Atomtronics deals with matter-wave circuits of ultracold atoms manipulated through magnetic or laser-generated guides with different shapes and intensities. In this way, new types of quantum networks can be constructed in which coherent fluids are controlled with the know-how developed in the atomic and molecular physics community. In particular, quantum devices with enhanced precision, control, and flexibility of their operating conditions can be accessed. Concomitantly, new quantum simulators and emulators harnessing on the coherent current flows can also be developed. Here, the authors survey the landscape of atomtronics-enabled quantum technology and draw a roadmap for the field in the near future. The authors review some of the latest progress achieved in matter-wave circuits' design and atom-chips. Atomtronic networks are deployed as promising platforms for probing many-body physics with a new angle and a new twist. The latter can be done at the level of both equilibrium and nonequilibrium situations. Numerous relevant problems in mesoscopic physics, such as persistent currents and quantum transport in circuits of fermionic or bosonic atoms, are studied through a new lens. The authors summarize some of the atomtronics quantum devices and sensors. Finally, the authors discuss alkali-earth and Rydberg atoms as potential platforms for the realization of atomtronic circuits with special features.

## I. INTRODUCTION

Quantum technologies are enabling important innovations in the 21st century with applications in areas as diverse as computation, simulation, sensing, and communication. The core of these new technological developments is the ability to control quantum systems all the way from the macroscopic scale down to the single quantum level. The latter has been achieved in physical systems ranging from atomic and spin systems to artificial atoms in the form of superconducting circuits.^{1,2}

In this article, we mostly focus on cold atom systems, where recent technological developments have delivered a collection of magnetic or laser-generated networks and guides in which atomic matter-waves can be controlled and manipulated coherently.^{3,4} *Atomtronics* exploits the state of the art in this field to realize matter-wave circuits of ultracold atoms.^{4,5} Some key aspects of this emerging field give atomtronic circuits great promise as a quantum technology. First, since atomtronic circuits employ matter-waves of neutral atoms, spurious circuit-environment interactions, which might, e.g., lead to decoherence, are expected to be less serious than in networks employing electrically charged fluids sensitive to Coulomb forces. Second, atomtronic networks can realize new types of circuits with current carriers having bosonic and/or fermionic quantum statistics along with tunable particle–particle interactions ranging from short-range to long-distance and from attractive to repulsive. Third, recent progress in the manipulation of optical guiding potentials enables engineering of time-dependent circuits whose topology can be reconfigured while they operate.^{3,6–9}

The name *Atomtronics* is inspired by the analogy between circuits with ultracold atomic currents and those formed by electron-based networks of conductors, semiconductors, or superconductors. For example, a Bose–Einstein condensate (BEC) confined in a linear optical lattice with a suitable abrupt variation of the particle density can exhibit behavior very similar to that of an electronic diode.^{10,11} As another example, a BEC in suitable optical ring trap is the atomic counterpart of the superconducting SQUID of quantum electronics,^{11–13} displaying the SQUID's defining characteristics of quantum interference^{15} and hysteresis.^{16} It is important to note that because atomtronics is entirely based on flexible potential landscapes and not limited to material properties, it is expected to be possible to create quantum devices and simulators with new architectures and functionalities that have no analog in conventional electronics.

The quantum nature of ultracold atoms as coherent matter waves enables interferometric precision measurements and new platforms for quantum information processing with applications in fundamental science and technology.^{17,18} At the same time, atomtronic circuits can serve as powerful probes of many-body quantum regimes: analogous to solid state I-V characteristics, and many-body cold atom systems can be probed by monitoring the current flowing in them while changes are made to external parameters and applied (effective) fields. In this way, atomtronic platforms can be thought of as extensions to the scope of conventional quantum simulators, revisiting textbook scenarios in many-body physics, such as frustration effects, topological constraints, and edge state formation, with the advantages of tunable boundary conditions and minimal finite size effects. Another interesting domain in which atomtronics can play an important role is mesoscopic physics.^{19–21} Important themes in the field of mesoscopic physics, such as persistent currents in ring-shaped structures and problems of quantum coherent transport, can be explored with a new twist.

For the implementation of the program sketched above, an important challenge to face in the years to come is to optimize the control of the matter-wave currents in complex networks as, for example, optical lattices, guiding circuits for matter waves based on optical or magnetic fields, or cold atoms-solid state hybrid circuits. On one hand, such a step would be instrumental to harness current and transport for investigations on quantum many-body physics and artificial matter in both the static and dynamic conditions. In particular, Rydberg atoms and ultracold fermionic systems with SU(*N*) symmetry provide novel interesting directions to go to. Experimental challenges for this goal are to design improved schemes for controlling the resulting matter-wave interactions and for including advanced schemes for their detection. On the other hand, the control of complex quantum networks would be opening the way to work out new types of devices based on integrated atomtronic circuits. In particular, new chips integrating different technologies, for example, silicon-based electronics and the various atomtronics approaches, would provide a milestone in quantum technology. Concerning potential applications, a certainly important direction pursued in the current research in *Atomtronics* is devoted to interferometry and inertial sensing with enhanced performance, but quantum simulation and computation as well as all other aspects of quantum technology are accessible. In this context, stabilizing the atomic coherence on small-to-intermediate spatial scales, for example, by smoothing the wave guides, are important challenges to be solved in order to harness the full power of cold-matter-wave quantum technology.

In this review, we summarize recent activities in *Atomtronics* and discuss the future of the field. In Secs. II–IV, we review fabrication principles for atomtronic platforms, ranging from reconfigurable optical potentials employing acousto-optic deflectors, digital micromirror devices, and liquid-crystal spatial light modulators to micro-optical systems and hybrid solid state–cold atom systems circuits where a scanning focused laser beam modifies the current density of a superconducting chip to create the desired trapping potential. These new capabilities open the way to addressing the dynamics of many-body systems, as described in Secs. V and VI. Sections VII and VIII deal with persistent currents in toroidal and ring-shaped condensates. These systems, the simplest atomtronic circuits with a closed architecture, enable the study of basic questions in many-body physics in a variety of new and different conditions. Atomtronic quantum sensors and devices are discussed in Sec. IX. Ring-shaped bosonic circuits are investigated as ideal platforms for matter-wave SQUIDs [the Atomtronic QUantum Interference Device (AQUID)] and flux qubits in Sec. X. These studies have also touched upon a number of fundamental questions, such as macroscopic quantum coherence, the nature of superfluidity in restricted geometries, and vortex dynamics. Transport in fermionic and bosonic circuits are discussed in Sec. XI and XII, respectively. Section XIII deals with bosonic ladders. In addition to their potential relevance to basic research in many-body physics, we envisage that they will be instrumental to the fabrication of coupled atomtronic circuits. In Sec. XIV, we discuss atomtronic circuits that exploit bright solitons both for studying fundamental questions in many-body quantum dynamics and for realizing quantum devices with enhanced performances. Sections XV and XVI deal with alkali-earth atoms with SU(*N*) symmetry and Rydberg atoms. To date, the latter have received little attention, but we believe that they offer great promise as an atomtronic quantum technology.

