Quantum information science possesses promising potentialities for developing new secure and efficient ways to transfer and process information. Prodigious efforts have been and are being devoted, however, a scalable architecture is still a considerable challenge. Among the potential candidates, photons are assumed to be good carriers, and neutral atoms good hosts. In this thesis, photon-matter entanglement sources, which exploit the advantages of both, are employed to demonstrate a teleportation between two atomic-ensemble quantum memory nodes and an efficient entanglement swapping, which are essential for scalable quantum networks. Later, the entanglement source is further developed, and an active feedforward one-way quantum computation is presented \nwith stationary quantum memories, which are necessary for scalable quantum computers. To fulfill another feature of scalability, the ease of integration, ultracold atoms in \ntwo-dimensional optical superlattices are studied and an experiment is designed to generate four-qubit entangled states. With the help of the photon-matter entanglement \nsources, serving as quantum memories and nodes of quantum networks, realization of large-scale quantum networks is foreseeable. Meanwhile, by applying controllable interactions, large entangled states could be generated in optical superlattices, which might help to construct quantum processors that outperform the classical counterparts.
Quantum information science possesses promising potentialities for developing new secure and efficient ways to transfer and process information. Prodigious efforts have been and are being devoted, however, a scalable architecture is still a considerable challenge. Among the potential candidates, photons are assumed to be good carriers, and neutral atoms good hosts. In this thesis, photon-matter entanglement sources, which exploit the advantages of both, are employed to demonstrate a teleportation between two atomic-ensemble quantum memory nodes and an efficient entanglement swapping, which are essential for scalable quantum networks. Later, the entanglement source is further developed, and an active feedforward one-way quantum computation is presented \nwith stationary quantum memories, which are necessary for scalable quantum computers. To fulfill another feature of scalability, the ease of integration, ultracold atoms in \ntwo-dimensional optical superlattices are studied and an experiment is designed to generate four-qubit entangled states. With the help of the photon-matter entanglement \nsources, serving as quantum memories and nodes of quantum networks, realization of large-scale quantum networks is foreseeable. Meanwhile, by applying controllable interactions, large entangled states could be generated in optical superlattices, which might help to construct quantum processors that outperform the classical counterparts.
Quantum teleportation and quantum memory are two crucial elements for large-scale quantum networks. With the help of prior distributed entanglement as a "quantum channel," quantum teleportation provides an intriguing means to faithfully transfer quantum states among distant locations without actual transmission of the physical carriers [Bennett CH, et al. (1993) Phys Rev Lett 70(13):1895-1899]. Quantum memory enables controlled storage and retrieval of fast-flying photonic quantum bits with stationary matter systems, which is essential to achieve the scalability required for large-scale quantum networks. Combining these two capabilities, here we realize quantum teleportation between two remote atomic-ensemble quantum memory nodes, each composed of ∼10(8) rubidium atoms and connected by a 150-m optical fiber. The spin wave state of one atomic ensemble is mapped to a propagating photon and subjected to Bell state measurements with another single photon that is entangled with the spin wave state of the other ensemble. Two-photon detection events herald the success of teleportation with an average fidelity of 88(7)%. Besides its fundamental interest as a teleportation between two remote macroscopic objects, our technique may be useful for quantum information transfer between different nodes in quantum networks and distributed quantum computing.
Quantum networks are composed of quantum nodes which coherently interact by way of quantum channels. They offer powerful capabilities for quantum computation, communication, and metrology. A generic requirement for these realizations is the capability to generate and store quantum states among multiple quantum nodes, and to disseminate these resources throughout the network via the quantum channels. In this thesis, I describe a series of experiments whereby single excitations in atomic ensembles are strongly coupled to optical modes and provide efficient means for the coherent control of entangled states between matter and light. By following the seminal proposal by Duan et al., we have generated measurement-induced entanglement of an excitation between two cold atomic ensembles. Using this system, we investigated the relationship for the global bipartite entanglement and local correlations in its subsystems. In addition, we achieved functional quantum nodes for entanglement distribution. Two pairs of remote ensembles at two quantum nodes were prepared into entangled states in a heralded and asynchronous fashion by the conditional controls of the entanglement. The quantum states of the ensembles were then distributed into polarization entangled states of photons. We also prepared an analogous quantum state and transferred the nonlocal coherence between two pairs of heralded entangled atomic ensembles, providing a step towards entanglement connection. Beyond such probabilistic approaches, we demonstrated an experiment where entanglement between two quantum memories is created by the reversible and deterministic mapping of an entangled state of light via dynamic electromagnetically induced transparency. This experiment opens novel prospects of integrating hybrid quantum systems by way of reversible quantum interfaces between light and matter. Then, we extended our work to multipartite quantum systems. We theoretically investigated the characterization of multipartite mode-entangled states by way of quantum uncertainty relations, and introduced theoretical tools to verify the entanglement orders in multipartite systems. In particular, we achieved entanglement for one delocalized photon among multiple optical modes (N > 2). Finally, we have achieved measurement-induced entanglement of spin waves among four quantum memories. The individual atomic components for the entangled W state of the four ensembles were then coherently converted into four propagating entangled beams of light via superradiant emissions. We observed the statistical and dynamic transitions for the multipartite entangled spin waves. Experiments described in this thesis thereby represent significant advances of experimental and theoretical capabilities to generate, store, transfer, and characterize entanglement of matter and light over quantum networks.
