Constructing a quantum memory for a photonic entanglement is vital for realizing quantum communication and network [1][2][3][4]. Besides enabling the realization of high channel capacity communication [5], entangled photons of high-dimensional space are of great interest because of many extended applications in quantum information and fundamental physics fields [6][7][8][9]. Photons entangled in a two-dimensional space had been stored in different system [10][11][12][13], but there have been no any report on the storage of a photon pair entangled in a high-dimensional space. Here, we report the first experimental realization of storing an entangled orbital angular momentum (OAM) state through a far off-resonant two-photon transition (FORTPT) in a cold atomic ensemble. We reconstruct the matrix density of an OAM entangled state postselected in a two-dimensional subspace with a fidelity of 90.3%±0.8% and obtain the Clauser, Horne and Shimony and Holt inequality parameter S of 2.41±0.06 after a programmed storage time. All 2 results clearly show the preservation of entanglement during the storage. Besides, we also realize the storage of a true-single-photon via FORTPT for the first time.The establishment of quantum network in the future needs distribution of quantum entangled photons over channels between different nodes [14,15]. To overcome the exponential scaling of the error rate with the channel length, the concept of quantum repeater is introduced [16], which combines entanglement swapping and quantum memory to efficiently extend the achievable distance of quantum communication. During the last years, important progresses have been made towards the realization of an efficient and coherent quantum memory based on gas and solid atomic ensemble [17][18][19][20][21], photons encoded in a two-dimensional space spanned for example by orthogonal polarizations or different paths had been stored [10][11][12][13]. Moreover, many groups and researchers are active in storing light encoded using a high-dimensional space in different physical systems [22][23][24][25][26][27][28][29][30][31][32]. In quantum information and quantum optics fields, a photon encoded in a high-dimensional space [33][34][35][36] could carry 2 log d bits information, where d is the number of orthogonal basis vectors of the Hilbert space. In such a way, the transmission rate of quantum communications is increased greatly [37], and the capacity of channel could be also significantly improved [5]. Moreover, it affords quantum key distribution a more secure flux of information [38], etc. Because of the inherent infinite dimension of orbital angular momentum (OAM) space [39][40][41], a light is usually encoded in OAM space to offer the higher-information-density coding. Therefore, the preparation of a high-dimensional OAM entangled state plays a vital role inquantum information and communication fields, and usually was realized by using the spontaneous parametric down-conversion in a crystal [41] or spontaneous Raman scattering (SRS) in an atomic ens...

The storage of photonic entanglement is central to the achievement of long-distance quantum communication based on quantum repeaters and scalable linear optical quantum computation. Among the memory protocols reported to date, the Raman scheme has the advantages of being broadband and high-speed, resulting in a huge potential in quantum networks. To date there have been no reports on the storage of photonic polarized entanglement using the Raman protocol. Here, two storage experiments using the Raman scheme are reported: (1) heralded single-photon entanglement of the path and polarization storage in a cold atomic ensemble, and (2) polarization entanglement storage in two cold atomic ensembles. The experimental data clearly show that the quantum entanglement is preserved in this memory platform. Our work shows great promise for the establishment of quantum networks in high-speed communications.L ong-distance quantum communication requires the distribution of quantum entanglement among different nodes 1-3 , and quantum memory is an indispensable component of this. A quantum repeater could be implemented by combining the entanglement swapping operation and quantum memory, thereby overcoming the problem of transmission error rates scaling exponentially with channel length 4,5 . The realization of quantum memory requires coherent interactions 3 between the information carrier (usually a photon) and the matter acting as the storage medium. A photon could be encoded with its intrinsic degrees of freedom, such as polarization, orbital angular momentum, the path and time bins. Photons encoded with polarization are reliable and robust for the transmission of information in optical fibres 6 , so it is efficient to build a quantum interface that connects different quantum nodes using photonic polarization entanglement. It is thus unsurprising that people have begun to work on different physical systems for storing polarization entanglement 7,8 .There are many protocols for photon storage, including electromagnetically induced transparency (EIT) 9-11 , far off-resonant twophoton transitions 6,12-19 , controlled reversible inhomogeneous broadening 20-23 , atomic frequency combs 24 , photon echoes 25,26 , optomechanical storage 27 and the off-resonant Faraday effect 28,29 . Of these, the far off-resonant two-photon transition protocol, or 'Raman quantum memory', uses the off-resonance atomic configuration in which there is an excited virtual energy level near the twophoton resonance. The linewidth of the excited virtual energy level could be increased by either enlarging the effective optical depth of the medium or by increasing the intensity of the control laser. Thus, this storage protocol has the ability to store a short time pulse and can operate at high speeds. Theoretically, Raman quantum memory can work over a large range of frequencies because the single photon detuning of the control laser can be varied and is also insensitive to inhomogeneous broadening. All of these properties show that the Raman scheme has huge pote...

Light-carrying orbital angular momentum (OAM) has great potential in enhancing the information channel capacity in both classical and quantum optical communications. Long distance optical communication requires the wavelengths of light are situated in the low-loss communication windows, but most quantum memories currently being developed for use in a quantum repeater work at different wavelengths, so a quantum interface to bridge the wavelength gap is necessary. So far, such an interface for OAM-carried light has not been realized yet. Here, we report the first experimental realization of a quantum interface for a heralded single photon carrying OAM using a nonlinear crystal in an optical cavity. The spatial structures of input and output photons exhibit strong similarity. More importantly, single-photon coherence is preserved during up-conversion as demonstrated.

