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Versatile interfaces with strong and tunable light–matter interactions are essential for quantum science1 because they enable mapping of quantum properties between light and matter1. Recent studies2–10 have proposed a method of controlling light–matter interactions using the rich interplay of photon-mediated dipole–dipole interactions in structured subwavelength arrays of quantum emitters. However, a key aspect of this approach—the cooperative enhancement of the light–matter coupling strength and the directional mirror reflection of the incoming light using an array of quantum emitters—has not yet been experimentally demonstrated. Here we report the direct observation of the cooperative subradiant response of a two-dimensional square array of atoms in an optical lattice. We observe a spectral narrowing of the collective atomic response well below the quantum-limited decay of individual atoms into free space. Through spatially resolved spectroscopic measurements, we show that the array acts as an efficient mirror formed by a single monolayer of a few hundred atoms. By tuning the atom density in the array and changing the ordering of the particles, we are able to control the cooperative response of the array and elucidate the effect of the interplay of spatial order and dipolar interactions on the collective properties of the ensemble. Bloch oscillations of the atoms outside the array enable us to dynamically control the reflectivity of the atomic mirror. Our work demonstrates efficient optical metamaterial engineering based on structured ensembles of atoms4,8,9 and paves the way towards controlling many-body physics with light5,6,11 and light–matter interfaces at the single-quantum level7,10. A single two-dimensional array of atoms trapped in an optical lattice shows a tunable cooperative subradiant optical response, acting as a single-monolayer optical mirror with controllable reflectivity. [ABSTRACT FROM AUTHOR]

Trapped quantum gases can be cooled to impressively low temperatures, but it is unclear whether their entropy is low enough to realize phenomena such as d-wave superconductivity and magnetic ordering. Estimated critical entropies per particle for quantum magnetic ordering are ∼0.3kB and ∼0.03kB for bosons in three- and two-dimensional lattices, respectively, with similar values for Néel ordering of lattice-trapped Fermi gases. Here we report reliable single-shot temperature measurements of a degenerate Rb gas by imaging the momentum distribution of thermalized magnons, which are spin excitations of the atomic gas. We record average temperatures fifty times lower than the Bose-Einstein condensation temperature, indicating an entropy per particle of ∼0.001kB at equilibrium, nearly two orders of magnitude lower than the previous best in a dilute atomic gas and well below the critical entropy for antiferromagnetic ordering of a Bose-Hubbard system. The magnons can reduce the temperature of the system by absorbing energy during thermalization and by enhancing evaporative cooling, allowing the production of low-entropy gases in deep traps. [ABSTRACT FROM AUTHOR]

Optomechanical systems, in which light drives and is affected by the motion of a massive object, will comprise a new framework for nonlinear quantum optics, with applications ranging from the storage and transduction of quantum information to enhanced detection sensitivity in gravitational wave detectors. However, quantum optical effects in optomechanical systems have remained obscure, because their detection requires the object's motion to be dominated by vacuum fluctuations in the optical radiation pressure; so far, direct observations have been stymied by technical and thermal noise. Here we report an implementation of cavity optomechanics using ultracold atoms in which the collective atomic motion is dominantly driven by quantum fluctuations in radiation pressure. The back-action of this motion onto the cavity light field produces ponderomotive squeezing. We detect this quantum phenomenon by measuring sub-shot-noise optical squeezing. Furthermore, the system acts as a low-power, high-gain, nonlinear parametric amplifier for optical fluctuations, demonstrating a gain of 20?dB with a pump corresponding to an average of only seven intracavity photons. These findings may pave the way for low-power quantum optical devices, surpassing quantum limits on position and force sensing, and the control and measurement of motion in quantum gases. [ABSTRACT FROM AUTHOR]

Ultracold atoms are emerging as an important platform for precision sensing and measurement, quantum information science, and simulations of condensed-matter phenomena. Microscopic imaging is a powerful tool for measuring cold-atom systems, enabling the readout of ultracold atomic simulators and registers, the characterization of inhomogeneous environments, and the determination of spatially varying thermodynamic quantities. Cold-atom microscopy has recently been demonstrated with imaging resolution sufficient to detect and address single or multiple atoms at individual optical-lattice sites with lattice spacings of micrometres and below. However, those methods, which rely either on the fluorescence or ionization of atoms, destroy the quantum states being measured and have limited dynamic range. Here we demonstrate magnetic resonance imaging of atomic gases in optical lattices, obtained by dispersively coupling atoms to a high-finesse optical cavity. We achieve state-sensitive, single-lattice-site images with high dynamic range. We also apply this technique to measure the non-equilibrium transport dynamics of the gas. [ABSTRACT FROM AUTHOR]

Current research on micromechanical resonators strives for quantum-limited detection of the motion of macroscopic objects. Prerequisite to this goal is the observation of measurement backaction consistent with quantum metrology limits. However, thermal noise currently dominates measurements and precludes ground-state preparation of the resonator. Here, we establish the collective motion of an ultracold atomic gas confined tightly within a Fabry–Perot optical cavity as a system for investigating the quantum mechanics of macroscopic bodies. The cavity-mode structure selects a particular collective vibrational motion that is measured by the cavity’s optical properties, actuated by the cavity optical field and subject to backaction by the quantum force fluctuations of this field. Experimentally, we quantify such fluctuations by measuring the cavity-light-induced heating of the intracavity atomic ensemble. These measurements represent the first observation of backaction on a macroscopic mechanical resonator at the standard quantum limit. [ABSTRACT FROM AUTHOR]

We study the ground-state behavior of a Bose-Einstein Condensate (BEC) in a Raman-laser-assisted one-dimensional (1D) optical lattice potential forming a multilayer system. We find that, such system can be described by an effective model with spin-orbit coupling (SOC) of pseudospin (N-1)/2, where N is the number of layers. Due to the intricate interplay between atomic interactions, SOC and laser-assisted tunnelings, the ground-state phase diagrams generally consist of three phases-a stripe, a plane wave and a normal phase with zero-momentum, touching at a quantum tricritical point. More important, even though the single-particle states only minimize at zero-momentum for odd N, the many-body ground states may still develop finite momenta. The underlying mechanisms are elucidated. Our results provide an alternative way to realize an effective spin-orbit coupling of Bose gas with the Raman-laser-assisted optical lattice, and would also be beneficial to the studies on SOC effects in spinor Bose systems with large spin. [ABSTRACT FROM AUTHOR]