined, plus less data manipulation and shorter computation time are required to obtain equivalent quality images under the same conditions. This TCDGI method offers a general approach applicable to all GI techniques, especially when objects with continuous gray tones are involved. DOI: 10.1103/PhysRevA.87.033813

General monogamy property of global quantum discord and the application

We provide a family of general monogamy inequalities for global quantum discord (GQD), which can be considered as an extension of the usual discord monogamy inequality. It can be shown that those inequalities are satisfied under the similar condition for the holding of usual monogamy relation. We find that there is an intrinsic connection among them. Furthermore, we present a different type of monogamy inequality and prove that it holds under the condition that the bipartite GQDs do not increase when tracing out some subsystems. We also study the residual GQD based on the second type of monogamy inequality. As applications of those quantities, we investigate the GQDs and residual GQD in characterizing the quantum phase transition in the transverse field Ising model.

Quantum-enhanced metrology for multiple phase estimation with noise

We present a general quantum metrology framework to study the simultaneous estimation of multiple phases in the presence of noise as a discretized model for phase imaging. This approach can lead to nontrivial bounds of the precision for multiphase estimation. Our results show that simultaneous estimation (SE) of multiple phases is always better than individual estimation (IE) of each phase even in noisy environment. The utility of the bounds of multiple phase estimation for photon loss channels is exemplified explicitly. When noise is low, those bounds possess the Heisenberg scale showing quantum-enhanced precision with the O(d) advantage for SE, where d is the number of phases. However, this O(d) advantage of SE scheme in the variance of the estimation may disappear asymptotically when photon loss becomes significant and then only a constant advantage over that of IE scheme demonstrates. Potential application of those results is presented.

Quantifying Coherence in Infinite Dimensional Systems

We study the quantification of coherence in infinite dimensio nal systems, especially the infinite dimensional bosonic systems in Fock space. We show that given the energy constraints, the relative entropy of coherence serves as a well-defined quantification of coherence in infini te dimensional systems. Via using the relative entropy of coherence, we also generalize the problem to multi-mode Fock space and special examples are considered. It is shown that with a finite average particle nu mber, increasing the number of modes of light can enhance the relative entropy of coherence. With the mean energy constraint, our results can also be extended to other infinite-dimensional systems. Quantum coherence arising from quantum superposition principle is a fundamental aspect of quantum physics [1]. The laser [2] and superfluidity [3] are examples of quantum coherence, whose effects are evident at the macroscopic scale. However, the framework of quantification of coherence has only been methodically investigated recently. The first attempt to address the classification of quantum coherence as physical resources by T. Baumgratz et. al., who have established a rigorous framework for the quantification of coherence based on distance measures in finite dimensional setting [4]. With such a fundational framework for coherence, one can find the appropriate distance measures to quantify co herence in a fixed basis by measuring the distance between the quantum state ˆ � and its nearest incoherent state. After the framework for coherence has been proposed, it receives increasing attentions. A. Streltsov et. al. have used entanglement to quantify quantum coherence, which provides the operational quantification of coherence [8]. S. Du et. al. focused on the interconversion of coherent states by means of incoherent operations using the concept of majorization relations [7]. Z. Xi et. al. have given a clear quantitative analysis and operational connections between relative entropy of coherence, quantum discord and one-way quantum deficit in the bipartite quantum system [6]. T. Bromley et. al. have found freezing conditions in which coherence remains unchanged during the nonunitary dynamics [5]. Up to now, all the results for quantifying the quantum coherence are assumed the finite dimensional setting, which is neither necessary nor desirable. In consideration of the relevant physical situations such as q uantum optics states of light, it must require further investig ations on infinite dimensional systems. In this paper, we aim to investigate the quantification of coherence in infinite dimensional systems. Specificly, we fo cus on the infinite dimensional bosonic systems in Fock space [10] which are used to describe the most notable quantum optics states of light [11] and Gaussian states [12‐14]. We show that when considering the energy constraints, the relative en

A double-threshold technique for fast time-correspondence imaging

We present a robust imaging method based on time-correspondence imaging and normalized ghost imaging (GI) that sets two thresholds to select the reference frame exposures for image reconstruction. This double-threshold time-correspondence imaging protocol always gives better quality and signal-to-noise ratio than previous GI schemes, and is insensitive to surrounding noise. Moreover, only simple add and minus operations are required while less data storage space and computing time are consumed; thus, faster imaging speeds are attainable. The protocol offers a general approach applicable to all GI techniques and marks a further step forward towards real-time practical applications of correlation imaging.

Coherence susceptibility as a probe of quantum phase transitions

We introduce a coherence susceptibility method, based on the fact that it signals quantum fluctuations, for identifying quantum phase transitions, which are induced by quantum fluctuations. This method requires no prior knowledge of order parameter, and there is no need for careful considerations concerning the choice of a bipartition of the system. It can identify different types of quantum phase transition points exactly. At finite temperatures, where quantum criticality is influenced by thermal fluctuations, our method can pinpoint the temperature frame of quantum criticality, which perfectly coincides with recent experiments.

Zeno dynamics in quantum open systems

Quantum Zeno effect shows that frequent observations can slow down or even stop the unitary time evolution of an unstable quantum system. This effect can also be regarded as a physical consequence of the statistical indistinguishability of neighboring quantum states. The accessibility of quantum Zeno dynamics under unitary time evolution can be quantitatively estimated by quantum Zeno time in terms of Fisher information. In this work, we investigate the accessibility of quantum Zeno dynamics in quantum open systems by calculating noisy Fisher information when a trace preserving and completely positive map is assumed. We firstly study the consequences of non-Markovian noise on quantum Zeno effect and give the exact forms of the dissipative Fisher information and the quantum Zeno time. Then, for the operator-sum representation, an achievable upper bound of the quantum Zeno time is given with the help of the results in noisy quantum metrology. It is of significance that the noise reducing the accuracy in the entanglement-enhanced parameter estimation can conversely be favorable for the accessibility of quantum Zeno dynamics of entangled states.

Demonstration of entanglement-enhanced phase estimation in solid

Precise parameter estimation plays a central role in science and technology. The statistical error in estimation can be decreased by repeating measurement, leading to that the resultant uncertainty of the estimated parameter is proportional to the square root of the number of repetitions in accordance with the central limit theorem. Quantum parameter estimation, an emerging field of quantum technology, aims to use quantum resources to yield higher statistical precision than classical approaches. Here we report the first room-temperature implementation of entanglement-enhanced phase estimation in a solid-state system: the nitrogen-vacancy centre in pure diamond. We demonstrate a super-resolving phase measurement with two entangled qubits of different physical realizations: an nitrogen-vacancy centre electron spin and a proximal 13C nuclear spin. The experimental data shows clearly the uncertainty reduction when entanglement resource is used, confirming the theoretical expectation. Our results represent an elemental demonstration of enhancement of quantum metrology against classical procedure.

High-Pressure and High-Temperature in situ X-Ray Diffraction Study of FeP2 up to 70 GPa

The high-pressure and high-temperature structural behavior of FeP2 is investigated by means of synchrotron x-ray powder diffraction combined with a laser heating technique up to 70GPa and at least 1800 K. No phase transition of FeP2 occurs up to 68 GPa at room temperature. While a new phase of FeP2 assigned to the CuAl2-type structure (I4/mcm, Z = 4) is observed at 70 GPa after laser-heating. This new phase presents a quenchable property on decompression to ambient conditions. Our results update previous experimental data and are consistent with theoretical studies.