Single Trapped Atom Research Articles

Optimized coplanar waveguide resonators for a superconductor–atom interface

We describe the design and characterization of superconducting coplanar waveguide cavities tailored to facilitate strong coupling between superconducting quantum circuits and single trapped Rydberg atoms. For initial superconductor–atom experiments at 4.2 K, we show that resonator quality factors above 104 can be readily achieved. Furthermore, we demonstrate that the incorporation of thick-film copper electrodes at a voltage antinode of the resonator provides a route to enhance the zero-point electric fields of the resonator in a trapping region that is 40 μm above the chip surface, thereby minimizing chip heating from scattered trap light. The combination of high resonator quality factor and strong electric dipole coupling between the resonator and the atom should make it possible to achieve the strong coupling limit of cavity quantum electrodynamics with this system.

Long working distance objective lenses for single atom trapping and imaging.

We present a pair of optimized objective lenses with long working distances of 117 mm and 65 mm, respectively, that offer diffraction limited performance for both Cs and Rb wavelengths when imaging through standard vacuum windows. The designs utilise standard catalog lens elements to provide a simple and cost-effective solution. Objective 1 provides NA = 0.175 offering 3 μm resolution whilst objective 2 is optimized for high collection efficiency with NA = 0.29 and 1.8 μm resolution. This flexible design can be further extended for use at shorter wavelengths by simply re-optimising the lens separations.

Preface: special topic on cold atoms

For several decades, Atomic,Molecular, andOptical Physics has undergone and is still going through a renaissance. Research on cold atoms has played a key role in a profound transformation of this traditional field. Ultracold atoms allow us to reach down to the core of AMO physics where detailed knowledge of atomic andmolecular structure forms the foundation for understanding light–matter interactions. This understanding then fuels the development of increasingly sophisticated control of individual quantum systems. The increasing ability for quantum control has stimulated the simultaneous, rapid development of the state-of-the-art lasers. These activities have not only dramatically advanced the frontier of precision measurement, but also enabled explorations of increasingly complex quantumphenomenawhere fundamental connectionsbetween few-bodyand many-body physics are probed. The desire for improved control of atomic motions for spectroscopy applications motivated the early development of laser cooling in 1980s. In a personal interview for this special topic, W.Phillips, oneof the pioneers of laser cooling, gives an intimate accountof how thefield started alongwith thebroad rangeof scientific and technological innovations that have ensued. For example, one imaginative andpractical application of cold atoms is radio-krypton dating. As described in a perspective by Z.-T. Lu, the exquisite isotope selectivity and detection efficacy for single trapped atoms have provided a unique background-free–tracedetection capability for earth science applications. In another application where high-quality atomic transitions in the optical domain are used for the new-generation optical atomic clocks, preserving the maximum coherence between atomic states and anoptical field requires that the atomic center-of-massmotion is controlled at the scale of an optical wavelength and that the optical probe of internal atomic states do not introduce measurement uncertainty fromphoton recoils.Meeting these challenges demands preparing atoms at ultralow temperatures and confining them in traps that decouple the atomic internal and external degrees of freedom.The full control of both internal and external dynamics is critical for precisionmeasurement, frequency metrology (discussed in a review by X. Zhang and J. Ye), and quantum information science. It is now routine to prepare, manipulate, and measure cold atoms in the formof quantumdegenerate gases inwhich striking quantumbehavior dominates the observation of systemdynamics. Two important tools have been invented to enrich quantum physics in strongly interacting regimes: the first powerful tool is the control of interatomic scattering resonances, and thus interactions, via the Feshbach resonance. A perspective by C. Chin beautifully illustrates the transition from a weakly interacting quantum system to a strongly interacting one, where new universal principles can be explored that govern correlated quantum behavior. The second useful tool is the introduction of optical lattices to regulate atomicmotions, which provides an intimate interface betweenultracold atoms and condensed-matter systems. A perspective by S. Kuhr shows the powerful in-situ visualization of individual atoms confined in an optical lattice via a recently developed technique of a quantum gas microscope, which is opening a new approach to the study of many-body quantum systems. With these tools closely integratedwithwell prepared atomic quantum systems, we are ready to take on challenging and outstandingexperiments toprobe the connectionbetween twoand few-body physics and to understand the emergence of quantum many-body correlations. A research highlight by A. M. Rey discusses the importance of introducing synthetic gauge fields to ultracold atoms and turning them into a versatile playground for simulating and gaining insights to new classes of quantummaterials. A reviewbyM.Weidemuller explores recent developments of resonant three-body Efimov effects in ultracold heteronuclear atomic mixtures, helping illuminate universal three-body physics and atom-molecule interactions. Plus, polar molecules have been cooled to quantum gases. The rich internal structure of these molecules provides a unique experimental testing ground for the fundamental laws of nature. They also provide new opportunities for the study of novel many-body quantum systems with tunable, long-range, and anisotropic interactions. The strong interdisciplinary character of these new atomic andmolecular systems facilitates powerful connections to other scientific fields, including chemistry, quantum information, condensed-matter physics, and high-energy physics. Many new research opportunities are emerging. I sincerely thank the contributing authors who invested time and effort for this special topic, and I hope it will serve to inspire young researchers to develop more creative ideas and approaches.