The present article was inspired by the Atomtronics@Benasque conference series. The Benasque staff is warmly acknowledged for their invaluable help in the organization of these workshops, and we thank the Benasque director Jose-Ignacio Latorre for his constant support of this line of research.

## II. DYNAMICALLY SCULPTED LIGHT

** M. Baker, G. Gauthier, T. W. Neely, H. Rubinsztein-Dunlop, F. Tosto, R. Dumke, P. Ireland, D. Cassettari **

In recent years, many experiments have been carried out with cold neutral atoms in arbitrary, reconfigurable optical potentials. Single atoms have been trapped in arbitrary-shaped arrays,^{22–26} which have subsequently led to the demonstration of topological phases of interacting bosons in one-dimensional lattices.^{27} Various configurations of atomtronic circuits have been demonstrated, namely, closed waveguides and Y-junctions,^{28} oscillator circuits,^{29} atomtronic transistors,^{30} rings, and atomtronic SQUIDs (AQUIDs).^{12,31,32} Reconfigurable optical potentials have also been used to realize Josephson junctions in rubidium condensates^{12} and in fermionic lithium superfluids in the BCS-BEC crossover.^{33} They have even been used for the optimization of rapid cooling to quantum degeneracy.^{34} Finally, another area of interest is the realization and study of quantum gases in uniform potentials.^{35,36} Some of these experiments are described in detail in Secs. II A–II E.

Static holographic potentials, as opposed to reconfigurable, also play an important role in atomtronics and have been implemented with great success.^{37–42} In particular, static holograms can provide substantial advantages for the generation of Laguerre–Gaussian and higher order Hermite–Gaussian modes.^{40,43,44} Static hologram techniques, such as optical nanofiber evanescent wave trapping,^{45} structured nanosurfaces to create trapping potentials,^{46,47} and the use of engineered quantum forces^{48} (also know as London–van der Waals or Casimir forces), are promisingly emerging technologies that will benefit the field of atomtronics. However, this section focuses on recently adopted *dynamic* technologies that have opened new avenues of research.

More generally, we note that sculpted light has many more applications beyond cold atom physics, e.g., to microscopy, optical tweezers, and quantum information processing with photonic systems.^{3} In this section, we review the tools and techniques that underpin all these experiments: scanning acousto-optic deflectors (AODs), digital micromirror devices (DMDs), and liquid-crystal spatial light modulators (SLMs).

### A. Fast-scanning AODs

By rapidly scanning a trapping laser beam much faster than the trapping frequencies for the atoms, the atoms experience the time-average of the optical potential. Under these conditions, despite the modulated scanning action of the beam, the density of the atom cloud remains constant in time. The spatial location of the beam can be scanned in arbitrary 2D patterns, “painting” the potential landscape, simply by modulating the RF frequencies driving the crystal.^{7} The control over the RF power at each scan location allows local control over the potential depth. This feature can be used to error-correct, ensuring smooth homogeneous potentials, or can be deliberately engineered to implement barriers, wells, or gradients in the trap. The trapping geometry can be dynamically changed with the use of deep-memory arbitrary waveform generators or field programmable gate array (FPGA) technology, which combined with nondestructive measurement allows for the real-time correction of the potential. Given the weak axial confinement provided by the scanned beam, this is best used in conjunction with an orthogonal light sheet, which provides tight confinement along the axis of the scanned beam, and ensuring excitation and phase fluctuations in the axial dimension is minimized.^{7}

#### 1. Feed-forward control

The diffraction efficiency of AODs can change with the drive frequency. In order to correct this, it is generally necessary to use feed-forward to compensate by adjusting the RF power of the AOD crystal and, hence, the beam intensity at each (x,y) location. To correct imperfections in other elements of the trapping potential, one can measure the atomic density distribution in the trap using absorption imaging and apply iterative correction to the RF power at each (x,y) location.^{32}

#### 2. Phase evolution in time-averaged potentials

A full treatment time-averaged potentials need to include the phase evolution of the condensate under the effect of the scanning beam. The time-varying potential $ V ( x , y , t )$ acts to imprint a phase $\varphi $ with the evolution $ \u210f \u2202 \varphi ( x , y , t ) / \u2202 t = V ( x , y , t )$. For sufficiently fast scan rates, the imprinted phase effect is negligible, but at slower scan rates, this phase imprinting action can accumulate local phase, leading to residual micromotion in the condensate, and the signatures of which have been observed.^{49} This is an important consideration for atomtronic applications where the phase is an observable of interest, such as for guided Sagnac interferometry.^{50}

#### 3. Atomtronics with time-averaged optical traps

The time-averaged optical dipole traps are extremely versatile, allowing a variety of geometries to be generated, and dynamically changed in structure by real-time adjustment of the scanning pattern. In the context of atomtronic geometries, BECs have been trapped into flat bottom line-traps, rings,^{32,51} lattices,^{52} and dumbbell reservoirs (Fig. 1). Additionally, single mode matter-wave propagation and coherent phase splitting have been demonstrated in circuit elements such as waveguides and beamsplitters.^{28} The time-averaged optical beams can be used to introduce multiple repulsive barriers and stirring elements to study persistent currents and superfluid transport in atomtronic circuits.^{12}

### B. Optical potentials with liquid-crystal SLMs

A liquid-crystal SLM spatially modulates the phase of the light. The phase pattern on the SLM acts as a generalized diffraction grating so that in the far field, an intensity pattern is formed, which is used to trap atoms. In practice, the far field is obtained by focusing the light with a lens so that the intensity pattern that traps the atoms is created in a well-defined “output plane” coinciding with the lens focal plane. The SLM acts effectively as a computer-generated hologram, and the light field in the output plane is the Fourier transform of the light field in the SLM plane.

The first experiments with these holographic traps go back over ten years ago.^{53,54} A reason for the use of phase-only SLMs, rather than amplitude modulators, is that the former does not remove light from the incident beam. This is advantageous from the point of view of light-utilization efficiency. Moreover, as is shown below, a phase-only SLM allows the control of both the amplitude and phase on the output plane.

The calculation of the appropriate phase modulation to give the required output field is a well-known inverse problem, which, in general, requires numerical solution. Iterative Fourier Transform Algorithms (IFTAs) are commonly used, and variants that control both phase and amplitude have been recently demonstrated.^{55,56} The removal of the singularities (e.g., vortices), which particular pattern optimization techniques can introduce, is widely researched due to their importance for controlled beam shaping^{57–60} and, in particular, to confine BECs in uniform potentials.^{35,61} One such example is a conjugate gradient minimization technique that efficiently minimizes a specified cost function.^{60,62} The cost function can be defined to reflect the requirements of the chosen light pattern, such as removing optical vortices from the region of interest.

The intensity patterns obtained with this method are shown in the first row of Fig. 2. They are taken at a wavelength of 1064 nm, i.e., red-detuned relative to the rubidium transition, causing rubidium atoms to be trapped in the regions of high intensity. The SLM light is focused on the atoms by f = 40 mm lens, giving a diffraction limit of the optical system of 6 *μ*m at 1064 nm.

