Physical Realization Of Quantum Network

The advent of Internet facilitates the lives of people, and advances our society con-siderably. With the prosperous development of quantum information, the interdiscinarysubject which integrates quantum mechanics and information science, people begin topicture the future of quantum network, and more and more scientists strive to make itreal through hard work.Quantum network refers to the information network comprised of quantum trans-mission channels and quantum nodes. Quantum channels enables the transmission ofrandom quantum states and quantum entanglements between each nodes, while thosenodes must possess powerful enough capability to store and process quantum infor-mation. Every single quantum node constitutes a quantum computer, and quantumchannels connect different quantum computers. Existing theories proves that quantumnetwork has significance in both scientific research and application in the areas of in-formation security, distributed quantum computing, and quantum simulations of newmaterials. However, quantum network requires a strong ability to implement quantumcontrol techniques, which still remains a huge challenge. As a result, I choose to studythe physical realization of quantum network as the subject of my PhD thesis. The mainresults of the dissertation are as follow:1. Quantum State Storage, Transfer and ExchangeA scheme was proposed to implement a heralded quantum memory for single-photon polarization qubits,In this scheme, an injected photon only exchanges quan-tum state with the atom, but not absorbed, so that the heralded storage can be achievedby detecting the output photon, which can be used for realizing the heralded quantum s-tate transfer and exchange between different nodes. In the scheme, storage and retrievalof photon polarization states have a high fidelity. In a realistic application operation er-rors due to all sources of photon loss, including atomic spontaneous emission, quantumchannel absorption, and photon collection, are always signaled by the absence of a pho-ton count. As a result, the photon loss only decreases the success probability but hasno contribution to the gate infidelity if the operation succeeds (i.e., if a photon count isregistered). The ability to detect whether the operation has succeeded or not, is crucial for practical application.2. Single-photon Source, Multiple-photon Entanglement Source and Long-distanceEntanglement DistributionA method is provided for single-photon source, multiple-photon entanglementsource and long-distance entanglement distribution with individual atoms and atom-ic ensembles. In the scheme, two local optical cavities are connected by a short opticalfiber. The individual atoms and atomic ensembles, respectively, trapped in the cavitiesact as the control qubits and memory qubits. Deterministic and storable single-photonsource, multiple-photon entanglement source can be obtained by adiabatic evolution ofdark states. We also show that this method can be used for largely enhancing the ef-ficiency for long-distance entanglement distribution. No strong-coupling condition forthe cavities is required in our scheme due to collective interference effects of atomicensembles. Thus this scheme opens an alternate avenue for a scalable quantum network.3. Quantum Computation With Multiqubit Geometric GatesA scheme was provided for implementing geometric quantum computation withmultiqubit gates in cavity QED. Under certain conditions, the atoms have small proba-bility of being populated in the excited states and the cavity remains in the vacuum state,thus both the spontaneous emission and the cavity decay are efficiently suppressed. TheN-qubit (N≥2) geometric phase gates can be realized just by one step and the requiredtime does not increase with the number of qubits, which will significantly reduce thenumber of operations for realizing quantum algorithms by geometric phases.4. Scalable Quantum ComputerA method is provided for a large-scale one-way quantum computer with spin-12physical qubits in a 2D array of coupled cavities. After coherent displacements of thequantum state of cavity fields in a phase space, only the qubits in nearest-neighbor cav-ities can accumulate a nontrivial phase shift, which is key importance for a large-scale2D cluster state created within a short time. We show the feasibility of our method forin various practical systems. It seem that our scheme is most suitable for such solid-state system, where the photons in the cavities have a long coherence time, and effectivepreparation of large-scale 2D cluster states can be achieved within a short time.