Controlling the quantum entanglement between parts of a many-body system is key to unlocking the power of quantum technologies such as quantum computation, high-precision sensing, and the simulation of many-body physics. The spin degrees of freedom of ultracold neutral atoms in their ground electronic state provide a natural platform for such applications thanks to their long coherence times and the ability to control them with magneto-optical fields. However, the creation of strong coherent coupling between spins has been challenging. Here we demonstrate a strong and tunable Rydberg-dressed interaction between spins of individually trapped caesium atoms with energy shifts of order 1 MHz in units of Planck's constant. This interaction leads to a ground-state spin-flip blockade, whereby simultaneous hyperfine spin flips of two atoms are inhibited owing to their mutual interaction. We employ this spin-flip blockade to rapidly produce single-step Bell-state entanglement between two atoms with a fidelity ≥81(2)%. P ristine quantum control of many-body correlations is fundamental to realizing the power of quantum information processors. Steady progress has continued in various platforms ranging from solid-state spintronics 1 and superconductors 2,3 to nanophotonics 4 and ultracold trapped atoms, both ionic 5-7 and neutral 8-10 . Cold neutral atoms are particularly attractive as the ability to create entanglement between atoms would allow for greatly increased precision of interferometers for applications in clocks 11-13 , and force sensors 14-16 . In addition, cold atoms provide a natural platform for quantum simulation of condensed-matter physics 17,18 and scalable digital quantum computers 19-21 . Controlled entanglement of neutral atoms, however, has been challenging, particularly if one seeks tunable interactions that are strong, coherent and long-range (∼µm).One mechanism to achieve strong, long-range coupling is the Rydberg blockade 22 . This has been successfully employed for implementing controlled entangling interactions between atoms 9,10,23 and quantum logic gates 24 . In the standard protocol, short pulses excite the population of one atom to the Rydberg state and optical excitation of a second atom is blockaded because of the electric dipole-dipole interaction 21 (EDDI). An alternative protocol is to adiabatically dress the ground state with the excited Rydberg state 25-27 . This Rydberg-dressed interaction enables tunable, anisotropic interactions that open the door to quantum simulations of a variety of exotic quantum phases 26,28,29 . In addition, it allows for quantum control of interacting atoms based solely on microwave/radiofrequency fields whose phase coherence is easily maintained. Applications include spin-squeezing for metrology 13,25 , and quantum computing 30,31 . Although the promise of Rydbergdressed interactions is great, experimental demonstration has been elusive. We present here a clear measurement of this interaction between two Rydberg-dressed atoms and employ coherent control in the ...

We describe an optical atomic clock based on quantum-logic spectroscopy of the 1 S0 ↔ 3 P0 transition in 27 Al + with a systematic uncertainty of 9.4 × 10 −19 and a frequency stability of 1.2 × 10 −15 / √ τ. A 25 Mg + ion is simultaneously trapped with the 27 Al + ion and used for sympathetic cooling and state readout. Improvements in a new trap have led to reduced secular motion heating, compared to previous 27 Al + clocks, enabling clock operation with ion secular motion near the three-dimensional ground state. Operating the clock with a lower trap drive frequency has reduced excess micromotion compared to previous 27 Al + clocks. Both of these improvements have led to a reduced time-dilation shift uncertainty. Other systematic uncertainties including those due to blackbody radiation and the second-order Zeeman effect have also been reduced.

We explore a single-photon approach to Rydberg state excitation and Rydberg blockade. Using detailed theoretical models, we show the feasibility of direct excitation, predict the effect of background electric fields, and calculate the required interatomic distance to observe Rydberg blockade. We then measure and control the electric field environment to enable coherent control of Rydberg states. With this coherent control, we demonstrate Rydberg blockade of two atoms separated by 6.6(3) µm. When compared with the more common two-photon excitation method, this singlephoton approach is advantageous because it eliminates channels for decoherence through photon scattering and ac Stark shifts from the intermediate state while moderately increasing Doppler sensitivity.