Going from left to right in Fig. 2, shown are a simple waveguide, a waveguide with a potential barrier halfway across, a ring trap, and a crosslike pattern. The latter has been proposed for the study of the topological Kondo effect.^{63} In all these light patterns, the phase is constrained by the algorithm. For the simple waveguide, the ring and the cross, a flat phase is programmed across the whole pattern. Controlling the phase this way leads to a well maintained intensity profile shape as it propagates out of the focal plane for up to $ \u223c 10$ times the Rayleigh range. By comparison, a pattern with random phase loses its shape much sooner.

Differently from the other three patterns, for the waveguide with the barrier, a sharp *π* phase change halfway across the line was programmed. In the resulting intensity profile, this phase discontinuity causes the intensity to vanish, hence creating the potential barrier whose width is close to the diffraction limit.

The second row of Fig. 2 shows Rb BECs trapped in the potential created by the SLM light patterns combined with an orthogonal light sheet that provides tight confinement along the axis of propagation of the SLM light.^{64,65} The clouds are imaged after a 2 ms time of flight and undergo mean-field expansion during this time, leading to a final density distribution that is more spread out compared to the transverse size of the SLM traps.

Controlling the phase of the light pattern opens new possibilities for the trapping and manipulation of ultracold atoms. Here, we have shown that the phase control gives an alternative way to create barriers close to the diffraction limit by using discrete phase jumps. Liquid-crystal SLMs were also used to transfer phase structure in a four-wave-mixing process in rubidium vapor, in particular trans-spectral orbital angular momentum transfer from near-infrared pump light to blue light.^{66} Additionally, they have enabled research into uniform 3D condensates. More recently, they were used in the realization of bottle beams that have been used to create 3D optical trapping potentials for confining Rydberg atoms.^{26} In addition to this, the phase control can also be useful for many atomtronics applications, for instance, phase imprint via a Raman transition,^{67} and the realization of artificial gauge fields.^{68,69}

### C. Direct imaged DMD optical potentials

A recent addition to the spatial light modulator family is the digital micromirror device (DMD). Developed for digital light processing (DLP) applications, DMDs consist of millions of individually addressable, highly reflective mirrors. Each hinged mirror, of typical size 7.56–10.8 *μ*m, is mounted on a silicon substrate on top of control electrodes. The application of a control voltage tilts the mirrors between two “on” or “off” angles, typically $ \xb1 12 \xb0$. The mirror array acts as a dynamical configurable amplitude mask for light reflected from their surface. The DMDs can be placed in the Fourier plane of the imaging/project system, similar to typical phase-based SLMs, where it can modulate both the phase and the amplitude of the light.^{72} If phase modulation is not required, the DMD can be used as a binary amplitude mask in the object plane, similar to its DLP applications.^{9,73} In the “DC” mode, the mirrors are fixed to the on angle and a static pattern can be projected. The true versatility of the device, however, lies in its dynamical (“AC”) capability with full frame refresh rates exceeding 20 kHz.

#### 1. Half-toning and time-averaging

The projected image from the DMD is binary in nature. Although this would appear as a significant limitation in producing arbitrary optical potentials, a number of techniques exist to overcome this issue. The first of these is half-toning, or error-diffusion, which takes advantage of the finite optical resolution of the projection optical system to increase the amplitude control. With suitable high magnification, such that the projected mirror size is smaller than the resolution, multiple mirrors contribute to each resolution spot in the projected plane.^{74} In this way, half-toning can be used to create intensity gradients in the light field, as shown in Fig. 3(a). Same as in the case of time-averaged AOD traps, feed-forward using the atomic density^{32,70,71} can be performed to correct for imperfections in the projection potential, as shown in Fig. 3(b).^{71}

One can also make use of the high-speed modulation of the mirrors to further improve the intensity control. The mirror array of the DMD is capable of switching speeds from DC to 20 kHz. By varying the on/off time of individual mirrors (pulse-width modulation), the time-average of the resulting light field can be utilized to improve the smoothness of the projected potentials.^{71}

#### 2. Atomtronics with DMDs

Atomtronics studies how to use neutral atom currents to create circuits that have properties similar to existing electrical devices. The advances in control and increased resolution of trapping potentials have been instrumental in the development of this field. The dynamic control over the potential given by DMDs have allowed time dependent implementations. Combined with other techniques such as the optical accordion lattice,^{73} which allows smooth transitions between quasi-2D and 3D systems, they open up further avenues of control for future studies. The high resolution projection of DMD optical potentials enables the creation of complex masks. These have facilitated the study of superfluid transport in a variety of traps. Figure 4 shows three relevant geometries for superfluid transport studies.

#### 3. Turbulence with DMDs

### D. Hybrid atomic-superconducting quantum systems

Superconducting (SC) atom chips have significant advantages in realizing trapping structures for ultracold atoms compared to conventional atom chips.^{4,80–85} These advantages have been extended further by the development of the ability to dynamically tailor the superconducting trap architecture. This is done by modifying the current density distribution in the SC film through local heating of the film using dynamically shaped optical fields. This allows for the creation of desired magnetic trapping potentials without having to change the chip or the applied electrical field.

Typically, a high-power laser and a DMD are used to create and shape the light field used to destroy the superconductivity and influence the shape and structure of a trap. Various trapping potentials have been realized using this technique, in particular, to split a single trap (see Fig. 6) or to transform it into a crescent or a ringlike trap (see Fig. 7). Since the atomic cloud evolves with the trapping potential, cold atoms can be used as a sensitive probe to examine the real-time magnetic field and vortex distribution. Simulations of the film heating, the corresponding redistribution of sheet current density, and the induced trapping potentials have been found to agree closely with experiments. Such simulations help us to better understand the process and can be used to design traps with the needed properties.

More complex structures can be achieved by increasing the heating pattern resolution. This method can be used to create magnetic trap lattices for ultracold atoms in quantum computing applications and, in particular, optically manipulated SC chips open new possibilities for ultracold atoms trapping and design of compact on-chip devices for investigation of quantum processes and applications in atomtronics.^{86–92}

### E. Concluding remarks and outlook

In this section, we have described the suite of technologies available to the experimenter for creating configurable optical potentials for ultracold atoms, primarily discussing AOMs, SLMs, and DMDs. A hybrid technique utilizing optical configurable potentials to shape magnetic potentials through superconducting quantum chips intermediary were also discussed. These technologies have drastically improved the control and manipulation of ultracold neutral atoms.

Although previously available static holograms technologies provided great control for the creation of optical potentials and are still usually better for 3D trapping potentials, the dynamic manipulation capabilities presented here have enabled new classes of experiments with ultracold atoms. For example, dynamically modulated DMDs have facilitated new studies of two-dimensional-quantum turbulence^{75,76,78} and condensate evolution in response to rapidly quenched trapping potentials.^{31,93} AOMs have enabled steerable arrays of single atoms,^{23,52,94} facilitating quantum simulation experiments. Furthermore, the rapid reconfigurability of DMD traps has enabled groundbreaking studies in the emerging field of atomtronics, where the system parameters can be easily tuned.^{29,33,77,95,99}

As the technology behind optical manipulation continues to mature and evolve through the increase in SLMs pixel array sizes and switching frequencies, these sculpted light and hybrid techniques are sure to have an even bigger impact on the development of atomtronics.

