Phase-Dependent Phonon Heat Transport in Nanoscale Gallium Oxide Thin Films.

Different phases of Ga2O3 have been regarded as superior platforms for making new-generation high-performance electronic devices. However, understanding of thermal transport in different phases of nanoscale Ga2O3 thin-films remains challenging, owing to the lack of phonon transport models and systematic experimental investigations. Here, thermal conductivity (TC) and thermal boundary conductance (TBC) of the ( 1 ¯ 010 ) $( {\bar 1010} )$ α-, ( 2 ¯ 01 ) $( {\bar 201} )\;$ ß-, and (001) κ-Ga2O3 thin films on sapphire are investigated. At ≈80 nm, the measured TC of α (8.8 W m-1 K-1) is ≈1.8 times and ≈3.0 times larger than that of ß and κ, respectively, consistent with model based on density functional theory (DFT), whereas the model reveals a similar TC for the bulk α- and ß-Ga2O3. The observed phase- and size-dependence of TC is discussed thoroughly with phonon transport properties such as phonon mean free path and group velocity. The measured TBC at Ga2O3/sapphire interface is analyzed with diffuse mismatch model using DFT-derived full phonon dispersion relation. Phonon spectral distribution of density of states, transmission coefficients, and group velocity are studied to understand the phase-dependence of TBC. This study provides insight into the fundamental phonon transport mechanism in Ga2O3 thin films and paves the way for improved thermal management of high-power Ga2O3-based devices.

Certifying Network Topologies and Nonlocalities of Triangle Quantum Networks.

Quantum networks promise unprecedented advantages in information processing and open up intriguing new opportunities in fundamental research, where network topology and network nonlocality fundamentally underlie these applications. Hence, the detections of network topology and nonlocality are crucial, which, however, remain an open problem. Here, we conceive and experimentally demonstrate to determine the network topology and network nonlocality hosted by a triangle quantum network comprising three parties, within and beyond Bell theorem, with a general witness operator for the first time. We anticipate that this unique approach may stimulate further studies toward the efficient characterization of large complex quantum networks so as to better harness the advantage of quantum networks for quantum information applications.

Observation of Non-Hermitian Edge Burst Effect in One-Dimensional Photonic Quantum Walk.

Non-Hermitian systems can exhibit unique quantum phases without any Hermitian counterparts. For example, the latest theoretical studies predict a new surprising phenomenon that bulk bands can localize and dissipate prominently at the system boundary, which is dubbed the non-Hermitian edge burst effect. Here we realize a one-dimensional non-Hermitian Su-Schrieffer-Heeger lattice with bulk translation symmetry implemented with a photonic quantum walk. Employing time-resolved single-photon detection to characterize the chiral motion and boundary localization of bulk bands, we determine experimentally that the dynamics underlying the non-Hermitian edge burst effect is due to the interplay of non-Hermitian skin effect and imaginary band gap closing. This new non-Hermitian physical effect deepens our understanding of quantum dynamics in open quantum systems.

Testing Heisenberg-Type Measurement Uncertainty Relations of Three Observables.

Heisenberg-type measurement uncertainty relations (MURs) of two quantum observables are essential for contemporary research in quantum foundations and quantum information science. Going beyond, here we report the first experimental study of MUR of three quantum observables. We establish rigorously MURs for triplets of unbiased qubit observables as combined approximation errors lower bounded by an incompatibility measure, inspired by the proposal of Busch et al. [Phys. Rev. A 89, 012129 (2014)PLRAAN1050-294710.1103/PhysRevA.89.012129]. We develop a convex programming protocol to numerically find the exact value of the incompatibility measure and the optimal measurements. We propose a novel implementation of the optimal joint measurements and present several experimental demonstrations with a single-photon qubit. We stress that our method is universally applicable to the study of many qubit observables. Besides, we theoretically show that MURs for joint measurement can be attained by sequential measurements in two of our explored cases. We anticipate that this work may stimulate broad interests associated with Heisenberg's uncertainty principle in the case of multiple observables, enriching our understanding of quantum mechanics and inspiring innovative applications in quantum information science.

Test of Genuine Multipartite Nonlocality.

While Bell nonlocality of a bipartite system is counterintuitive, multipartite nonlocality in our many-body world turns out to be even more so. Recent theoretical study reveals in a theory-agnostic manner that genuine multipartite nonlocal correlations cannot be explained by any causal theory involving fewer-partite nonclassical resources and global shared randomness. Here, we provide a Bell-type inequality as a test for genuine multipartite nonlocality in network by exploiting a matrix representation of the causal structure of a multipartite system. We further present experimental demonstrations that both four-photon GHZ state and generalized four-photon GHZ state significantly violate the inequality, i.e., the observed four-partite correlations resist explanations involving three-way nonlocal resources subject to local operations and common shared randomness, hence confirming that nature is boundless multipartite nonlocal.

Testing Real Quantum Theory in an Optical Quantum Network.

Quantum theory is commonly formulated in complex Hilbert spaces. However, the question of whether complex numbers need to be given a fundamental role in the theory has been debated since its pioneering days. Recently it has been shown that tests in the spirit of a Bell inequality can reveal quantum predictions in entanglement swapping scenarios that cannot be modeled by the natural real-number analog of standard quantum theory. Here, we tailor such tests for implementation in state-of-the-art photonic systems. We experimentally demonstrate quantum correlations in a network of three parties and two independent EPR sources that violate the constraints of real quantum theory by over 4.5 standard deviations, hence disproving real quantum theory as a universal physical theory.

Error-Disturbance Trade-off in Sequential Quantum Measurements.

We derive a state-dependent error-disturbance trade-off based on a statistical distance in the sequential measurements of a pair of noncommutative observables and experimentally verify the relation with a photonic qubit system. We anticipate that this Letter may further stimulate the study on the quantum uncertainty principle and related applications in quantum measurements.