Since quantum coherence is an undoubted characteristic trait of quantum physics, the quantification and application of quantum coherence has been one of the long-standing central topics in quantum information science. Within the framework of a resource theory of quantum coherence proposed recently, a fiducial basis should be preselected for characterizing the quantum coherence in specific circumstances, namely, the quantum coherence is a basis-dependent quantity. Therefore, a natural question is raised: what are the maximum and minimum coherences contained in a certain quantum state with respect to a generic basis? While the minimum case is trivial, it is not so intuitive to verify in which basis the quantum coherence is maximal. Based on the coherence measure of relative entropy, we indicate the particular basis in which the quantum coherence is maximal for a given state, where the Fourier matrix (or more generally, complex Hadamard matrices) plays a critical role in determining the basis. Intriguingly, though we can prove that the basis associated with the Fourier matrix is a stationary point for optimizing the $l_1$ norm of coherence, numerical simulation shows that it is not a global optimal choice.

We theoretically study the effects of loss on the phase sensitivity of an SU(1,1) interferometer with parity detection with various input states. We show that although the sensitivity of phase estimation decreases in the presence of loss, it can still beat the shot-noise limit with small loss. To examine the performance of parity detection, the comparison is performed among homodyne detection, intensity detection, and parity detection. Compared with homodyne detection and intensity detection, parity detection has a slightly better optimal phase sensitivity in the absence of loss but has a worse optimal phase sensitivity with a significant amount of loss with one-coherent state or coherent $\otimes$ squeezed state input.

Yellow luminescence (YL) of unintentionally doped GaN (u-GaN) peaking at about 2.2 eV has been investigated for decades, but its origin still remains controversial. In this study, ten u-GaN samples grown via metalorganic chemical vapor deposition (MOCVD) are investigated. It is observed from the room temperature (RT) photoluminescence (PL) measurements that the YL band is enhanced in the PL spectra of those samples if their MOCVD growth is carried out with a decrease of pressure, temperature, or flow rate of NH₃. Furthermore, a strong dependence of YL band intensity on the carbon concentration is found by secondary ion mass spectroscopy (SIMS) measurements, demonstrating that the increased carbon-related defects in these samples are responsible for the enhancement of the YL band.

Color centers in two-dimensional hexagonal boron nitride, as promising room-temperature quantum emitters, have attracted much research interest. Many methods have been performed to create color centers on hBN, but the induced color centers tend to aggregate at flake edges or grain boundaries. In this paper, fast neutron irradiation was implemented to generate color centers with a homogeneous and discrete distribution on 2D hBN multilayers. The density of induced color centers positively correlated with the neutron dose, and within an appropriate neutron dose range, the spectral uniformity of the induced color centers could be maintained well. In addition, first-principle calculations were performed to explore the atomic origin of the observed color centers, and the V_B3N1 defect was in good agreement with our experimental results.

Recently, there has been a renewed interest in the quantification of coherence or other coherencelike concepts within the framework of quantum resource theory. However, rigorously defined or not, the notion of coherence or decoherence has already been used by the community for decades since the advent of quantum theory. Intuitively, the definitions of coherence and decoherence should be two sides of the same coin. Therefore, a natural question is raised: How can the conventional decoherence processes, such as the von Neumann–Lüders (projective) measurement postulation or partially dephasing channels, fit into the bigger picture of the recently established theoretical framework? Here we show that the state collapse rules of the von Neumann or Lüders-type measurements, as special cases of genuinely incoherent operations (GIOs), are consistent with the resource theories of quantum coherence. New hierarchical measures of coherence are proposed for the Lüders-type measurement and their relationship with measurement-dependent discord is addressed. Moreover, utilizing the fixed-point theory for $C^{*}$ algebra, we prove that GIOs indeed represent a particular type of partially dephasing (phase-damping) channels which have a matrix representation based on the Schur product. By virtue of the Stinespring dilation theorem, the physical realizations of incoherent operations are investigated in detail and we find that GIOs in fact constitute the core of strictly incoherent operations and generally incoherent operations and the unspeakable notion of coherence induced by GIOs can be transferred to the theories of speakable coherence by the corresponding permutation or relabeling operators.