Quantum information technology has been developing rapidly in last two decades. Very recently, quite a few optical quantum key distribution networks have been reported, enabling preliminary practical applications in secure information transfer. Meanwhile, a number of significant progresses based on quantum network have also been made in the research of linear optical quantum computing, simulation and metrology. However, on the way towards large-scale optical quantum network serious problems occur. On the one hand, the distance of fiber-based quantum communication is limited, due to intrinsic fiber loss and decreasing of entanglement quality caused by the noisy environment. On the other hand, the probabilistic feature of single and entangled photon sources would also cause an exponentially increasing overhead for large-scale quantum network. To solve the above problems, quantum repeaters and/or transmission of optical quantum bits over free space channel can be efficiently exploited for future wide-area realization of quantum communication. In addition, memory built-in quantum repeaters would also enable scalable linear optical quantum computing, simulation and high precision measurement. In this talk I will present some recent experiments from our group, including eight-photon entanglement, topological quantum error-correction, quantum repeater and efficient and long-lived quantum memory, and entanglement distribution and quantum teleportation over 100km-scale free-space quantum channels. These experiments show the promising future possibility towards scalable quantum network with photons and atoms.
Reversible entanglement transfer between light and matter is a crucial requisite for the ongoing developments of quantum information technologies. Quantum networks and their envisioned applications, e.g., secure communications beyond direct transmission, distributed quantum computing, or enhanced sensing, rely on entanglement distribution between nodes. Although entanglement transfer has been demonstrated, a current roadblock is the limited efficiency of this process that can compromise the scalability of multi-step architectures. Here we demonstrate the efficient transfer of heralded single-photon entanglement into and out of two quantum memories based on large ensembles of cold cesium atoms. We achieve an overall storage-and-retrieval efficiency of 85% together with a preserved suppression of the two-photon component of about 10% of the value for a coherent state. Our work constitutes an important capability that is needed toward large scale networks and increased functionality.
Entanglement provides an essential resource for quantum computation, quantum communication, and quantum networks. How to conveniently and efficiently realize the generation, distribution, storage, retrieval, and control of multipartite entanglement is the basic requirement for realistic quantum information processing. Here, we present a theoretical proposal to efficiently and conveniently achieve a universal quantum information processor (QIP) via atomic coherence in an atomic ensemble. The atomic coherence, produced through electromagnetically induced transparency (EIT) in the Λ-type configuration, acts as the QIP and has full functions of quantum beam splitter, quantum frequency converter, quantum entangler, and quantum repeater. By employing EIT-based nondegenerate four-wave mixing processes, the generation, exchange, distribution, and manipulation of light-light, atom-light, and atom-atom multipartite entanglement can be efficiently and flexibly achieved in a deterministic way with only coherent light fields. This method greatly facilitates the operations in quantum information processing, and holds promising applications in realistic scalable quantum communication and quantum networks.
The ultimate goal of this research is to develop the essential physical ingredients required for the shared quantum gate processing unit (S-QGPU) architecture for distributed quantum computing. In this architecture, local quantum processors are connected through flying photonic qubits and a shared central gate processor, providing a route toward scalable quantum computation beyond the limitations of isolated local devices. Realizing such an architecture requires two key components: a reliable quantum link between distant nodes and a hybrid controlled gate capable of implementing nonlocal quantum operations.This thesis investigates these two ingredients in cold 87Rb atomic ensembles. The first part focuses on narrowband biphoton generation through spontaneous four-wave mixing in laser-cooled atoms. These biphotons serve as heralded single-photon sources and quantum links between distant nodes, while remaining naturally compatible with atomic quantum memories. A central result of this work is the discovery and analysis of symmetry-protected biphoton coherence time, in which the biphoton coherence becomes immune to loss associated with electromagnetically induced transparency. This effect provides a new mechanism for generating ultranarrow-band quantum light with improved robustness, and enhances both synchronization and light–matter interfacing in the S-QGPU architecture. The role of atomic-state preparation in biphoton generation is also investigated through opticalpumping-based configurations. The second part of this thesis explores atom–photon hybrid controlled gates based on the combination of quantum memory and Rydberg blockade. In these schemes, photonic qubits are mapped onto collective atomic excitations, conditional operations are implemented through strong Rydberg interactions, and the quantum states are subsequently converted back into photonic qubits. These hybrid gate schemes provide the physical mechanism for remotely controlled operations between distant quantum nodes. Together, the narrowband biphoton source and the atom–photon hybrid gate constitute the two essential building blocks of the S-QGPU architecture. This work establishes a physical foundation for distributed quantum computing with cold atomic systems and advances the development of scalable quantum networks based on efficient photon–atom interfaces.