Experiments with ultracold quantum gases provide a platform for creating many-body systems that can be well controlled and whose parameters can be tuned over a wide range. These properties put these systems in an ideal position for simulating problems that are out of reach for classical computers. This review surveys key advances in this field and discusses the possibilities offered by this approach to quantum simulation.

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Quantum simulations with ultracold quantum gases

This article reviews recent theoretical and experimental advances in the fundamental understanding and active control of quantum fluids of light in nonlinear optical systems. In the presence of effective photon-photon interactions induced by the optical nonlinearity of the medium, a many-photon system can behave collectively as a quantum fluid with a number of novel features stemming from its intrinsically nonequilibrium nature. A rich variety of recently observed photon hydrodynamical effects is presented, from the superfluid flow around a defect at low speeds, to the appearance of a Mach-Cherenkov cone in a supersonic flow, to the hydrodynamic formation of topological excitations such as quantized vortices and dark solitons at the surface of large impenetrable obstacles. While the review is mostly focused on a specific class of semiconductor systems that have been extensively studied in recent years (planar semiconductor microcavities in the strong light-matter coupling regime having cavity polaritons as elementary excitations), the very concept of quantum fluids of light applies to a broad spectrum of systems, ranging from bulk nonlinear crystals, to atomic clouds embedded in optical fibers and cavities, to photonic crystal cavities, to superconducting quantum circuits based on Josephson junctions. The conclusive part of the article is devoted to a review of the future perspectives in the direction of strongly correlated photon gases and of artificial gauge fields for photons. In particular, several mechanisms to obtain efficient photon blockade are presented, together with their application to the generation of novel quantum phases.

Quantum fluids of light

A new quantum gas microscope that bridges the gap between microscopic and macroscopic approaches to the study of quantum systems has been developed. It uses high-resolution optical imaging to detect single atoms held in a holographically generated optical lattice. Its potential is demonstrated by the production of images of single rubidium atoms confined to an optical lattice with spacings of just 640 nanometres between atoms. The approach should facilitate quantum simulation of condensed-matter systems and find possible application in addressing and read-out of large-scale quantum information systems based on ultracold atoms. There are two different approaches for creating complex atomic many-body quantum systems — the macroscopic and the microscopic — which have, until now, been fairly disconnected. A quantum gas 'microscope' is now demonstrated that bridges the two approaches and can be used to detect single atoms held in a Hubbard-regime optical lattice. This quantum gas microscope may enable addressing and read-out of large-scale quantum information systems based on ultracold atoms. Recent years have seen tremendous progress in creating complex atomic many-body quantum systems. One approach is to use macroscopic, effectively thermodynamic ensembles of ultracold atoms to create quantum gases and strongly correlated states of matter, and to analyse the bulk properties of the ensemble. For example, bosonic and fermionic atoms in a Hubbard-regime optical lattice1,2,3,4,5 can be used for quantum simulations of solid-state models6. The opposite approach is to build up microscopic quantum systems atom-by-atom, with complete control over all degrees of freedom7,8,9. The atoms or ions act as qubits and allow the realization of quantum gates, with the goal of creating highly controllable quantum information systems. Until now, the macroscopic and microscopic strategies have been fairly disconnected. Here we present a quantum gas ‘microscope’ that bridges the two approaches, realizing a system in which atoms of a macroscopic ensemble are detected individually and a complete set of degrees of freedom for each of them is determined through preparation and measurement. By implementing a high-resolution optical imaging system, single atoms are detected with near-unity fidelity on individual sites of a Hubbard-regime optical lattice. The lattice itself is generated by projecting a holographic mask through the imaging system. It has an arbitrary geometry, chosen to support both strong tunnel coupling between lattice sites and strong on-site confinement. Our approach can be used to directly detect strongly correlated states of matter; in the context of condensed matter simulation, this corresponds to the detection of individual electrons in the simulated crystal. Also, the quantum gas microscope may enable addressing and read-out of large-scale quantum information systems based on ultracold atoms.

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A quantum gas microscope for detecting single atoms in a Hubbard-regime optical lattice

The reliable detection of single quantum particles has revolutionized the field of quantum optics and quantum information processing. For several years, researchers have aspired to extend such detection possibilities to larger-scale, strongly correlated quantum systems, in order to record in situ images of a quantum fluid in which each underlying quantum particle is detected. Here we report fluorescence imaging of strongly interacting bosonic Mott insulators in an optical lattice with single-atom and single-site resolution. From our images, we fully reconstruct the atom distribution on the lattice and identify individual excitations with high fidelity. A comparison of the radial density and variance distributions with theory provides a precise in situ temperature and entropy measurement from single images. We observe Mott-insulating plateaus with near-zero entropy and clearly resolve the high-entropy rings separating them, even though their width is of the order of just a single lattice site. Furthermore, we show how a Mott insulator melts with increasing temperature, owing to a proliferation of local defects. The ability to resolve individual lattice sites directly opens up new avenues for the manipulation, analysis and applications of strongly interacting quantum gases on a lattice. For example, one could introduce local perturbations or access regions of high entropy, a crucial requirement for the implementation of novel cooling schemes.

