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arXiv:1704.08982 · Physical Review Letters

Cavity Carving of Atomic Bell States

Stephan Welte, Bastian Hacker, Severin Daiss, Stephan Ritter, Gerhard Rempe

Published May 26, 2017• 88 citations
arXivHTMLPDF

Abstract

We demonstrate entanglement generation of two neutral atoms trapped inside an optical cavity. Entanglement is created from initially separable two-atom states through carving with weak photon pulses reflected from the cavity. A polarization rotation of the photons heralds the entanglement. We show the successful implementation of two different protocols and the generation of all four Bell states with a maximum fidelity of (90±2)%. The protocol works for any distance between cavity-coupled atoms, and no individual addressing is required. Our result constitutes an important step towards applications in quantum networks, e.g., for entanglement swapping in a quantum repeater.

DOI
10.1103/physrevlett.118.210503

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We demonstrate entanglement generation of two neutral atoms trapped inside an optical cavity. Entanglement is created from initially separable two-atom states through carving with weak photon pulses reflected from the cavity. A polarization rotation of the photons heralds the entanglement. We show the successful implementation of two different protocols and the generation of all four Bell states with a maximum fidelity of (90±2)%. The protocol works for any distance between cavity-coupled atoms, and no individual addressing is required. Our result constitutes an important step towards applications in quantum networks, e.g., for entanglement swapping in a quantum repeater.
Summary form only given. Neutral atoms trapped inside an optical cavity provide an ideal platform for the implementation of quantum networks [1]. In such a network, nodes containing multiple atomic qubits are essential for the construction of a quantum repeater as they allow for entanglement swapping and thus the generation of entanglement between qubits over long distances. Here we will show the realisation of such a multi-qubit network node containing two87Rb atoms (see Figure. 1) in an optical cavity. Local entanglement between the two atoms is created with an experimental technique called quantum state carving. It relies on the reflection of weak coherent pulses from the cavity and subsequent measurement of the photons polarisation [2]. Postselection on the cases where the photon changed its linear polarisation by 90° after the reflection projects the atomic state onto an entangled state. This measurement technique is intrinsically probabilistic, but heralded. We will show the generation of all four maximally entangled atomic Bell states, a key resource for a plethora of quantum communication protocols. We will give an overview over the necessary experimental toolbox including the collective manipulation and readout of the two atomic qubits.
The so-called state-carving protocol generates high-fidelity entangled states at an atom-cavity interface without requiring high cavity cooperativity. However, this protocol is limited to 50% efficiency, which restricts its applicability. We propose a simple modification to the state-carving protocol to achieve efficient entanglement generation, with unit probability in principle. Unlike previous two-photon schemes, ours employs only one photon which interacts with the atoms twice-avoiding separate photon detections which causes irrecoverable probability loss. We present a detailed description and performance evaluation of our protocol under nonideal conditions. High fidelity of 0.999 can be achieved with cavity cooperativity of only 34. Efficient state carving paves the way for large-scale entanglement generation at cavity interfaces for modular quantum computing, quantum repeaters, and creating arbitrary shaped atomic graph states, essential for one-way quantum computing.
Abstract One of the most fascinating aspects of quantum networks is their capability to distribute entanglement as a nonlocal communication resource1. In a first step, this requires network-ready devices that can generate and store entangled states2. Another crucial step, however, is to develop measurement techniques that allow for entanglement detection. Demonstrations for different platforms3–13 suffer from being not complete, destructive or local. Here, we demonstrate a complete and nondestructive measurement scheme14–16 that always projects any initial state of two spatially separated network nodes onto a maximally entangled state. Each node consists of an atom trapped inside an optical resonator from which two photons are successively reflected. Polarization measurements on the photons discriminate between the four maximally entangled states. Remarkably, such states are not destroyed by our measurement. In the future, our technique might serve to probe the decay of entanglement and to stabilize it against dephasing via repeated measurements17,18.
Based on the interference effect of polarized photons leaking out of separate cavities, we propose schemes for the generation of the N -atom Greenberger–Horne–Zeilinger (GHZ) state, three-atom W state and a genuine four-atom entangled state |χ⟩. In these schemes, each of the atoms is trapped separately in a remote optical cavity, and the possible spontaneous channels induced by the excited atoms lead to the coherent superposition of the states of the atoms. The desired multipartite entangled states can be generated with a certain success probability by the subsequent detection of the polarized photons in different modes. The schemes would be useful steps towards long-distance quantum communication, distributed quantum computation and constructing remote quantum information processing networks.
Neutral-atom quantum processors are a promising platform for large-scale quantum computing. Integrating them with optical cavities enables fast nondestructive qubit readout and access to fast remote entanglement generation for quantum networking. In this work, we introduce a platform for coupling single atoms in optical tweezers to a Fabry-Perot fiber cavity. Leveraging the strong atom-cavity coupling, we demonstrated fast qubit-state readout with [Formula: see text] fidelity and two methods for cavity-mediated entanglement generation with integrated error detection. First, we used cavity-carving to generate a Bell state with 91(4)% fidelity and a 32(1)% success rate (the number in parentheses is the standard deviation). Second, we performed a cavity-mediated gate with a deterministic entanglement fidelity of 52.5(18)%, increased to 76(2)% with error detection. Our approach provides a route toward modular quantum computing and networking.