D. L. Moehring

Entanglement of single-atom quantum bits at a distance.

Quantum information science involves the storage, manipulation and communication of information encoded in quantum systems, where the phenomena of superposition and entanglement can provide enhancements over what is possible classically. Large-scale quantum information processors require stable and addressable quantum memories, usually in the form of fixed quantum bits (qubits), and a means of transferring and entangling the quantum information between memories that may be separated by macroscopic or even geographic distances. Atomic systems are excellent quantum memories, because appropriate internal electronic states can coherently store qubits over very long timescales. Photons, on the other hand, are the natural platform for the distribution of quantum information between remote qubits, given their ability to traverse large distances with little perturbation. Recently, there has been considerable progress in coupling small samples of atomic gases through photonic channels, including the entanglement between light and atoms and the observation of entanglement signatures between remotely located atomic ensembles. In contrast to atomic ensembles, single-atom quantum memories allow the implementation of conditional quantum gates through photonic channels, a key requirement for quantum computing. Along these lines, individual atoms have been coupled to photons in cavities, and trapped atoms have been linked to emitted photons in free space. Here we demonstrate the entanglement of two fixed single-atom quantum memories separated by one metre. Two remotely located trapped atomic ions each emit a single photon, and the interference and detection of these photons signals the entanglement of the atomic qubits. We characterize the entangled pair by directly measuring qubit correlations with near-perfect detection efficiency. Although this entanglement method is probabilistic, it is still in principle useful for subsequent quantum operations and scalable quantum information applications.

Manipulation and detection of a trapped Yb+ hyperfine qubit

We demonstrate the use of trapped ytterbium ions as quantum bits for quantum information processing. We implement fast, efficient state preparation and state detection of the first-order magnetic field-insensitive hyperfine levels of

Quantum interference of photon pairs from two remote trapped atomic ions

^{171}\mathrm{Yb}^{+}

Photon-photon entanglement with a single trapped atom.

, with a measured coherence time of

Experimental Bell Inequality Violation with an Atom and a Photon

2.5\phantom{\rule{0.3em}{0ex}}\mathrm{s}

Phase shaping of single-photon wave packets

. The high efficiency and high fidelity of these operations is accomplished through the stabilization and frequency modulation of relevant laser sources.

Lossless State Detection of Single Neutral Atoms

Trapped atomic ions are among the most attractive implementations of quantum bits for applications in quantum-information processing, owing to their long trapping lifetimes and long coherence times. Although nearby trapped ions can be entangled through their Coulomb-coupled motion1,2,3,4,5,6, it seems more natural to entangle remotely located ions through a coupling mediated by photons, eliminating the need to control the ion motion. A promising way to entangle ions via a photonic channel is to interfere two photons emitted from the ions and then detect appropriate photon coincidence events7,8,9. Here, we report the pivotal element of this scheme in the observation of quantum interference between pairs of single photons emitted from two atomic ions residing in independent traps.

Probabilistic quantum gates between remote atoms through interference of optical frequency qubits

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.

Quantum networking with photons and trapped atoms (Invited)

We report the measurement of a Bell inequality violation with a single atom and a single photon prepared in a probabilistic entangled state. This is the first demonstration of such a violation with particles of different species. The entanglement characterization of this hybrid system may also be useful in quantum information applications.

Fast Excitation and Photon Emission of a Single-Atom-Cavity System

While the phase of a coherent light field can be precisely known, the phase of the individual photons that create this field, considered individually, cannot [1]. Phase changes within singlephoton wave packets, however, have observable effects. In fact, actively controlling the phase of individual photons has been identified as a powerful resource for quantum communication protocols [2, 3]. Here we demonstrate the arbitrary phase control of a single photon. The phase modulation is applied without affecting the photon’s amplitude profile and is verified via a two-photon quantum interference measurement [4, 5], which can result in the fermionic spatial behaviour of photon pairs. Combined with previously demonstrated control of a single photon’s amplitude [6, 7, 8, 9, 10], frequency [11], and polarisation [12], the fully deterministic phase shaping presented here allows for the complete control of single-photon wave packets. Consider two identical photons mode-matched at the two input ports (A and B) of a 50/50 non-polarising beam splitter (NPBS), represented by the initial state |Ψi〉 = |1A1B〉 (see Fig. 1). Due to the indistinguishability of the photons, the detection of one photon in output port C or D at time t0 projects the initial product state |Ψi〉 into the “which path” superposition state |Ψ±(t0)〉 = (|1A, 0B〉 ± |0A, 1B〉)/ √ 2 of the remaining photon. As first demonstrated by Hong, Ou and Mandel [4], the bosonic nature of photons always results in the detection of the second photon in the same output port as the first. However, we can alter this coalescence behaviour by introducing an arbitrary differential phase ∆φ between the two components of |Ψ±〉. This results in a phase-dependent wave function of the remaining single photon