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Single-atom-resolved fluorescence imaging of an atomic Mott insulator

Quantum simulation, a subdiscipline of quantum computation, can provide valuable insight into difficult quantum problems in physics or chemistry. Ultracold atoms in optical lattices represent an ideal platform for simulations of quantum many-body problems. Within this setting, quantum gas microscopes enable single atom observation and manipulation in large samples. Ultracold atom–based quantum simulators have already been used to probe quantum magnetism, to realize and detect topological quantum matter, and to study quantum systems with controlled long-range interactions. Experiments on many-body systems out of equilibrium have also provided results in regimes unavailable to the most advanced supercomputers. We review recent experimental progress in this field and comment on future directions.

http://science.sciencemag.org/content/sci/357/6355/995.full.pdf

Quantum simulations with ultracold atoms in optical lattices

Trapping and optically interfacing laser-cooled neutral atoms are essential requirements for their use in advanced quantum technologies. Here we simultaneously realize both of these tasks with cesium atoms interacting with a multicolor evanescent field surrounding an optical nanofiber. The atoms are localized in a one-dimensional optical lattice about 200 nm above the nanofiber surface and can be efficiently interrogated with a resonant light field sent through the nanofiber. Our technique opens the route towards the direct integration of laser-cooled atomic ensembles within fiber networks, an important prerequisite for large scale quantum communication schemes. Moreover, it is ideally suited to the realization of hybrid quantum systems that combine atoms with, e.g., solid state quantum devices.

Optical Interface Created by Laser-Cooled Atoms Trapped in the Evanescent Field Surrounding an Optical Nanofiber

Our knowledge of ultracold quantum gases is strongly influenced by our ability to probe these objects. In situ imaging combined with single-atom sensitivity is an especially appealing scenario, as it can provide direct information on the structure and the correlations of such systems. For a precise characterization a high spatial resolution is mandatory. In particular, the perspective to study quantum gases in optical lattices makes a resolution well below one micrometre highly desirable. Here, we report on a novel microscopy technique, which is based on scanning electron microscopy and allows for the detection of single atoms inside a quantum gas with a spatial resolution of better than 150 nm. We document the great functionality of this technique by precise density measurements of a trapped Bose–Einstein condensate and the first experimental demonstration of single-site addressability in a submicrometre optical lattice. Electron microscopes are regularly used to resolve atoms in solid samples. It turns out that they can also be used to image atoms in a Bose–Einstein condensate—remarkably, without destroying the coherent properties of the condensate.

High-resolution scanning electron microscopy of an ultracold quantum gas

We dispersively interface an ensemble of 1000 atoms trapped in the evanescent field surrounding a tapered optical nanofiber. This method relies on the azimuthally asymmetric coupling of the ensemble with the evanescent field of an off-resonant probe beam, transmitted through the nanofiber. The resulting birefringence and dispersion are significant; we observe a phase shift per atom of $\ensuremath{\sim}1\text{ }\text{ }\mathrm{mrad}$ at a detuning of 6 times the natural linewidth, corresponding to an effective resonant optical density per atom of 0.027. Moreover, we utilize this strong dispersion to nondestructively determine the number of atoms.

Dispersive Optical Interface Based on Nanofiber-Trapped Atoms

We report the production of a F=1 spinor condensate of 87Rb atoms in a single beam optical dipole trap formed by a focused CO2 laser. The condensate is produced 13 mm below the tip of a scanning electron microscope employing standard all-optical techniques. The condensate contains up to 120000 atoms and we achieve a duty cycle of less than 10 s.

All-optical formation of a Bose–Einstein condensate for applications in scanning electron microscopy

We report on the production of a F=1 spinor condensate of 87Rb atoms in a single beam optical dipole trap formed by a focused CO2 laser. The condensate is produced 13mm below the tip of a scanning electron microscope employing standard all-optical techniques. The condensate fraction contains up to 100,000 atoms and we achieve a duty cycle of less than 10s.

Daniel Reitz

Papers

Optical Interface Created by Laser-Cooled Atoms Trapped in the Evanescent Field Surrounding an Optical Nanofiber

High-resolution scanning electron microscopy of an ultracold quantum gas

Dispersive Optical Interface Based on Nanofiber-Trapped Atoms

All-optical formation of a Bose–Einstein condensate for applications in scanning electron microscopy

All-optical formation of a Bose-Einstein condensate for applications in scanning electron microscopy