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Science

Single-Atom Single-Photon Quantum Interface

Tatjana Wilk, S. C. Webster, Axel Kuhn, Gerhard Rempe

Published Jun 22, 2007• 509 citations
Publisher

Abstract

A major challenge for a scalable quantum computing architecture is the faithful transfer of information from one node to another. We report on the realization of an atom-photon quantum interface based on an optical cavity, using it to entangle a single atom with a single photon and then to map the quantum state of the atom onto a second single photon. The latter step disentangles the atom from the light and produces an entangled photon pair. Our scheme is intrinsically deterministic and establishes the basic element required to realize a distributed quantum network with individual atoms at rest as quantum memories and single flying photons as quantum messengers.

DOI
10.1126/science.1143835

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A major challenge for a scalable quantum computing architecture is the faithful transfer of information from one node to another. We report on the realization of an atom-photon quantum interface based on an optical cavity, using it to entangle a single atom with a single photon and then to map the quantum state of the atom onto a second single photon. The latter step disentangles the atom from the light and produces an entangled photon pair. Our scheme is intrinsically deterministic and establishes the basic element required to realize a distributed quantum network with individual atoms at rest as quantum memories and single flying photons as quantum messengers.
A prerequisite for the realization of quantum networks is a coherent interface between flying and stationary qubits. So far, most of these interfaces have been based on the exchange of information between photons and collective atomic excitations. A promising alternative is the development of an interface between single photons and a single atom, employing an optical cavity to achieve sufficient coupling strength. This approach has fundamental advantages, as it allows for the individual manipulation of the atomic qubit and the implementation of a heralding scheme based on quantum non-demolition (QND) measurements of the atomic state.
Summary form only given. Implementation of a quantum network with single atoms as quantum nodes and single photons as channels between the nodes requires their interfacing in receiver mode, whereby the photonic qubit is mapped onto the internal degrees of freedom of the atom [1]. Combining such a receiver interface with a source of entangled photon pairs enables the distribution of entanglement between network nodes. We implemented the heralded state-mapping protocol of [2] between a narrowband entangled-photon source and a single trapped40Ca+ion and transferred photon-photon to atom-photon entanglement.
A single atom strongly coupled to a high-finesse optical-cavity is a versatile tool for quantum information processing. Utilized as a single-photon source, it allows one to generate single photons very efficiently in a well defined spatio-temporal mode. In a first experiment, polarization-control over the photons is shown. A time-resolved two-photon interference experiment proves the indistinguishability of these photons - required in various quantum information processing schemes. Moreover, in a second experiment, entanglement between the polarization of the emitted photon and the population of the atomic Zeeman levels is created. Subsequent state mapping of the atomic state onto another photon results in a pair of polarization-entangled photons emitted one after the other from the cavity. Although these schemes are in principle possible in free space, the cavity boosts the efficiency by several orders of magnitude.
Distributed quantum computing requires a quantum network with robust links between physically separated quantum nodes [1]. Hybrid networks where nodes from different types of stationary qubits (such as neutral atoms or solid state defects) are linked, can be used to take advantage of the strengths of various systems. An efficient interface for fast and reliable exchange of quantum information between stationary qubits at nodes with photons, or traveling qubits, is needed for distributing entanglement between nodes. A high rate of generating and distributing light-matter entangled states will scale up the speed of computation distributed between the nodes of a network.
An experiment is performed where a single rubidium atom trapped within a high-finesse optical cavity emits two independently triggered entangled photons. The entanglement is mediated by the atom and is characterized both by a Bell inequality violation of S=2.5, as well as full quantum-state tomography, resulting in a fidelity exceeding F=90%. The combination of cavity-QED and trapped atom techniques makes our protocol inherently deterministic--an essential step for the generation of scalable entanglement between the nodes of a distributed quantum network.