Entangled quantum states in high-dimensional space show many advantages compared with entangled states in two-dimensional space. The former enable quantum communication with higher channel capacity, enable more efficient quantum-information processing and are more feasible for closing the detection loophole in Bell test experiments. Establishing high-dimensional entangled memories is essential for long-distance communication, but its experimental realization is lacking. We experimentally established high-dimensional entanglement in orbital angular momentum space between two atomic ensembles separated by 1 m. We reconstructed the density matrix for a three-dimensional entanglement and obtained an entanglement fidelity of (83.9±2.9)%. More importantly, we confirmed the successful preparation of a state entangled in more than three-dimensional space (up to seven-dimensional) using entanglement witnesses. Achieving high-dimensional entanglement represents a significant step toward a high-capacity quantum network.

Nonclassical multi-photon and number states are attractive because of their wide applicability, and experimental generation of these states is a longstanding aim. We prepare photon triplets using a spontaneous Raman scattering process in a hot atomic ensemble cascaded by a spontaneous parametric downconversion process in a periodically poled nonlinear waveguide; strong temporal correlations are observed between the photons. This represents the first combination of nonlinear processes of different orders with different physical systems, and demonstrates the scheme's feasibility. The triplets exist in the telecommunications band, which is promising for testing of quantum mechanical laws for multi-photon entanglement over large distances.Multipartite entanglement and correlations have numerous potential applications in the fundamental quantum mechanics and quantum information fields [1], and many physical systems have therefore been used to generate triplets or eight-photon states [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Significant progress has been made with successful demonstrations of quantum communication complexity reduction [2,10], topological error correction [11], and fundamental testing of nonlocality [3,12]. Also, approximately 10,000 entangled modes in the continuous variables field have been prepared [17].Many techniques and physical systems have been proposed for direct generation of photon triplets. The methods include triexcitations in quantum dots [18], combinations of second-order nonlinear processes [19], and high-energy electron-position collisions [20]. More recently, a technique that uses cascaded photon pair sources prepared in a nonlinear spontaneous parametric downconversion (SPDC) process in a nonlinear crystal has been used to prepare genuine tripartite photons [14,15]. Also, a predicted two-photon photonic polarized entangled state has been realized without post-selection [16] using this method. Another way to generate tripartite photons is through the use of two SPDC processes cascaded by a sum-frequency process [21][22][23], where single photon nonlinear interaction is demonstrated. However, we have noted that all protocols used to prepare triplets use the same physical systems (e.g., two pp crystals in [14][15][16], and three pp crystals in [23]) and the same order of nonlinear process (e.g., the second order in [14][15][16]23,24], and the third order in [25]). In the future, quantum networks will have to include many physical systems and interaction processes; generation of a photon triplet using combinations of physical systems is therefore interesting and merits investigation.In this Letter, we realize hybrid-cascaded preparation of tripartite photons for the first time, to the best of our knowledge, when using a combination of two physical systems: a hot atomic ensemble and a nonlinear waveguide. In our work, the spontaneous Raman scattering (SRS) process in a hot Rb atomic cell in a collinear configuration is initially used to prepare the nonclassical photon pair ...

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.

Light with helical phase structures, carrying quantized orbital angular momentum (OAM), has many applications in both classical and quantum optics, such as high-capacity optical communications and quantum information processing. Frequency conversion is a basic technique to expand the frequency range of fundamental light. The frequency conversion of OAM-carrying light gives rise to new physics and applications such as up-conversion detection of images and high dimensional OAM entanglements.Quasi-phase matching (QPM) nonlinear crystals are good candidates for frequency conversion, particularly for their high-valued effective nonlinear coefficients and no walk-off effect. Here we report the first experimental second-harmonic generation (SHG) of OAM light with a QPM crystal, where a UV light with OAM of 100  is generated. OAM conservation is verified using a specially designed interferometer. With a pump beam carrying an OAM superposition of opposite sign, we observed interesting interference phenomena in the SHG light; specifically, a photonics gear-like structure is obtained that gives direct evidence of OAM conservation, which will be very useful for ultra-sensitive angular measurements. We also develop a theory to reveal the underlying physics of the phenomena.The methods and theoretical analysis shown here are also applicable to other frequency conversion processes, such as sum frequency generation and difference-frequency generation, and may also be generalized to the quantum regime for single photons.PACS numbers: 42.65. Ky,42.50.Tx, 42.25.Hz, 42.70.Mp Orbital angular momentum (OAM) in light is a very useful degree of freedom that has no dimensional limitation, and has been widely studied in both classical and quantum optics fields since The interaction of OAM light with matter, such as nonlinear crystals [12,[20][21][22] and atomic vapors [23][24][25], produces many interesting phenomena in contrast to those obtained using Gaussian beams.Allen [20,21] has demonstrated the OAM transformation and conservation in frequency conversion in LBO crystal. Zeilinger's group [12] has realized high-dimensional OAM entanglement in the spontaneous parametric down-conversion processes. In all these nonlinear interaction processes, the total OAM conservation of light plays a very important role. The frequency conversion of OAM lights will be very useful in up-conversing detection of images [26] and generating of OAM light from a fundamental OAM light at special wavelengths (in the UV or mid-infrared frequency domains), which are hard to produce them with traditional method. For nonlinear processes with crystals, the benefits from quasi-phase matching (QPM) when compared with birefringence phase matching make QPM crystals good candidates for frequency conversion of OAM light, particularly for their high-valued effective nonlinear coefficients and no walk-off effect. Then some important questions are coming naturally: can we use QPM crystals for nonlinear frequency conversion of OAM light? Whether the total OAM of light i...

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.