### ACKNOWLEDGMENTS

The UQ group has been funded by the ARC Centre of Excellence for Engineered Quantum Systems (Project No. CE1101013) and ARC Discovery Projects under Grant No. DP160102085. G.G. acknowledges support of ARC Discovery Project under No. DP200102239, and T.W.N. acknowledges the support of ARC Future Fellowship under No. FT190100306. The St Andrews group acknowledges funding from the Leverhulme Trust (No. RPG-2013-074) and from EPSRC (Nos. EP/G03673X/1 and EP/L015110/1).

## III. IMPLEMENTING RING CONDENSATES

** M. Baker, T. A. Bell, T. W. Neely, A. L. Pritchard, G. Birkl, H. Perrin, L. Longchambon, M. G. Boshier, B. M. Garraway, S. Pandey, W. von Klitzing **

The many interesting properties of degenerate quantum gases, such as phase coherence, superfluidity, and vortices, naturally make the geometry of these systems of great interest. Ring systems are of particular interest, as the simplest multiply connected geometry for coherent matter-wave guiding and as a potential building block for circuital atomtronic devices. In addition, ring systems have interesting properties such as persistent flow, quantum hall states, and the potential for Sagnac interferometry.

Advances in the control of quantum gases have seen the development of atom waveguides formed from both magnetic trapping and magnetic resonance, and optical dipole trapping, and more recent implementations using hybrids of both. These approaches satisfy the criteria needed for coherent quantum matter-wave flow: i.e., the waveguides are *smooth* and can form *loops* that are *dynamically* controllable.

### A. General features of ring traps

*R*is expressed in terms of radial and vertical trapping frequencies $ \omega \rho $ and

*ω*, respectively,

_{z}*T*for a 3D harmonically trapped gas, yielding

_{c}^{97}

^{,}

*N*

_{0}is the atom number. For sufficiently elongated geometries, such as cigar traps, or ring traps with long azimuthal length, a regime of thermally driven phase-fluctuations in the condensate can exist

^{98,101}even at temperatures below

*T*. These phase-fluctuations are suppressed when the correlation length is larger than the system size, which for a ring geometry is half the azimuthal circumference, or

_{c}*πR*. As we are typically interested in fully phase coherent ring traps, we can define this transition temperature $ T \varphi $,

^{97}

^{100}

*a*is the s-wave scattering length.

_{s}In this section, we will discuss the experimental and theoretical developments in all three types of waveguide approach. In what follows, in Sec. III B, we discuss approaches primarily involving magnetic and radio-frequency fields; and in Sec. III C, we we will discuss optical and hybrid approaches to implementing ultracold atoms and condensates in rings before concluding in Sec. III E.

### B. Techniques based on magnetic traps

Experimental techniques for trapping atoms in magnetic fields are well developed since the first BECs, and it is natural to consider such an approach, and build on that approach, to make ring waveguides. Nevertheless, this brings particular challenges because of the need to satisfy Maxwell's equations for fields trapping in a ring geometry, the need to avoid the loss of atoms from Majorana spin flips, occurring in the vicinity of field zeros, and the desire, for some experiments, to have trapping systems with high symmetry.

The earliest examples of waveguides for ultracold atoms were produced using static magnetic fields, where DC current carrying wires were used to create large area ring^{101} and stadium^{102} geometries which initially trapped thermal atoms. With Ref. 111, we had the first demonstrations of a ring waveguide with a Bose-condensed gas. Subsequent experimental developments can be divided into systems which principally use macroscopic coils for generating the magnetic trap, and those systems which employ microfabricated structures in an *atom chip* to generate the spatially varying potentials. We will briefly discuss the latter next and the former in Secs. III B 1–III B 3.

The appeal of atom-chip traps is their compact footprint, potential portability, and the ability to fabricate quite complex geometries, switches, and antenna components into a compact package.^{104,105} Additionally, the close proximity of the wires allows high trapping frequencies to be achieved, even for modest currents. However, trapping in close proximity to a surface brings with it its own challenges. Foremost of these are the corrugations in the magnetic guiding potential, which arise from imperfectly directed currents in the conducting material. An additional challenge is the perturbing effect of the end connections to supply current in and out of the conducting ring. Although these problems can be alleviated to some degree by the use of AC fields,^{106} which provides a smooth time-averaged current in the wire, as well as switching elements at the end connections to minimize the perturbative phase effects on the ring condensate,^{107} they cannot be removed completely. A comprehensive survey on the implementation of ring traps based on atom-chips, and their applications, is covered in detail in Ref. 108.

Here, we will focus our attention on ring traps derived from a combination of static magnetic traps with RF and modulated fields. Using macroscopically large conducting elements requires the use of high currents and occupies a greater size, but there are significant gains in the resulting trap smoothness, as the conducting elements are far from the trapping region. This makes such magnetic traps ideal for producing corrugation free toroidal waveguides for coherent matter, which is detailed in this section.

However, the complexity of the fields requires an atom-chip approach to a pure magnetic waveguide system^{104,105} and this brings a difficult problem for the perfect ring waveguide because of the need to get the currents into, and out of, the wires that define the waveguide. We can try to live with this,^{108} but asymmetry seems inevitable. We can think of tricks, for example, as the atoms go around the ring, and we can switch the current between different sets of 130 wires as in Ref. 107. This would avoid the bumps and humps in the waveguide, which occur in the places where current enters and leaves the defining structures at the expense of potential losses and heating as the guides are switched over.

#### 1. RF dressing and bubbles

It is not obvious that micrometer-scale trapping structures for ultracold atoms can be created using macroscopic scale magnetic coils. However, by means of the addition of radio-frequency coils, magnetic traps with a simple trapping geometry can be transformed into ring traps and other topologies. The theoretical basis is to treat the atom and radio-frequency field with adiabatic following and the dressed-atom theory.^{109} Originally introduced in the optical domain by Cohen-Tannoudji and Reynaud,^{109} we adapt it here in the radio-frequency domain where it has found several applications (see also Secs. III B 2, III B 3, and III D). The approach is suitable for ultracold atoms in magnetic traps where the trap potential is governed by the spatially varying Zeeman energy and the spatially varying energy difference between Zeeman levels can be in the radio-frequency range.^{110,111} The method relies on the adiabatic following of local eigenstates, and it is notable that the superpositions of Zeeman states can provide some resilience to temporal noise and surface roughness.^{106} The combination of static magnetic fields and radio-frequency fields with their different spatial and vector variation allows flexibility in the resulting potentials for the creation of shell potentials, rings, tubes, and toroidal surfaces among others.^{110,111}

*x*-

*y*plane. This field is often generated by a pair of coils with current circulating in opposite directions. When an atom interacts with this static field via its magnetic dipole moment $\mu $, we obtain the ubiquitous interaction energy,

*g*-factor

*g*and Bohr magneton

_{F}*μ*. For our example static field [Eq. (5)], the resulting potential is $ U ( r ) = m F \u210f \alpha x 2 + y 2 + 4 z 2$, where $ \alpha = g F \mu B b \u2032 / \u210f$.

