The orbital angular momentum (OAM) carried by photons defines an infinitely dimensional discrete Hilbert space. With OAM modes, high-dimensional quantum states can be achieved for quantum communication and cryptography. Here we demonstrate a heralded single-photon source with a switchable OAM mode, which consists of a heralded single-photon source and an integrated OAM emitter as the mode converter. As the first step, the heralded single-photon source is based on the dispersion-shifted fiber. In this work, the OAM mode (quantized by topological charge l ) carried by the heralded single photon (at fixed wavelength of 1555.75 nm) can be switched within the range of l = 3 – 7 while the mode purity is more than 80%.
Twisted photons, carrying spin and orbital angular momentum (OAM) in an unbounded-dimensional Hilbert space, has recently sparked significant interest, leading to the discovery of new physics in both classical and quantum optics, furthering our understanding and incubating applications in classical and quantum communication. At the heart of this emerging field is the requirement for versatile, efficient, and robust generation of novel photonic spin-orbit states involving both spin and OAM degrees of freedom (DoFs). However, so far, the generation and manipulation of spin–orbit photons have primarily relied on free-space bulk optics, which, due to their lack of compactness and stability, are unsuitable for large-scale applications. In this work, we address this challenge by dynamically generating and manipulating the photonic spin-orbit states in both integrated microlaser and single photon source systems, leveraging non-Hermitian photonics and chiral quantum optics. We further investigated the generation and control of quantum entanglement between the spin and OAM DoFs of photons via integrated photonic circuits. Moreover, we successfully generated photonic spin-orbit states with intricate Skyrmionic topological structures and demonstrate the robustness of their topological properties during the quantum teleportation process. The integrated spin-orbit control toolbox for various structured photons, combined with the robustness of their topological properties, holds significant potential for developing large-channel capacity, noise-resilient information systems within a unique multidimensional space.
Engineering single-photon states endowed with orbital angular momentum (OAM) is a powerful tool for quantum information photonic implementations. Indeed, due to its unbounded nature, OAM is suitable for encoding qudits, allowing a single carrier to transport a large amount of information. Most of the experimental platforms employ spontaneous parametric down-conversion processes to generate single photons, even if this approach is intrinsically probabilistic, leading to scalability issues for an increasing number of qudits. Semiconductor quantum dots (QDs) have been used to get over these limitations by producing on-demand pure and indistinguishable single-photon states, although only recently they have been exploited to create OAM modes. Our work employs a bright QD single-photon source to generate a complete set of quantum states for information processing with OAM-endowed photons. We first study hybrid intraparticle entanglement between OAM and polarization degrees of freedom of a single photon whose preparation was certified by means of Hong–Ou–Mandel visibility. Then, we investigate hybrid interparticle OAM-based entanglement by exploiting a probabilistic entangling gate. The performance of our approach is assessed by performing quantum state tomography and violating Bell inequalities. Our results pave the way for the use of deterministic sources for the on-demand generation of photonic high-dimensional quantum states.
(0) ≈ 0.22, carrying entangled spin and OAM states and forming two spatially separated entangled radiation channels with different polarization properties. The OAM-encoded single photons are generated by efficiently outcoupling diverging surface plasmon polaritons excited with a deterministically positioned quantum emitter via Archimedean spiral gratings. Our OAM single-photon source platform bridges the gap between conventional OAM manipulation and nonclassical light sources, enabling high-dimensional and large-scale photonic quantum systems for quantum information processing.
Quantum memory is a key component of quantum networks. A successful development in enabling the distribution of quantum information requires the storage of single-photon light. Usually quantum information is encoded using a 2D space, spanned, for example, by the orthogonal polarization states of a photon. In this case, each photon carries a quantum bit of information. If the photon could be encoded in a high-dimensional space instead, the information carried by each photon could be increased from a bit to |
log2d| bits, where d is the dimension of the Hilbert space in which the photon is described. The channel capacity of the network and the transmission efficiency would then be greatly enlarged [1]. Moreover, storing high-dimensional states in quantum memory can reduce its sensitivity to memory coherence time [2], leading to significant improvements in storage capacity [3]. It is thus no surprise that many research groups are actively working on different physical systems for storing light encoded using a high-dimensional space. The degrees of freedom for position, linear momentum, time-bin or orbital angular momentum (OAM) of a photon can all be considered as a high-dimensional space for encoding. The photon's OAM has the most potential for encoding [4]. Electromagnetic waves have both spin and OAM. Spin angular momentum produces polarization. Photons with OAM can be regarded as helices with left and right handedness twisted to varying degrees (Fig. 1). Information is encoded using this handedness and the amount of twist. Because of the inherent infinite degrees of freedom for OAM, a photon can in principle carry an arbitrarily large amount of information, and hence the eagerness to store light encoded in OAM space. Although some work on this has been reported for different physical systems, what is lacking is a way of storing and releasing at some later time a true single photon, including its shape and structure [5–7]. Constructing this basic