Kelly Cooper Younge

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.

Quantum interference of photon pairs from two remote trapped atomic ions

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.

Rotary echo tests of coherence in Rydberg-atom excitation

Rotary echoes are employed to study excitation dynamics in many-body Rydberg systems. In this method, a phase reversal of a narrow-band excitation field is applied at a variable time during the excitation pulse. The visibility of the resulting echo signal reveals the degree of coherence of the excitation process. Rotary echoes are measured for several nD5/2 Rydberg levels of rubidium with principal quantum numbers near n=43, where the strength of electrostatic Rydberg-atom interactions is modulated by a Forster resonance. The Rydberg-atom interactions are shown to diminish the echo visibility, in agreement with recent theoretical work. The equivalence of echo signals with spectroscopic data is demonstrated.

A model system for examining the radial distribution function

The radial distribution function provides a means of characterizing an amorphous material and is a measure of the spatial distribution of a system of particles. We introduce an experiment suitable for the undergraduate laboratory that illustrates the meaning and application of the radial distribution function to a two-dimensional system of hard spheres comprised of varying area fractions. Larger area fractions lead to an increase in the correlation length and the magnitude of the underlying particle–particle correlations.

Adiabatic potentials for Rydberg atoms in a ponderomotive optical lattice

We calculate the adiabatic potentials and adiabatic wavefunctions of Rydberg atoms in a one-dimensional ponderomotive optical lattice. All lattice- induced couplings between the degenerate high-angular-momentum Rydberg states are taken into account. To obtain insight into the underlying physics, we analyze the numerical results in terms of effective electric and magnetic fields produced by the lattice. Near the inflection points of the lattice potential and for sufficiently low principal quantum numbers (n . 35 in the cases studied), the adiabatic level structure resembles that of the dc Stark effect, and an effective electric-field model can be used to model the lattice-induced perturbation. Near the nodes and anti-nodes of the lattice field, the adiabatic level structure exhibits a combination of adjacent rotational and vibrational energy level sequences. Here, an analogy between the ponderomotive optical lattice and the diamagnetic problem works well to interpret the lattice-induced perturbation in terms of an effective magnetic field.

Rydberg-atom trajectories in a ponderomotive optical lattice

Using semiclassical simulations, we investigate the trajectories and the microwave spectra of Rydberg atoms excited in a ponderomotive optical lattice. We relate distinct features found in the microwave spectra to characteristic types of trajectory. Several methods are presented that are designed to greatly improve the trapping efficiency of the lattice and to generalize the trapping from one to three dimensions.

Measurement of the lifetime of the6pP21/2olevel ofYb+

We present a precise measurement of the lifetime of the 6p 2P_1/2 excited state of a single trapped ytterbium ion (Yb+). A time-correlated single-photon counting technique is used, where ultrafast pulses excite the ion and the emitted photons are coupled into a single-mode optical fiber. By performing the measurement on a single atom with fast excitation and excellent spatial filtering, we are able to eliminate common systematics. The lifetime of the 6p 2P_1/2 state is measured to be 8.12 +/- 0.02 ns.

Quantum Interference of Electromagnetic Fields from Two Remote Trapped Atomic Ions

We observe quantum Hong Ou Mandel interference between electromagnetic fields emitted from two remote trapped ytterbium ions. This result points the way toward scaling to large entangled networks of remote qubits.