_{B}^{110,111}In the linear Zeeman regime, the local Larmor frequency is given by

^{110,111}in terms of a Rabi frequency $ \Omega 0 ( r )$,

^{110,111}we obtain the local eigenenergies, or dressed potentials,

*m*described above.

_{F}The result of this is that slow atoms are confined by the potential (10), which in a typical configuration, and to a first approximation, confines atoms to an iso-*B* surface defined by $ \u210f \omega rf \u2212 \u210f \omega L ( r ) = 0$, which approximately reduces the value of $ U ( r )$ in Eq. (10). The term $ g F \mu B | B rf \u22a5 ( r ) | / 2$ also plays a role, and in particular, it can be zero at certain locations on the trapping surface allowing the escape of atoms. This latter effect prevents the trapping of atoms in a shell potential by using the static quadrupole field (5). However, shell potentials are possible with different field arrangements such as those arising from the Ioffe–Pritchard trap and variations,^{110–115} which have become candidates for experiments on the International Space Station.^{116} The requirement is simpe for a local extremum in the *magnitude* of the field $ B 0 ( r )$ together with a nonzero $ B rf \u22a5 ( r )$. The reason for the interest in shell potentials in the earth orbit is that on the earth's surface a gravitational term *mgz* should be added to Eq. (10), which plays an important role for larger and interesting shells (e.g., see Sec. III B 3).

Although the matter-wave *bubbles* produced by shell potentials have become an object of great interest, the shell potentials themselves are the building blocks for other potentials of interest such as ring traps: we will see an example in Sec. III D. Another example is in Sec. III B 2, where a modulated bias field is used to make a ring trap: then $ B 0 ( r ) \u2192 B 0 ( r ) + B m ( r , t )$ and $ B m ( r , t )$ is a field varying in space, and time, but typically at a frequency rather lower than the radio-frequency case.

#### 2. Waveguides formed from time-averaged adiabatic potential (TAAP)

Time averaged adiabatic potentials (TAAPs) allow the generation of extremely smooth matterwave guides,^{117} which can be shaped into a half-moon or ring (see Fig. 8). They are an excellent candidate for matterwave optics, long-distance transport experiments, and interferometry in an atomtronic circuit.^{117–119} TAAPs are formed by applying an oscillating homogeneous potential to the adiabatic bubble traps described in Sec. III B 1. If the modulation frequency $ ( \omega m = 2 \pi f m )$ is small compared to the Larmor frequency, but fast compared to the trapping frequency of the bubble trap, then the effective potential for the atoms is the bubble potential time-averaged over one oscillation period.^{120} Let us consider TAAP potentials formed from a quadrupole bubble trap and an oscillating homogeneous field of the form $ B m = { 0 , 0 , B m \u2009 sin \u2009 \omega m t}$. The modulation field simply displaces the quadrupole (and thus the bubble trap) by $ z m = \alpha \u2212 1 B m \u2009 sin \u2009 \omega m t$ at an instant in time. In order to find the *effective* potential that the atoms are subjected to by this method, one calculates the time-average. Time-averaging of a concave potential increases the energy of the bottom of the trap, as is readily illustrated by taking the time average of a harmonic potential jumping between two positions: the curvature does not change since it is everywhere the same; however, the energy of the trap bottom increases since it is at exactly the crossing point between the two harmonic potentials. Returning to the modulated bubble trap, one notices that the modulation is orthogonal to the shell at the poles of the shell $ ( x = y = 0 )$, but tangential to the shell on the equator (*z* = 0). Therefore, the time averaging causes a larger increase in the trapping potential at the poles rather than the equator and, therefore, creates a ringlike structure.

Assuming that $ \omega RF$ is modulated such as to stay resonant on the ring and to keep $ \Omega RF$ constant, the vertical and radial trapping frequencies can be controlled via the relative amplitude of the modulation $ \beta = g F \mu B B m / \u210f \omega RF$ as $ \omega \rho = \omega 0 \u2009 ( 1 + \beta 2 ) \u2212 1 / 4$ and $ \omega z = 2 \omega 0 1 \u2212 ( 1 + \beta 2 ) \u2212 1 / 2$, where the radial trapping frequency of the bare bubble trap is $ \omega 0 = m F g F \mu B \u2009 \alpha \u2009 ( m \u2009 \u210f \Omega RF ) \u2212 1 / 2$ with the mass of the atom *m*, the $ g F$ is the Landé *g*-factor of the considered hyperfine manifold, $ \mu B$ is the Bohr magneton, and $ \Omega RF$ the Rabi frequency of the dressing RF. In order to achieve large RF field strengths ( $ \u2248 0.3$–1 G) and Rabi frequencies, $ ( \Omega RF )$, one usually has to use RF-resonators, which make it very difficult to tune the RF frequency, and which results in a somewhat weaker confinement in the axial (i.e., vertical) direction. Trapping frequencies of the order of a hundred hertz are readily achieved.

In many cases, it is also desirable to confine the atoms azimuthally. This is readily achieved either by tilting the ring away from being perfectly horizontal or by modifying the polarization of the rf-field. The half-moon shaped BEC in Fig. 8(b) was formed this way. A gravito-magnetic trap results from tilting the direction of the $ B m$ and thus tilting the ring against gravity.^{118} The gravito-magnetic potential forms a single minimum much like a tilted rigid pendulum. One can also create a trap by changing the polarization of the dressing RF: tilting a linear polarization from the z-axes will cause due to its projection on the local B-field, and a sinusoidal modulation of the Rabi frequency along the ring resulting in a two minima on opposite sides of the ring. Alternatively an elliptical RF polarization creates a single minimum. Combining these modulation techniques permits the creation of two arbitrary placed traps along the ring or, more generally, any longitudinal confinement of the form $ a 1 \u2009 sin \u2009 ( \varphi + \varphi 1 ) + a 2 \u2009 sin \u2009 ( 2 \varphi + \varphi 2 )$, where $\varphi $ is the azimuthal angle and $ \varphi 1$ and $ \varphi 2$ are phase offsets. Note that there are no angular spatial Fourier components higher than $ 2 \varphi $ present in the system.

Thermal atoms and BECs are readily loaded into the gravito-magnetic TAAPs from a trapping-frequency-matched dipole trap. This can be done fully adiabatically by ramping down the dipole confinement and at the same time ramping up the TAAP trap. With a sufficiently high level of control on the rf-fields, one can also load them from a TOP trap via a tilted dumbbell-shaped trap.^{118} Once in the ring, one can then manipulate the atoms with a simple manipulation of the time-averaging fields: The depth of the azimuthal trap can be changed by modifying the degree of tilt applied to the modulation field $ ( B m )$. By changing the direction of the tilt (i.e., the phase between the modulation fields in the x and y directions), one can move the trap along the ring. This can be used, e.g., to accelerate the atoms along the ring with angular momenta of $ 40 \u2009 000 \u2009 \u210f$ per atom being readily achieved.^{117} They can then travel in the waveguides over distances of tens of centimeters without any additional heating associated with the propagation. One can also remove the azimuthal confinement and allow the condensates to expand around the ring. Viewed in the corotating frame at high angular momenta, the atoms see an exceptionally flat potential with the largest resulting density fluctuations corresponding to an energy difference of a few hundred picokelvin: this is equivalent to a few nanometers in height.^{117} Current experiments have been performed with BECs in the Thomas–Fermi regime with about 20 transverse vibration modes occupied. The 1D regime is readily accessible simply by reducing the atom number and increasing the radius of the ring.

