As an emerging channel resource for modern optics, big data, internet traffic and quantum technologies, twisted photons carrying orbital angular momentum (OAM) have been extended their applicable boundary in different media, such as optical fiber and atmosphere. Due to the extreme condition of loss and pressure, underwater transmission of twisted photons has not been well investigated yet. Especially, single-photon tests were all limited at a level of a few meters [1,2], and it is in practice unclear what will happen for longer transmission distances. Here we experimentally demonstrate the transmission of singlephoton twisted light over an underwater channel up to 55 meters, which reach a distance allowing potential real applications. For different order OAM states and their superposition, a good preservation of modal structure and topological charge are observed. Our results for the first time reveal the real transmission performance of twisted photons in a long-distance regime, representing a step further towards OAM-based underwater quantum communication.

Quantum correlation and its measurement are essential in exploring fundamental quantum physics problems and developing quantum enhanced technologies. Quantum correlation may be generated and manipulated in different spaces, which demands different measurement approaches corresponding to position, time, frequency and polarization of quantum particles. In addition, after early proof-of-principle demonstrations, it is of great demand to measure quantum correlation in a Hilbert space large enough for real quantum applications. When the number of modes goes up to several hundreds, it becomes economically unfeasible for single-mode addressing and also extremely challenging for processing correlation events with hardware. Here we present a general and large-scale measurement approach of Correlation on Spatially-mapped Photon-Level Image (COSPLI). The quantum correlations in other spaces are mapped into the position space and are captured by single-photon-sensitive imaging system. Synthetic methods are developed to suppress noises so that single-photon registrations can be faithfully identified in images. We eventually succeed in retrieving all the correlations with big-data technique from tens of millions of images. We demonstrate our COSPLI by measuring the joint spectrum of parametric down-conversion photons. Our approach provides an elegant way to observe the evolution results of large-scale quantum systems, representing an innovative and powerful tool added into the platform for boosting quantum information processing.Quantum correlation, as one of the unique features of quantum theory, plays a crucial role in quantum information applications. After experiencing quantum evolutions, e.g. quantum interference, quantum particles can be correlated in more diverse ways than the classical counterpart. For example, Hong-Ou-Mandel (HOM) interference can reveal the nonclassical bunching properties of photons [1]. These correlation characteristics are crucial in quantum computing [2][3][4][5], quantum simulation[6-9] and quantum communication [10][11][12][13].In theory, especially in the field of quantum computing, if we manage to send enough entangled photons into plenty of modes and operate their superposition states simultaneously, we would be able to obtain sufficiently large quantum state space that may enable a higher computational power than classical computers. In practice, it may still be acceptable to place single photon detector behind each spatial mode for comparably small systems [9, 14-16]. However, it will become both technically challenging and economically unfeasible to address thousands of modes simultaneously with single photon detectors, and therefore become a decisive bottleneck preventing from detecting state spaces large enough for real quantum applications.Thankfully, recent advances of charge-coupled device (CCD) cameras make it possible to directly image spatial output results at a single-photon level [17][18][19][20][21][22][23][24]. HOM interference was also successfully verified by low-noise corre...

We present an approach to achieve bi-directional conversion between microwave and optical photons based on a hybrid waveguide-QED structure. The proposed converter combines both the merits of optomechanical system and superconducting circuits. Through analysis, we obtain two different impedance matching conditions to realize an ideal conversion efficiency close to unity. One of the matching conditions is owing to atom–phonon hybrid modes, which is different from the well known one due to dark modes. The underlying physical mechanism is discussed in detail. The performance of the setup under different matching conditions is also compared. The device studied here may be realizable in the future experiments, suitable for on-chip integrations, and may have potential applications in the future quantum networks.