element of quantum memory has been a major challenge. The wavefront of an OAM light is a helical structure (left and center), with an optical vortex at its center in the intensity distribution of light (right). The vortex of light can carry non-zero OAM of any order m. OAM states of a different order are orthogonal. A group at the Key Laboratory of Quantum Information, Chinese Academy of Science, at the University of Science and Technology of China finally resolved this problem [8]. They reported on the first experimental realization of a true single-photon-carrying OAM stored via electromagnetically induced transparency (EIT) in a cold atomic ensemble (Fig. 2). With such a suitable candidate, this work indicates a means of constructing high-dimensional quantum memory. After this, similar results were reported by Laurat's group at Laboratoire Kastler Brossel, at Université Pierre et Marie Curie in France [9], where an extremely weak laser at single-photon level carrying OAM was also stored via EIT in a cold atomic ensemble. Ding et al. [8] first prepared two cold 85Rb atomic clouds in two magnetic-optical traps (MOT) using laser cooling and trapping techniques. They generated non-classical correlated photon pairs using spontaneous four-wave mixing in an atomic cloud. One photon of each pair was used as a trigger; the other mapped and stored in a second atomic cloud via EIT. Each photon to be stored carried one OAM unit. After a programed storage time, the photon was retrieved. What they achieved during storage included (1) proof that the non-classical correlation between the trigger photon and the retrieved photon is retained by demonstrating a strong violation of the Cauchy–Schwarz inequality, (2) a demonstration that the single-photon property of the signal photon is maintained by performing the Hanbury–Brown and Twiss experiment on the trigger photon, (3) proof that the spatial structure of the photon is also very well preserved by showing a high similarity between input and output, and (4) evidence, gathered by Sagnac interferometry, that the coherence of a single photon is also preserved. A heralded single photon is generated through spontaneous four-wave mixing in MOT 1 (a); this single photon carrying OAM is then stored through EIT in MOT 2 (b); (c) shows an image of the photon carrying OAM. Experimental measurements in the transverse position of the image carried by the photon before and after storage are presented in (d) and (e), respectively; (f) shows the simplified experimental setup for storage of a superposition of OAM. Interference patterns taken at different angle settings of the half-wave plate are shown in (g). The left column represents the experimental angle settings of the half-wave and the quarter-wave plates, the middle column gives the experimental results, and the right column gives results from theoretical simulations; (h) and (i) present the interference patterns of the input signal photon and the retrieved signal with plate angle, respectively. This is a significant breakthrough towards the realization of a quantum internet and, potentially, quantum computing. This work addresses the ability to generate, store, retrieve on demand an arbitrary single-photon-level photonic state encoded in OAM space. It signifies an important step in the development of practical quantum-enabled technologies, opens up the possibility for realizing high-dimensional quantum memories based on this degree of freedom of light, and will have a large impact in a broad physical community, especially in the quantum information and quantum optics communities. The Technology Review website of MIT comments on their work very positively with an article entitled ‘First Quantum Memory That Records the Shape of a Single Photon Unveiled in China’. Further, Nature Photonics selected this work as a research highlight. Clearly, memory that can preserve the spatial structure of photons will make the internet far more flexible and capable. Of course, there are still many basic problems remaining that need to be solved to build the future quantum internet, one of which is the construction of quantum memory exploiting entanglement. This derives from the fact that a quantum network needs the distribution of quantum-entangled photons over channels between different nodes [10]. A high-dimensional quantum internet needs to store OAM-entangled states and perform entanglement swapping to efficiently extend the achievable communication distances. While Ding et al. [8] reported the storage of light with OAM at the single-photon level, the photon was, however, still encoded in a 2D space. Therefore, whether it is possible to construct a high-dimensional quantum memory remains an open issue that needs to be investigated further.
Quantum light sources are one of key devices for quantum information processing, and they are also the important foundation for applications such as in quantum computing, quantum communication, and quantum simulation. Improving the capacity of quantum information coding by using the quantum light source is a major challenge in the development of quantum information technology. Photons with a helical phase front can carry a discrete, unlimited but quantized amount of orbital angular momentum (OAM). The infinite number of states with different OAMs can greatly increase the capacity of optical communication and information processing in quantum regimes. To date photons carrying OAM have mainly been generated by using bulk crystals, which limits the efficiency and the scalability of the source. With the advancement of quantum photonic technology, many significant quantum photonic devices can now be realized on integrated chips. However, creating high-dimensional OAM quantum states at a micro-nano scale is still a challenge. And the research of harnessing high-dimensional OAM mode by using integrated quantum photonic technologies is still in its infancy. Here, the authors review the recent progress and discuss the integrated quantum light sources with OAM. The authors introduce the research progress of using OAM for both single photons and entangled photons and emphasize the exciting work on pushing boundaries in high-dimensional quantum states. This may pave the way for the research and practical applications of high-dimensional quantum light sources.