The complete lack of any roughness combined with a picokelvin level control of the trapping parameters make the TAAP waveguides a very good candidate for guided matterwave interferometry and the study of ultralow energy phenomena such as long-distance quantum tunneling. A remaining challenge is to completely fill the ring with a phase coherent condensate. Current experiments allowed a condensate to expand along the ring, which converts the chemical potential of the BEC into kinetic energy. When the condensate touches itself at the opposite side of the ring, the two ends have a finite velocity in opposite directions, resulting in a spiral BEC, i.e., a BEC wrapped around itself. Using atom-optical manipulation of the expansion process, kinetic energies in the pico-kelvin range (a few hundred micrometers per second) can readily be achieved. It will be interesting to study the very low energy collisions that will lead to a thermalization of this system. A promising approach for a fully phase-coherent ring-shaped condensate is to first fill a small ring and then increase its radius. This should not induce any additional phase fluctuations, despite the fact that the lowest excitation has an energy of $ E = \u210f 2 / ( 2 m r 2 )$, which for a ring of 1 mm radius is 3 fK.

#### 3. Dynamical ring in an rf-dressed adiabatic bubble potential

There is a formal analogy between the Hamiltonian of a neutral gas in rotation and the one of a quantum system of charged particles in a magnetic field. This makes rotating superfluids natural candidates to simulate condensed matter problems such as type II superconductors or the quantum Hall effect.^{121,122} For a quantum gas confined in a harmonic trap of radial frequency *ω _{r}* and rotating at angular frequency Ω approaching

*ω*, the ground state of the system reaches the atomic analog of the lowest Landau level (LLL) relevant in the quantum Hall regime.

_{r}^{123–125}Reaching these fast rotation rates is experimentally challenging in a harmonic trap because the radial effective trapping potential in the rotating frame vanishes due to the centrifugal force. To circumvent this limit, higher-order confining potentials have been developed,

^{126}which allow to access the regime where Ω even exceeds

*ω*.

_{r}The adiabatic bubble trap has many features that make it a very good candidate to explore this regime. Indeed, it is very smooth, and easy control of its anisotropy is possible through the dressing field polarization.^{127} This allows us to deform the bubble and rotate the deformation around the vertical axis in a very controlled way, allowing us to inject angular momentum into the cloud. The curved geometry of the bubble provides naturally the anharmonicity required to rotate the atoms faster than the trapping frequency *ω _{r}* at the bottom.

In the experiment at LPL,^{128} the atoms are placed in a quadrupole magnetic field of symmetry axis *z* dressed by a radio-frequency (rf) field of maximum coupling $ \Omega 0$ at the bottom of the shell. Here, the equilibrium properties in the absence of rotation (Ω = 0) are well known:^{127} the minimum of the trapping potential is located at *r* = 0 and *z* = *z*_{0}, and around this equilibrium position, the potential is locally harmonic with vertical and radial frequencies $ \omega z = 2 \pi \xd7 356 \u2009 Hz$ and $ \omega r = 2 \pi \xd7 34 \u2009 Hz$ without measurable in-plane anisotropy. This trap is loaded with a pure BEC of $ 2.5 \xd7 10 5$ ^{87}Rb atoms with no discernible thermal fraction. This atomic cloud has a chemical potential of $ \mu / \u210f = 2 \pi \xd7 1.8 \u2009 kHz$, which is much greater than *ω _{r}* and

*ω*and well in the three-dimensional Thomas-Fermi (TF) regime. In addition to the dressing field, a radio-frequency knife with frequency $ \omega kn$ is used to set the trap depth to approximately $ \omega kn \u2212 \Omega 0$ by outcoupling the most energetic atoms in the direction transverse to the ellipsoid.

_{z}^{129,130}

In a frame rotating at frequency Ω, the effective dressed trap potential is the usual trap described above with the addition of a $ \u2212 1 2 M \Omega 2 r 2$ term taking into account the centrifugal potential. In this frame, the atomic ground state consists of an array of vortices of quantized circulation, each vortex accounting for $\u210f$ of angular momentum per atom. When only a few vortices are present, the velocity field differs strongly from the one of a classical fluid, but for a sufficiently large number of vortices, the superfluid rotates as a solid body with a rotation rate Ω. When $ \Omega < \omega r$, the equilibrium position remains on the axis *r* = 0 at *z* = *z*_{0}, and the only difference is a renormalization of the radial trapping frequency: $ \omega r eff = \omega r 2 \u2212 \Omega 2$. Of course, as this frequency decreases, the trap anharmonicity becomes more important in the determination of the cloud shape.

For $ \Omega > \omega r$, the trap minimum is located at a nonzero radius. In this situation, a hole grows at the trap center above a critical rotation frequency Ω_{h,}^{131} leading to an annular two-dimensional density profile [Fig. 9(a)], which we will refer to as a “dynamical ring.”^{128} Moreover, the velocity of the atomic flow is expected to be supersonic,^{132} i.e., exceeding by far the speed of sound. For increasing Ω, one expects the annular gas to sustain vortices in its bulk up to a point where the annulus width is too small to host them. The gas should then enter the so-called “giant vortex” regime^{132,133} where all the vorticity gathers close to the center of the annulus.

The experimental sequence is the following: angular momentum is injected into the cloud by rotating the trap with an ellipsoidal anisotropy at a frequency $ \Omega = 31 \u2009 Hz$. The trap rotation is then stopped and isotropy is restored. At this moment, which we take as *t* = 0, the cloud shape goes back to circular with an increased radius due to its higher angular momentum. An additional evaporation process, selective in angular momentum, continuously accelerates the superfluid and increases its radius.^{128} Due to this size increase, the chemical potential is reduced and the gas enters the quasi-2D regime $ \mu \u2264 \u210f \omega z$. After a few seconds, a density depletion is established at the center of the cloud which is a signature of Ω now exceeding *ω _{r}*. After a boost in selective evaporation due to a lowering of the frequency of the rf knife, a macroscopic hole appears in the profile, indicating that Ω is now above Ω

_{h}and that a fast rotating dynamical ring with a typical radius of $ \u223c 30 \u2009 \mu m$ has formed as can be seen in Fig. 9(b). The rotation keeps increasing, and a ring is still observable after $ t = 80 \u2009 s$. Rotational invariance is critical in that regard and is ensured at the $ 10 \u2212 3$ level by a fine tuning of the dressing field polarization and of the static magnetic field gradients.

