Long-Distance Quantum Communication with Neutral Atoms

The architecture proposed by Duan, Lukin, Cirac, and Zoller (DLCZ) for long-distance quantum communication with atomic ensembles is analyzed. Its fidelity and throughput in entanglement distribution, entanglement swapping, and quantum teleportation is derived within a framework that accounts for multiple excitations in the ensembles as well as loss and asymmetries in the channel. The DLCZ performance metrics that are obtained are compared to the corresponding results for the trapped-atom quantum communication architecture that has been proposed by a team from the Massachusetts Institute of Technology and Northwestern University (MIT/NU). Both systems are found to be capable of high-fidelity entanglement distribution. However, the DLCZ scheme only provides conditional teleportation and repeater operation, whereas the MIT/NU architecture affords full Bell-state measurements on its trapped atoms. Moreover, it is shown that achieving unity conditional fidelity in DLCZ teleportation and repeater operation requires ideal photon-number resolving detectors. The maximum conditional fidelities for DLCZ teleportation and repeater operation that can be realized with non-resolving detectors are 1/2 and 2/3, respectively.Comment: 15 pages, 10 figure

Quantum Key Distribution over Probabilistic Quantum Repeaters

A feasible route towards implementing long-distance quantum key distribution (QKD) systems relies on probabilistic schemes for entanglement distribution and swapping as proposed in the work of Duan, Lukin, Cirac, and Zoller (DLCZ) [Nature 414, 413 (2001)]. Here, we calculate the conditional throughput and fidelity of entanglement for DLCZ quantum repeaters, by accounting for the DLCZ self-purification property, in the presence of multiple excitations in the ensemble memories as well as loss and other sources of inefficiency in the channel and measurement modules. We then use our results to find the generation rate of secure key bits for QKD systems that rely on DLCZ quantum repeaters. We compare the key generation rate per logical memory employed in the two cases of with and without a repeater node. We find the cross-over distance beyond which the repeater system outperforms the non-repeater one. That provides us with the optimum inter-node distancing in quantum repeater systems. We also find the optimal excitation probability at which the QKD rate peaks. Such an optimum probability, in most regimes of interest, is insensitive to the total distance.Comment: 12 pages, 6 figures; Fig. 5(a) is replace

Long-distance quantum communication with neutral atoms

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2006.Includes bibliographical references (p. 132-138).In this thesis, we develop quantitative performance analyses for a variety of quantum communication/computation systems that have the common feature of employing neutral atoms for storage/processing and photons for qubit transmission. For most of these systems, there is a lack of a precise performance analysis to enable a comparison between different scenarios from a top-level system standpoint. One main goal of this thesis is to fill that gap, thus providing quantum system designers with realistic estimates of system performance that can guide and inform the design process. For many applications in quantum communication and distributed quantum processing, we need to share, in advance, an entangled state between two parties. Thus, entanglement distribution is at the core of long-distance quantum communication systems. It not only includes generation and transmission of entangled states, but it also requires storing them for further processing purposes. Whereas the photons are the prime candidate for the former task, they are not appropriate for long-time storage and processing. Metastable levels in some alkali atoms, e.g., rubidium, are attractive venues for quantum storage. In this thesis, we study several basic quantum memory modules-all based on single trapped atoms in high-finesse optical cavities-and analytically evaluate how efficiently they can be loaded with (entangled) quantum states.(cont.) We propose a non-adiabatic mechanism for driving off-resonant Raman transitions that can be used in loading trapped-atom quantum memories. Our method is more flexible than its adiabatic counterpart in that it allows use of larger cavities and a larger class of driving sources. We also describe two proposed implementations for long-distance quantum communication-one that uses trapped atoms as quantum memories and another that employs atomic ensembles for quantum storage. We provide, for the first time, a detailed quantitative performance analysis of the latter system, which enables us to compare these two systems in terms of the fidelity and the throughput that they achieve for entanglement distribution, repeater operation, and quantum teleportation. Finally, we study quantum computing systems that use the cross-Kerr nonlinearity between single-photon qubits and a coherent mode of light. The coherent beam serves a mediating role in coupling two weak single-photon beams. We analytically study this structure using a continuous-time formalism for the cross-Kerr effect in optical fibers. Our results establish stringent conditions that must be fulfilled for the system's proper operation.by Mohsen Razavi.Ph.D

Continuous Variable Quantum Key Distribution in Multiple-Input Multiple-Output Settings

We investigate quantum key distribution (QKD) in optical multiple-input-multiple-output (MIMO) settings. Such settings can prove useful in dealing with harsh channel conditions as in, e.g., satellite-based QKD. We study a $2\times2$ setting for continuous variable (CV) QKD with Gaussian encoding and heterodyne detection and reverse reconciliation. We present our key rate analysis for this system and compare it with single-mode and multiplexed CV QKD scenarios. We show that we can achieve multiplexing gain using multiple transmitters and receivers even if there is some crosstalk between the two channels. In certain cases, when there is nonzero correlated excess noise in the two received signals, we can even surpass the multiplexing gain