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arXiv:1307.0348 · Journal of Physics B Atomic Molecular and Optical Physics

Centre-of-mass motion-induced decoherence and entanglement generation in a hybrid quantum repeater

József Zsolt Bernád, H Frydrych, G Alber

Published Nov 7, 2013• 9 citations
arXivHTMLPDF

Abstract

Quantum communication over long distances relies on the ability to create entanglement between two remote quantum nodes. Recent proposals aiming at experimental realization propose a hybrid quantum repeater setup where two distant material qubits are entangled by light-matter interaction. Motivated by these developments, we investigate possible decoherence effects originating from the centre- of-mass motion of the spatially well separated trapped qubits. Within the Lamb-Dicke regime we use photon exchange involving coherent states of the radiation field to entangle the two material qubits. Optimal generalized photonic field measurements are used to achieve entangled qubit pairs with high fidelities and high success probabilities. We demonstrate that the quality of the achievable two-qubit entanglement crucially depends on the trap frequencies involved. Furthermore, dynamical decoupling schemes are proposed which are capable of suppressing centre-of-mass-motion-induced decoherence effects significantly and which involve only local operations acting on the spatially well-separated material qubits.

DOI
10.1088/0953-4075/46/23/235501

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Quantum communication over long distances relies on the ability to create entanglement between two remote quantum nodes. Recent proposals aiming at experimental realization propose a hybrid quantum repeater setup where two distant material qubits are entangled by light-matter interaction. Motivated by these developments, we investigate possible decoherence effects originating from the centre- of-mass motion of the spatially well separated trapped qubits. Within the Lamb-Dicke regime we use photon exchange involving coherent states of the radiation field to entangle the two material qubits. Optimal generalized photonic field measurements are used to achieve entangled qubit pairs with high fidelities and high success probabilities. We demonstrate that the quality of the achievable two-qubit entanglement crucially depends on the trap frequencies involved. Furthermore, dynamical decoupling schemes are proposed which are capable of suppressing centre-of-mass-motion-induced decoherence effects significantly and which involve only local operations acting on the spatially well-separated material qubits.
We propose a hybrid quantum repeater based on ancillary coherent field states and material qubits coupled to optical cavities. For this purpose, resonant qubit-field interactions and postselective field measurements are determined which are capable of realizing all necessary two-qubit operations for the actuation of the quantum repeater. We explore both theoretical and experimental possibilities of generating near-maximally-entangled qubit pairs (F>0.999F>0.999F>0.999) over long distances. It is shown that our scheme displays moderately low repeater rates, between $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ and 23 pairs per second, over distances up to 900 km, and it relies completely on current technology of cavity quantum electrodynamics.
We describe a quantum repeater protocol for long-distance quantum communication. In this scheme, entanglement is created between qubits at intermediate stations of the channel by using a weak dispersive light-matter interaction and distributing the outgoing bright coherent-light pulses among the stations. Noisy entangled pairs of electronic spin are then prepared with high success probability via homodyne detection and postselection. The local gates for entanglement purification and swapping are deterministic and measurement-free, based upon the same coherent-light resources and weak interactions as for the initial entanglement distribution. Finally, the entanglement is stored in a nuclear-spin-based quantum memory. With our system, qubit-communication rates approaching 100 Hz over 1280 km with fidelities near 99% are possible for reasonable local gate errors.
We present a hybrid quantum repeater protocol for the long-distance distribution of atomic entangled states beyond qubits. In our scheme, imperfect noisy entangled pairs of two qudits, i.e., two discrete-variable ddd-level systems, each of, in principle, arbitrary dimension ddd, are initially shared between the intermediate stations of the channel. This is achieved via local, sufficiently strong light-matter interactions, involving optical coherent states and their transmission after these interactions, and optical measurements on the transmitted field modes, especially efficient continuous-variable homodyne detections (``hybrid'' here refers to the simultaneous exploitation of discrete- and continuous-variable degrees of freedom for the local processing and storage of entangled states as well as their nonlocal distribution, respectively) and unambiguous state discrimination. For qutrits we quantify the light-matter entanglement that can be effectively shared through an elementary lossy channel, and for a repeater spacing of up to 10 km we show that the realistic (lossy) qutrit entanglement is even larger than any ideal (loss-free) qubit entanglement. After including qudit entanglement purification and swapping procedures, we calculate the long-distance entangled-pair distribution rates and the final entangled-state fidelities for total communication distances of up to 1280 km. For example, employing unambiguous state discrimination, with three rounds of purification, entangled qudit pairs of near-unit fidelity can be distributed over 1280 km at an ideal maximal rate (assuming perfect gate operations) on the order of 100 Hz.
The recently introduced quantum gravity-induced entanglement of masses (QGEM) protocol aims to test the quantum nature of gravity by witnessing the entanglement produced by the virtual exchange of a graviton between two spatially superposed masses. Shortly after the original proposal, further improvements upon the experiment were suggested, including the addition of a third mass, showing that three-qubit setups can be more resilient to higher rates of decoherence caused by the interaction of the system with the environment. In this work, we investigate the type of tripartite entanglement generated in these three-qubit QGEM experiments when considering the effects of decoherence. We show that the gravitational interaction between the qubits is able to generate genuine tripartite entanglement between them, studying the corresponding parameter spaces and comparing the performance of the possible experimental configurations of the three qubits at allowing for the detection of genuine entanglement via an entanglement witness.
We describe a practical quantum repeater protocol for long-distance quantum communication. In this scheme, entanglement is created between the intermediate nodes of the quantum channel by using a weak dispersive light-matter interaction and distributing the outgoing intense coherent light pulses among the nodes. Noisy entangled pairs of electronic spin are then prepared with high success probability via homodyne detection including postselection. The local gates for entanglement purification and swapping are deterministic and measurement-free, based upon the same coherent-light resources and weak interactions as for the initial entanglement distribution. Finally, the entanglement is stored in a nuclear-spin-based quantum memory. Simulations of the system show qubit-communication rates approaching 100 Hz and fidelities above 99 % for reasonable local gate errors.
We explore the possibilities of entangling two distant material qubits with the help of an optical radiation field in the regime of strong quantum electrodynamical couplingwith almost resonant interaction. For this purpose the optimum generalized field measurements are determined which are capable of preparing a two-qubit Bell state by postselection with minimum error. It is demonstrated that in the strong-coupling regime some of the recently found limitations of the nonresonant weak-coupling regime can be circumvented successfully due to characteristic quantum electrodynamical quantum interference effects. In particular, in the absence of photon loss it is possible to postselect two-qubit Bell states with fidelities close to unity by a proper choice of the relevant interaction time. Even in the presence of photon loss this strong-coupling regime offers interesting perspectives for creating spatially well-separated Bell pairs with high fidelities, high success probabilities, and high repetition rates which are relevant for future realizations of quantum repeaters.