^{111}

A Thomas–Fermi profile convoluted with the imaging resolution is much better at reproducing the experimental density profile than a semiclassical Hartree–Fock profile, demonstrating that the samples are well below the degeneracy temperature. Using the Thomas–Fermi model, we can estimate the properties of the cloud. For example, the ring obtained at $ t = 35 \u2009 s$ has a chemical potential of $ \mu / \u210f \u2243 2 \pi \xd7 84 \u2009 Hz$ and an averaged angular momentum per particle $ \u27e8 L \u0302 z \u27e9 / N \u2243 \u210f \xd7 317$. Interestingly, the estimated peak speed of sound $ c = \mu / M \u2243 0.62 \u2009 mm / s$ at the peak radius $ r peak$ is much smaller than the local fluid velocity $ v = \Omega r peak \u2243 6.9 \u2009 mm / s$: the superfluid is, therefore, rotating at a supersonic velocity corresponding to a Mach number of 11. Moreover, due to the continuous acceleration of the rotation, the dynamical ring radius grows gradually with time which results in a decrease in the chemical potential and an increase in the Mach number. For $ t > 45 \u2009 s$, the chemical potential is below $ 2 \u210f \omega r$ and the highest measured Mach number is above 18.

Superfluidity in the dynamical ring has also been evidenced by the observation of quadrupolelike collective modes. After the ring formation, the rotation rate, while accelerating, crosses a value where the quadrupole collective mode is at zero frequency, such that any elliptical static anisotropy can excite it resonantly. A very small bubble anisotropy is enough to excite this mode characterized by an elliptic ring shape rotating with a period of approximately $ 10 \u2009 s$ in the direction opposite to the superfluid flow (Fig. 10). This counterpropagating effect is not predicted by a mean-field theory and has been confirmed by resonant spectroscopy of the quadrupole mode during the ring acceleration.^{128}

The persistence of superfluidity at such hypersonic velocity raises fundamental questions about the decay of superfluidity in the presence of obstacles, and how superfluidity can be preserved at such speeds: nonlinear effects, the presence of vortices and the dependence on temperature would be particularly interesting to study experimentally and compare with theoretical predictions.^{134–138} This hypersonic superflow is not yet a giant vortex, but it is an important step toward this long-sought regime whose transition rotation frequency is not theoretically clearly identified. Moreover, the well-known elementary excitation spectrum for a connected rotating superfluid is strongly modified when the ring appears, and the important discrepancies observed between the experimental results and a mean-field theoretical approach for a quadrupolelike collective mode highlight the need to refine the description of fast rotating superfluids in anharmonic traps.

An alternative way of generating large angular momentum states in rf-dressed adiabatic bubble potentials is to first generate them in a TAAP ring and then reduce the vertical modulation, thus adiabatically transferring the atoms into the bubble.

### C. Trapping in rings with optical potentials

Potentials for ultracold atoms can be formed through the use of focused far-detuned optical beams.^{139} Since the potential is directly proportional to the intensity of the optical field, ring-shaped condensates may be created through the implementation of ring-shaped optical patterns. The most significant advantage in optical dipole ring traps is the insensitivity to the hyperfine state, allowing multicomponent and spinor BECs to be trapped. Additional advantages include the imprinting of superfluid flow, either through phase imprinting or through Raman transitions that can directly transfer angular momentum to the cloud. The advent of spatial light modulator technologies means the optical ring trap has become highly configurable, allowing more complex geometries to be generated.

#### 1. Optical trapping

*I*(

*R*) is the intensity profile of the light and Γ is the transition linewidth. Since the trapping force is determined through the gradient of Eq. (11), a trapping potential requires a nonuniform optical intensity, obtained by shaping and focusing the intensity profile $ I ( r )$. Ring traps, can either be created from attractive (red-detuned) or repulsive (blue-detuned) light, usually by combining the ring shaped intensity profile with a perpendicular light sheet that provides confinement along the propagation direction of the projected ring pattern.

#### 2. Optical ring traps

We begin by looking at some of the optical beam techniques for ring traps that are in use and outline their potential for atomtronic applications.

##### a. Laguerre–Gauss beams

One of the first proposed methods for a ring optical dipole trap was the use of Laguerre–Gaussian (LG) modes having circular symmetry.^{140} For far-off-resonance light, these provide the spatial structure for a toroidal trap. An additional advantage of such LG modes is that they also carry orbital angular momentum. With pulses of near-resonant light, the LG modes can be tailored to provide two-photon Raman transitions that transfer exact quanta of circulation to the condensate.

_{0N}modes are typically generated by phase transformation of a Gaussian TEM

_{00}mode, which transforms the spatial profile of the beam into a donut mode carrying $ N \u210f$ units of orbital angular momentum. A number of methods exist, including spiral phase plates, computer generated holograms, or through the use of phase based spatial light modulators. The toroidal intensity profile of the LG

_{01}mode is given by

^{141}

*P*is the total laser power in the LG beam and

_{LG}*r*

_{0}is the radius at the peak intensity of the LG mode. Correction for imperfections in the spatial structure, and obtaining sufficient power in higher order modes, is typically a challenge. Ring traps and circulating currents using LG modes have been demonstrated in both single state and multicomponent spinor gases and were early demonstrations of all-optical trapping of BEC in a ring geometry.

^{67,142,143}To date, they have been used to realize small optical rings for the study of quantized superfluid flows.

##### b. Painted optical traps

An alternative to projecting a ring shaped beam is to build a time-averaged potential with a moving, red-detuned, focused laser beam. By rapidly steering a Gaussian beam in a circular orbit, a ring trap can be generated. This is achieved through the use of two acousto-optical deflectors (AOD) controlling the two axes of the painting beam by driving the deflectors with lists of frequency points that are repeatably iterated at high speed.^{7,32} This approach was used to create the first ring BEC,^{7} as shown in Fig. 11.

The advantages of this technique is that it allows adapting the intensity locally to create desired features in the potential landscape and to flatten imperfections due to possible laser inhomogeneity;^{28} the available laser power is used in an efficient way as only the relevant trapping locations are illuminated; the painting laser itself can be used as a stirrer to set the quantum fluid into rotation and demonstrate quantized superfluid flows;^{144} the technique also enables more complex geometries. As an example, the atomtronic analog of a Josephson junction has been demonstrated and used to realize a DC atomtronic SQUID.^{12} More recently, the dynamic potentials possible with painting were used to show that the atomtronic SQUID exhibits quantum interference.^{15}

The painting approach also comes with specific technical constraints that may need to be addressed. The phase of the time-averaged beam loop plays a role on the fine details of the potential, which results in imprinting of the condensate phase, and has to be compensated for.^{49} This is particularly relevant for the application of such traps in atom interferometry schemes.

##### c. Conical refraction

A novel approach to generating ring traps has been demonstrated with the use of conical refraction occurring in biaxial crystals. A focused Gaussian beam passing along the optical axis of the crystal transforms, at the focal plane, into one or more concentric rings of light. In the case of a double-ring, the light field encloses a ring of null intensity, called the Poggendorff dark ring.^{145} For a blue-detuned laser field, the atoms are trapped between the bright rings. The advantage of this configuration is that it minimizes spontaneous scattering of photons responsible for heating when the laser beam is not very far detuned from resonance. Further advantages include the high conversion efficiency of the incoming Gaussian beam to the ring-trap light field and the access to different ring configurations. The ring diameter is defined by the refractive indices of the biaxial crystal and its length. The width of each ring is given by the focal waist of the focused Gaussian beam. A variation of the ratio of these numbers (e.g., by changing the focal waist) allows for a variation of the resulting light field topology from a single bright ring to a bright ring with a central bright spot and further to bright double rings of increasing diameter. As with LG modes, there are challenges in alignment of the optical beams through the biaxial crystal. On the other hand, the conversion efficiency from a Gaussian TEM_{00} mode to the ring pattern can be close to unity. The first results on BECs transferred into a Poggendorff ring have been reported.^{145} Ongoing work is directed toward implementing quantum sensors (e.g., Sagnac interferometers) for rings with large diameter and atomtronic SQUIDs for small rings.

