Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes — stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium — an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single stepQ-qubit GHZ states with success probability psuccess ~ ηQ/2, where η is the combined detection and source efficiency. This is significantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer.
Thesis chapter. The fragility of quantum information is a fundamental constraint faced by anyone trying to build a quantum computer. A truly useful and powerful quantum computer has to be a robust and scalable machine. In the case of many qubits which may interact with the environment and their neighbors, protection against decoherence becomes quite a challenging task. The scalability and decoherence issues are the main difficulties addressed by the distributed model of quantum computation. A distributed quantum computer consists of a large quantum network of distant nodes — stationary qubits which communicate via flying qubits. Quantum information can be transferred, stored, processed and retrieved in decoherence-free fashion by nodes of a quantum network realized by an atomic medium — an atomic quantum memory. Atomic quantum memories have been developed and demonstrated experimentally in recent years. With the help of linear optics and laser pulses, one is able to manipulate quantum information stored inside an atomic quantum memory by means of electromagnetically induced transparency and associated propagation phenomena. Any quantum computation or communication necessarily involves entanglement. Therefore, one must be able to entangle distant nodes of a distributed network. In this article, we focus on the probabilistic entanglement generation procedures such as well-known DLCZ protocol. We also demonstrate theoretically a scheme based on atomic ensembles and the dipole blockade mechanism for generation of inherently distributed quantum states so-called cluster states. In the protocol, atomic ensembles serve as single qubit systems. Hence, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultra-cold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single stepQ-qubit GHZ states with success probability psuccess ~ ηQ/2, where η is the combined detection and source efficiency. This is significantly more efficient than any known robust probabilistic entangling operation. The GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer.
We demonstrate theoretically a scheme for cluster-state generation, based on atomic ensembles and the dipole-blockade mechanism. In the protocol, atomic ensembles serve as single-qubit systems. Therefore, we review single-qubit operations on qubit defined as collective states of atomic ensemble. Our entangling protocol requires nearly identical single-photon sources, one ultracold ensemble per physical qubit, and regular photodetectors. The general entangling procedure is presented, as well as a procedure that generates in a single step Q-qubit GHZ states with success probability ${p}_{\text{success}}\ensuremath{\sim}{\ensuremath{\eta}}^{Q∕2}$, where $\ensuremath{\eta}$ is the combined detection and source efficiency. This is significantly more efficient than any known robust probabilistic entangling operation. GHZ states form the basic building block for universal cluster states, a resource for the one-way quantum computer.
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 has been witnessing great science value and latent application since 1980's. The research work presented here consist of mainly two important parts: manipulations of multiphoton entanglement and atomic ensembles based quantum memory. In first part, the experimental technique multi-photon entanglement is further developed to study fundamental issues in quantum mechanics, remarkable applications to quantum communication and quantum computation. Specifically, we have demonstrated a bit-flip error-free transfer of quantum information, violation of Bell's inequality beyond Tsirelson's bound, teleportation of a two-qubit composite system, as well as the one-way computing by two-photon-four-qubit cluster state. To overcome un-scalability problem due to probabilistic feature in linear optical quantum information processing, we investigated in the second part the physics of atomic ensembles based quantum memory. We show that theoretically, entanglement between distant locations can be deterministically generated. The experimental work has thoroughly developed the necessary techniques and we have achieved deterministic single photon source, and interference of the photons from independent atomic ensembles, teleportation between photonic and atomic qubits, a novel way to create a robust entanglement between an atomic and a photonic qubit, and memory based entanglement swapping. We believe, the developed techniques here would dramatically facilitate progresses in many fields including global quantum communication, linear optical quantum computation and the foundations of quantum mechanics etc.
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.
Quantum information science and atom optics are among the most active fields in modem physics. In recent years, many theoretical efforts have been made to combine these two fields. Recent experimental progresses [1-3] have shown the in-principle possibility to perform scalable quantum information processing (QIP) with linear optics and atomic ensembles [4]. One of our main activities is to use atomic qubits as quantum memory and exploit photonic qubits for information transfer and processing to achieve efficient linear optics QIP. On the one hand, utilizing the interaction between laser pulses and atomic ensembles we experimentally investigate the potentials of atomic ensembles in the gas phase to build quantum repeaters for longdistance quantum communication [5], that is, to develop a new technological solution for quantum repeaters making use of the effective qubit-type entanglement of two cold atomic ensembles by a projective measurement of individual photons by spontaneous Raman processes. On the other hand, building on our long experience in research on multi-photon entanglement, we are also working on a number of experiments in the field of QIP with particular emphasis on fault-tolerant quantum computation [6], photon-loss-tolerant quantum computation [7] and cluster-state based quantum simulation [8]. In future, by combining the techniques developed in the above quantum memory and multi-photon interference experiments, we will experimentally investigate the possibility to achieve quantum teleportation between photonic and atomic qubits, quantum teleportation between remote atomic qubits and efficient entanglement generation via classical feed-forward. The techniques that are being developed will lay the basis for future large-scale realizations of linear optical QIP with atoms and photons.
The experiments discussed in this thesis investigate the application of atomic ensembles in building a quantum network. Specifically, the atomic ensembles refer to cesium atoms in magneto-optical traps. Chapter 1 gives an introduction to quantum networks, along with the difficulty in extending the range due to the optical loss of communication channels. Chapter 2 describes the protocol proposed by Duan, Lukin, Cirac, and Zoller (DLCZ) for overcoming the limit of channel loss on scaling up a quantum network, exploiting relatively simple setups with atomic ensembles. The protocol introduces many capabilities and simplifies many tasks in quantum information processing with atoms and light. Chapter 3 summarizes the first step in our lab toward realizing the DLCZ protocol. In particular, we observed nonclassical correlation between two optical fields generated from one atomic ensemble. Chapter 4 is a sidetrack apart from the DLCZ protocol. We demonstrated that the atomic ensemble can be used as a conditional source of single photons. In addition to a description of the experiment, details of the simple model we used to fit the data are also included. Chapter 5 augments chapter 4 in that the temporal behavior of the nonclassical correlation is investigated. We found that the correlation decayed rapidly, which is a major obstacle for further implementation of the DLCZ protocol. Chapter 6 describes our effort to fight the fast decay of correlation observed in the experiment. A theoretical model is used to better understand the source of decoherence. Chapter 7 is the follow-up in the direction of implementing the DLCZ protocol. Two atomic ensembles located in vacuum chambers on two optical tables are entangled in a heralded fashion. The details on controlling the phases of the interferometers and data processing are elaborated. Chapter 8 is a practical proposal on how to proceed further toward realization of the DLCZ protocol. Four atomic ensembles are involved in the proposed setup, which merely requires relative phase stability. Chapter 9 concludes the thesis and provides several possible directions toward building a large-scale quantum network through the DLCZ protocol.