We report results of a ground-state entanglement protocol for a pair of Cs atoms separated by 6 µm, combining the Rydberg blockade mechanism with a two-photon Raman transition to prepare the |Ψ + = (|10 +|01 )/ √ 2 Bell state with a loss-corrected fidelity of 0.81 (5), equal to the best demonstrated fidelity for atoms trapped in optical tweezers but without the requirement for dynamically adjustable interatomic spacing. Qubit state coherence is also critical for quantum information applications, and we characterise both ground-state and ground-Rydberg dephasing rates using Ramsey spectroscopy. We demonstrate transverse dephasing times T * 2 = 10(1) ms and T 2 = 0.14(1) s for the qubit levels and achieve long ground-Rydberg coherence times of T * 2 = 17(2) µs as required for implementing high-fidelity multi-qubit gate sequences where a control atom remains in the Rydberg state while applying local operations on neighbouring target qubits.

Single atom imaging requires discrimination of weak photon count events above background and has typically been performed using either EMCCD cameras, photomultiplier tubes or single photon counting modules. sCMOS provides a cost effective and highly scalable alternative to other single atom imaging technologies, offering fast readout and larger sensor dimensions. We demonstrate single atom resolved imaging of two siteaddressable optical traps separated by 10 µm using an sCMOS camera, offering a competitive signal-to-noise ratio at intermediate count rates to allow high fidelity readout discrimination (error < 10 −6 ) and sub-µm spatial resolution for applications in quantum technologies.

Quantum information processing using atomic qubits requires narrow linewidth lasers with longterm stability for high fidelity coherent manipulation of Rydberg states. In this paper, we report on the construction and characterization of three continuous-wave (CW) narrow linewidth lasers stabilized simultaneously to an ultra-high finesse Fabry-Perot cavity made of ultra-low expansion (ULE) glass, with a tunable offset-lock frequency. One laser operates at 852 nm while the two locked lasers at 1018 nm are frequency doubled to 509 nm for excitation of 133 Cs atoms to Rydberg states. The optical beatnote at 509 nm is measured to be 260(5) Hz. We present measurements of the offset between the atomic and cavity resonant frequencies using electromagnetically induced transparency (EIT) for high-resolution spectroscopy on a cold atom cloud. The long-term stability is determined from repeated spectra over a period of 20 days yielding a linear frequency drift of ∼ 1 Hz/s. I. INTRODUCTIONThe field of quantum information processing (QIP) is an intensive research area. Its attractiveness lies in the possibility of speeding up classical problems and modelling complex quantum systems [1]. Neutral atoms present an attractive candidate for scalable QIP [2, 3] combining long coherence times of weakly interacting hyperfine ground states [4] with strongly interacting Rydberg states [5] to create pair-wise entanglement [6,7], perform deterministic quantum gates [8,9] and even realize a quantum simulator for Ising models [10].Rydberg excitation is typically performed using two-photon excitation due to weak single photon matrix elements from the ground state [11] and inconvenient UV wavelengths. Using a resonant two-photon excitation, Rydberg electromagnetically induced transparency (EIT) [12] can be exploited for laser frequency stabilization [13] as well as precision metrology of Rydberg state energies [14,15] and lifetimes [16], dc electric fields [17] and RF field sensors operating at both microwave [18,19] and THz [20] frequency ranges. For dense cold atom samples, the strong atomic interactions can be mapped onto the optical field to create non-linearities at the single photon level [21][22][23][24].For robust Rydberg excitation of atomic qubits for gate operations the two-photon excitation must be * Electronic address: jonathan.pritchard@strath.ac.uk

High power lasers have become useful scientific tools, but their large size is determined by their low damage-threshold optical media. A more robust and compact medium for amplifying and manipulating intense laser pulses is plasma. Here we demonstrate, experimentally and through simulations, that few-millijoule, ultra-short seed pulses interacting with 3.5-J counter-propagating pump pulses in plasma, stimulate back-scattering of nearly 100 mJ pump energy with high intrinsic efficiency, when detuned from Raman resonance. This is due to scattering off a plasma Bragg grating formed by ballistically evolving ions. Electrons are bunched by the ponderomotive force of the beat-wave, which produces space-charge fields that impart phase correlated momenta to ions. They inertially evolve into a volume Bragg grating that backscatters a segment of the pump pulse. This, ultra-compact, two-step, inertial bunching mechanism can be used to manipulate and compress intense laser pulses. We also observe stimulated Compton (kinetic) and Raman backscattering.