##### d. Digital micromirror direct projection

Direct imaging of digital micromirror devices (DMDs) has recently emerged as a powerful tool for the all-optical configuration of BECs.^{9,73,146,147} Ring traps can be created by directly projecting the DMD-patterned light onto a vertically confining attractive light-sheet potential,^{9,146} similarly to Fig. 12, or onto a vertically oriented accordion lattice.^{73} This can be accomplished using a relatively simple optical system, usually consisting of an infinite conjugate pair. Due to the large magnification factors required to reduce the DMD image to the typical $ 100 \u2009 \mu $m scale of the BEC, the final element in the imaging system is typically an infinity corrected microscope objective.^{9,146} DMDs may also be used in the Fourier plane of the imaging system,^{72} where the DMD implements an amplitude-only hologram. A detailed discussion of holographic techniques is beyond the scope of this section, and the reader is referred to more complete reviews of the subject.^{148}

In Fig. 13, direct imaging of a DMD is used to create a ring trap, along with a central phase-uncorrelated reference BEC. By introducing a stirring barrier with the DMD, and circulating the barrier around the ring, a 21-quanta persistent current results, corresponding to an angular momentum of $ \u223c 132 \u2009 \u210f$ per atom. The winding number of the current is visualized through interference with the reference central BEC after a short 5 ms time of flight.^{149} The DMD technology can also be used to phase imprint an azimuthal light gradient such that angular momentum can be imparted to the atoms^{150} and a circulating current created.^{151}

##### e. Microfabricated optical elements

An approach combining flexibility, integrability, and scalability can be based on the application of microfabricated optical elements for the generation of complex architectures of dipole traps and guides.^{152} It draws its potential from the significant advancement in producing diffraction-limited optical elements with high quality on the micro- and nanometer scale. Lithographic manufacturing techniques can be used to produce many identical systems on one subtrate for a scalable configuration.^{153} On the other hand, state-of-the art direct laser writing gives high flexibility in producing unique integrated systems and allows for fast prototyping.^{154} Applications range from integrated waveguides and interferometer-type structures^{155} to arrays of dipole-traps for quantum information processing^{156} and single-atom atomtronics implementations.^{157} In combination with DMD-based control of the light field (see Sec. III C 2 d), access to dynamic reconfiguration becomes possbile. Integrability is not limited to the generation of light fields for dipole potentials but can be extended to the integration of light sources and detectors or even complex quantum-optical systems such as an entire magneto-optical trap.^{152}

#### 3. Imperfections in optical traps

Defects in the optical potential will influence the ability to sustain superfluid flow without dissipation, or may introduce unwanted phase perturbations on the condensate if the optical potential is time-varying. We can gain some measure of the significance, and the level of control required for optical traps useful in atomtronics, by considering the density of the BEC in a ring potential. In the Thomas–Fermi limit, with sufficient atom number in the trap, the interaction energies dominate over kinetic energy terms, leading to a simplified GPE equation $ [ V ( r ) + g | \Psi ( r ) | 2 ] \Psi ( r ) = \mu \Psi ( r )$, giving the density $ n ( r ) = | \Psi ( r ) | 2 = [ \mu \u2212 V ( r ) ] / g$, where *μ* is defined by Eq. (4). The density occupies the spatial profile of the ring trap. In the context of the intensity of the optical potential, assuming a fixed light sheet, the trap depth scales directly scales with the ring optical intensity *I*_{0}, while the chemical potential of the BEC more weakly follows as $ \mu \u221d I 0 1 / 4$. This means that for a typical condensate, the chemical potential is on the order of tens of nanokelvin, and is only weakly effected by the trapping intensity, while large optical trap depths on the order of $ 1 \u2009 \mu $K or larger may be easily achieved and utilized. Since the density of the condensate closely follows the optical potential, small perturbations in the optical field can result in significant fluctuations on at the energy scale of the condensate, and thus significant density fluctuations; variations in the optical intensity must typically be limited to less than 1% in order to avoid unwanted perturbations. The precision of the optical projection is thus a key consideration when implementing configurable optical potentials. These aspects, however, also mean that the condensate density provides a very sensitive probe of the optical potential, and the atom density can be used to feedforward corrections to the optical potential.^{32}

### D. Hybrid traps: RF bubble plus light sheet(s)

One can also combine optical potentials and magnetic trapping to produce a hybrid trap and exploit the advantages of each technique for ring trap generation. As mentioned above, optical potentials can achieve large trapping frequencies, while magnetic traps are very smooth due to the macroscopic size of the coils generating them. The bubble geometry described in Secs. III B 1 and III B 3 is particularly suited to create a ring trap: by combining the rf-dressed bubble trap and an optical light sheet as in Sec. III C 2, one can create a toroidal trap. The principle is depicted in Fig. 14: a horizontal light sheet is superimposed with a bubble trap which is rotationally invariant around the vertical direction.^{100,158} The light sheet is designed to achieve a strong optical confinement in the vertical direction, and the radial confinement is ensured by the bubble trap itself, made with the same rf-dressed quadrupole trap as in Sec. III B 3. Maximum radial trapping and maximum radius will be attained if the light sheet is located at the equator of the bubble, a situation which also ensures maximum decoupling between the vertical trapping frequency *ω _{z}* and the radial trapping frequency

*ω*.

_{r}Experimentally, the optical trap is formed between two horizontal light sheets, which are made repulsive by their large blue detuning from the atomic transition. The bubble radius is significantly smaller than the light sheets width and also the vertical Rayleigh length to minimize the azimuthal potential variations. The choice of a small radius also comes with a higher critical temperature and a larger chemical potential, which reduces the relative density fluctuations around the ring due to optical imperfections from residual light scattering of the vacuum glass cell (see Sec. III C 2). One then creates a trapped toroidal degenerate gas of approximately $ 10 5$ atoms [Fig. 14(b)]. With further reduction of optical imperfections in the light sheets, one could enter with 10^{4} atoms the quasi-1D condensate regime,^{159} where large-scale correlations and solitons play an essential role in the dynamics.

The gas can be set into rotation by different procedures, using either magnetic or optical means. The first method, used in our experiment in Ref. 160, consists in slightly deforming the bubble trap with an ellipsoidal anisotropy, rotate this magnetic deformation at a given fixed frequency and finally restore the circular symmetry. In a second method (Fig. 15), the rotation is induced by a rotating optical defect^{160,161} driven by a dual-axis acousto-optic modulator system as described in Sec. III B 1. Well-controlled circulation could also be imparted by direct optical phase imprinting onto the ring trap.^{150}