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Estimating properties of a quantum state is an indispensable task in various applications of quantum information processing. To predict properties in the postprocessing stage, it is inherent to first perceive the quantum state with a measurement protocol and store the information acquired. In this paper, we propose a general framework for constructing classical approximations of arbitrary quantum states with quantum reservoirs. A key advantage of our method is that only a single local measurement setting is required for estimating arbitrary properties, while most of the previous methods need an exponentially increasing number of measurement settings. To estimate M properties simultaneously, the size of the classical approximation scales as lnM. Moreover, this estimation scheme is extendable to higher-dimensional systems and hybrid systems with nonidentical local dimensions, which makes it exceptionally generic. We support our theoretical findings with extensive numerical simulations.

Layered materials, such as graphite/graphene, boron nitride, transition metal dichalcogenides, represent materials in which reduced size, dimensionality, and symmetry play critical roles in their physical properties. Here, we report on a comprehensive investigation of the phonon properties in the topological insulator Bi2Te3 and Bi2Se3 two-dimensional (2D) crystals, with the combination of Raman spectroscopy, first-principles calculations, and group theory analysis. Low frequency (<30cm-1) interlayer vibrational modes are revealed in few-quintuple-layer (QL) Bi2Te3/Bi2Se3 2D crystals, which are absent in the bulk crystal as a result of different symmetries. The experimentally observed interlayer shear and breathing mode frequencies both show blueshifts, with decreasing thickness in few-QL Bi2Te3 (down to 2QL) and Bi2Se3 (down to 1QL), in agreement with first-principles calculations and a linear chain model, from which the interlayer coupling force constants can be estimated. Besides, an intense ultralow (<12cm-1) frequency peak is observed in 2-4QL Bi2Te3, which is tentatively attributed to a substrate-induced interface mode supported by a linear chain model analysis. The high frequency Raman peaks exhibit frequency shifts and broadening from 3D to 2D as a result of the phonon confinement effect. Our studies shed light on a general understanding of the influence of dimensionality and crystal symmetry on the phonon properties in layered materials. [ABSTRACT FROM AUTHOR]

We report observations of coherent phonon oscillations in solution-processed polycrystalline stibnite (Sb2S3) photovoltaic thin films using transient absorption spectroscopy. Detailed optical spectroscopy correlated by extensive first-principles lattice dynamics calculations indicates that the coherent B3s longitudinal optical phonon mode with a frequency of 63.74 ± 0.05 cm -1 (or 1.911 ± 0.002 THz) at room temperature is generated via the impulsive stimulated-Raman-scattering mechanism. These strong electron-phonon interactions indicate a dominant energy-loss channel in these materials that could impose a fundamental limit on their solar energy conversion efficiency. [ABSTRACT FROM AUTHOR]

The layered dichalcogenide MoS2 has many unique physical properties in low dimensions. Recent experimental Raman spectroscopies report an anomalous blue shift of the in-plane E2g1 mode with decreasing thickness, a trend that is not understood. Here, we combine experimental Raman scattering and theoretical studies to clarify and explain this trend. Special attention is given to understanding the surface effect on Raman frequencies by using a force constants model based on first-principles calculations. Surface effects refer to the larger Mo-S force constants at the surface of thin film MoS2, which results from a loss of neighbours in adjacent MoS2 layers. Without surface effects, the frequencies of both out-of-plane A1g and in-plane E2g1 modes decrease with decreasing thickness. However, the E2g1 mode blue shifts while the A1g mode red shifts once the surface effect is included, in agreement with the experiment. Our results show that competition between the thickness effect and the surface effect determines the mechanical properties of two-dimensional MoS2, which we believe applies to other layered materials. [ABSTRACT FROM AUTHOR]

Bi2S3 has shown promise in thermoelectric and optoelectronic applications as well as biological and chemical sensors. We present here a comprehensive study on the lattice dynamics of Bi2S3 nanostructures probed by micro-Raman scattering spectroscopy and first-principles calculations. Bi2S3 nanowires are synthesized using a physical vapor transport method via a vapor-liquid-solid mechanism on silicon substrates. Oriented Bi2S3 nanosheets are also obtained on mica substrates. The structure of the nanowires is determined to be orthorhombic with a growth orientation of [110] by x-ray diffraction and high-resolution transmission electron microscopy. A Raman scattering study is conducted for as-prepared Bi2S3 nanostmctures, in which 33, 38, 46, and 53 cm-1 phonon modes are observed for the first time. We find several modes to be very sensitive to excitation wavelength and power. First-principles calculations of orthorhombic Bi2S3 predict a series of Raman modes, in good agreement with our experiments. Phonon-dispersion curves of Bi2S3 are also presented, and the effect of Born effective charges on the longitudinal-optical-transverse-optical splitting at the zone center is taken into account. [ABSTRACT FROM AUTHOR]

We report the direct observation of strong coupling between magnons and phonons in a two-dimensional antiferromagnetic semiconductor FePS3, via magneto-Raman spectroscopy at magnetic fields up to 30 Tesla. A Raman-active magnon at 121 cm-1 is identified through Zeeman splitting in an applied magnetic field. At a field-driven resonance with a nearby phonon mode, a hybridized magnon-phonon quasiparticle is formed due to strong coupling between the two modes. We develop a microscopic model of the strong coupling in the two-dimensional magnetic lattice, which enables us to elucidate the nature of the emergent quasiparticle. Our polarized Raman results directly show that the magnons transfer their spin angular momentum to the phonons and generate phonon spin through the strong coupling. [ABSTRACT FROM AUTHOR]

Organolead halide perovskites have emerged as potential building blocks for photovoltaic and optoelectronic devices. Yet the underlying fundamental physics is not well understood. There is lack of agreement on the electronic band structures and binding energies of coupled electron-hole pairs (excitons), which drive the photophysical processes. In this work, we conducted temperature-dependent reflectance and photoluminescence experiments on high-quality CH3 NH3 PbBr3 single crystals. Two direct optical transitions corresponding to intrinsic free-excitons are clearly resolved, showing excellent consistence between the low-temperature (T = 10 K) reflectance and photoluminescence spectra. Remarkably, the excitons have different binding energies and behave oppositely with temperature, suggesting distinctive origins. Moreover, the asymmetric photoluminescence profile is counterintuitively dominated by the high-energy exciton that is explained by a long relaxation time between levels and by the favorable generation rate of electron-hole pairs at the high-energy band. Our study opens access to the intrinsic properties of CH3 NH3 PbBr3 and sheds light to reconcile the large range of binding energies reported on these emergent direct band-gap semiconductors. [ABSTRACT FROM AUTHOR]