Level II.

Entanglement is a vital resource for realizing many tasks such as teleportation, secure key distribution, metrology, and quantum computations. To effectively build entanglement between different quantum systems and share information between them, a frequency transducer to convert between quantum states of different wavelengths while retaining its quantum features is indispensable. Information encoded in the photon's orbital angular momentum (OAM) degrees of freedom is preferred in harnessing the information-carrying capacity of a single photon because of its unlimited dimensions. A quantum transducer, which operates at wavelengths from 1558.3 to 525 nm for OAM qubits, OAM-polarization hybrid-entangled states, and OAM-entangled states, is reported for the first time. Nonclassical properties and entanglements are demonstrated following the conversion process by performing quantum tomography, interference, and Bell inequality measurements. Our results demonstrate the capability to create an entanglement link between different quantum systems operating in a photon's OAM degrees of freedom, which will be of great importance in building a high-capacity OAM quantum network.

Entanglement in multiple degrees of freedom has many benefits over entanglement in a single one. The former enables quantum communication with higher channel capacity and more efficient quantum information processing and is compatible with diverse quantum networks. Establishing multi-degree-of-freedom entangled memories is not only vital for high-capacity quantum communication and computing, but also promising for enhanced violations of nonlocality in quantum systems. However, there have been yet no reports of the experimental realization of multi-degree-of-freedom entangled memories. Here we experimentally established hyper- and hybrid entanglement in multiple degrees of freedom, including path (K-vector) and orbital angular momentum, between two separated atomic ensembles by using quantum storage. The results are promising for achieving quantum communication and computing with many degrees of freedom.

a b s t r a c tAre quantum states real? This most fundamental question in quantum mechanics has not yet been satisfactorily resolved, although its realistic interpretation seems to have been rejected by various delayedchoice experiments. Here, to address this long-standing issue, we present a quantum twisted double-slit experiment. By exploiting the subluminal feature of twisted photons, the real nature of a photon during its time in flight is revealed for the first time. We found that photons' arrival times were inconsistent with the states obtained in measurements but agreed with the states during propagation. Our results demonstrate that wavefunctions describe the realistic existence and evolution of quantum entities rather than a pure mathematical abstraction providing a probability list of measurement outcomes. This finding clarifies the long-held misunderstanding of the role of wavefunctions and their collapse in the evolution of quantum entities.

Vector beams (VBs) are widely investigated for its special intensity and polarization distributions, which is useful for optical micromanipulation, optical micro-fabrication, optical communication, and single molecule imaging. To date, it is still a challenge to realize nonlinear frequency conversion (NFC) and manipulation of such VBs because of the polarization sensitivity in most of nonlinear processes. Here, we report an experimental realization of NFC and manipulation of VBs which can be used to expand the available frequency band. The main idea of our scheme is to introduce a Sagnac loop to solve the polarization dependence of NFC in nonlinear crystals. Furthermore, we find that a linearly polarized vector beam should be transformed to an exponential form before performing the NFC. The experimental results are well agree with our theoretical model. The present method is also applicable to other wave bands and second order nonlinear processes, and may also be generalized to the quantum regime for single photons.

Spiral phase contrast is an important and convenient imaging processing technology in edge detection, and a broader field-of-view (FOV) of imaging is a long-pursuing aim to see more regions of the illumination objects. Compared with near-infrared (NIR) spectrum, the up-conversion imaging in visible spectrum benefits from the advantages of higher efficiency detection and lower potential speckle. FOV enhanced and spiral phase contrast up-conversion imaging processing methods by using second order nonlinear frequency up-conversion from NIR spectrum to visible spectrum in two different configurations are presented in this work. By changing the temperature of crystal, controllable spatial patterns of imaging with more than 4.5 times enhancement of FOV is realized in both configurations. Additionally, we present numerical simulations of the phenomenon, which agree well with the experimental observations. Our results provide a very promising way in imaging processing, which may be widely used in biomedicine, remote sensing and up-conversion monitoring.

Developing high-performance catalysts for oxygen evolution reaction (OER) is critical for the widespread applications of clean and sustainable energy through electrochemical devices such as zinc-air batteries and (photo) electrochemical water splitting. Constructing heterostructure and oxygen vacancies have demonstrated great promises to boost the OER performance. Herein, we report a facile strategy to fabricate hetero-structured NiFe 2 O 4 /Ni 3 S 4 nanorods, where NiFe 2 O 4 can be derived from Fe-based metalorganic frameworks (MOFs). The NiFe 2 O 4 /Ni 3 S 4 catalyst exhibited excellent OER performance, evidenced by an overpotential value of 357 mV at the current density of 20 mA cm À 2 , and a small Tafel slope of 87.46 mV dec À 1 in 1 M KOH, superior to the benchmark IrO 2 catalyst. Moreover, NiFe 2 O 4 /Ni 3 S 4 outperformed with regard to long-term durability for OER than IrO 2. Such outstanding OER performance is mainly accounted by the interface between NiFe 2 O 4 and Ni 3 S 4 , and the presence of rich oxygen vacancies. When employed as air-cathode in zinc-air batteries, the NiFe 2 O 4 / Ni 3 S 4 decorated battery had a high round-trip efficiency of 62.1% at 10 h, and possessed long-term stability of > 50 h. This study may pave the way for fabricating non-noblemetal-based cost-effective, efficient and durable electrocatalysts for OER, zinc-air batteries, and beyond.

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