Highly uniform holographic microtrap arrays for single atom trapping using a feedback optimization of in-trap fluorescence measurements.

We report on the novel optimization method to realize highly uniform microtrap arrays for single atom trapping with a spatial light modulator (SLM). This method consists of two iterative feedback loops with the measurements of both diffracted light intensities and in-trap fluorescence intensities from each microtrap. By applying this method to the single 87Rb atom trapping, we can reduce the variance of trap depths from 20.8% to 1.7% for 4 × 4 square arrays and less than 4% for various arrays with up to 62 sites. The detection error of individual single atoms is also reduced from 1.7% to 0.0054% on average.

Quantum memory for photons

The quantum state of a photon can be transferred to a single trapped atom or to a bunch of atoms in a gas or solid and be stored for later release on demand.

Trapping and Cooling of Single Atoms in an Optical Microcavity by a Magic-Wavelength Dipole Trap**Supported by the National Basic Research Program of China under Grant No 2012CB921601, and the National Natural Science Foundation of China under Grant Nos 11125418, 61121064, 61275210, 61227902 and 91336107.

We present trapping and cooling of single cesium atoms inside a microcavity by means of an intracavity far-off-resonance trap (FORT). By the ‘magic’ wavelength FORT, we achieve state-insensitive single-atom trapping and cooling in a microcavity. The cavity transmission of the probe beam strongly coupled to single atoms enables us to continuously observe the intracavity atom trapping. The average atomic localization time inside the bright FORT is about 7 ms by introducing cavity cooling with appropriate detuning. This experiment presents great potential in coherent state manipulation for strongly coupled atom–photon systems in the context of cavity quantum electrodynamics.

Universal quantum gates for atomic systems assisted by Faraday rotation

Both cavity quantum electrodynamics and photons are promising candidates for quantum information processing. Here we present two efficient schemes for universal quantum gates, that is, Fredkin gates and gates on atomic systems, assisted by Faraday rotation catalyzed by an auxiliary single photon. These gates are achieved by successfully reflecting an auxiliary single photon from an optical cavity with a single-trapped atom. They do not require additional qubits and they only need some linear-optical elements besides the nonlinear interaction between the flying photon and the atoms in the optical cavities. Moreover, these two universal quantum gates are robust. A high fidelity can be achieved in our schemes with current experimental technology. They may be very useful in quantum information processing in future, with the great progress on controlling atomic systems.

Single-Atom Trapping in Holographic 2D Arrays of Microtraps with Arbitrary Geometries

Holding single ultracold atoms in reconfigurable arrangements makes it possible to study quantum systems. Researchers demonstrate two-dimensional arrays of optical tweezers containing up to 100 traps that can be configured in arbitrary geometries.

A quantum gate between a flying optical photon and a single trapped atom

The steady increase in control over individual quantum systems supports the promotion of a quantum technology that could provide functionalities beyond those of any classical device. Two particularly promising applications have been explored during the past decade: photon-based quantum communication, which guarantees unbreakable encryption but which still has to be scaled to high rates over large distances, and quantum computation, which will fundamentally enhance computability if it can be scaled to a large number of quantum bits (qubits). It was realized early on that a hybrid system of light qubits and matter qubits could solve the scalability problem of each field--that of communication by use of quantum repeaters, and that of computation by use of an optical interconnect between smaller quantum processors. To this end, the development of a robust two-qubit gate that allows the linking of distant computational nodes is "a pressing challenge". Here we demonstrate such a quantum gate between the spin state of a single trapped atom and the polarization state of an optical photon contained in a faint laser pulse. The gate mechanism presented is deterministic and robust, and is expected to be applicable to almost any matter qubit. It is based on reflection of the photonic qubit from a cavity that provides strong light-matter coupling. To demonstrate its versatility, we use the quantum gate to create atom-photon, atom-photon-photon and photon-photon entangled states from separable input states. We expect our experiment to enable various applications, including the generation of atomic and photonic cluster states and Schrödinger-cat states, deterministic photonic Bell-state measurements, scalable quantum computation and quantum communication using a redundant quantum parity code.

ALPHA: antihydrogen and fundamental physics

Detailed comparisons of antihydrogen with hydrogen promise to be a fruitful test bed of fundamental symmetries such as the CPT theorem for quantum field theory or studies of gravitational influence on antimatter. With a string of recent successes, starting with the first trapped antihydrogen and recently resulting in the first measurement of a quantum transition in anti-hydrogen, the ALPHA collaboration is well on its way to perform such precision comparisons. We will discuss the key innovative steps that have made these results possible and in particular focus on the detailed work on positron and antiproton preparation to achieve antihydrogen cold enough to trap as well as the unique features of the ALPHA apparatus that has allowed the first quantum transitions in anti-hydrogen to be measured with only a single trapped antihydrogen atom per experiment. We will also look at how ALPHA plans to step from here towards more precise comparisons of matter and antimatter.

High-numerical-aperture microlensed tip on an air-clad optical fiber

We show that a hemispherically shaped tip on an air-clad optical fiber simultaneously works as a high-numerical-aperture lens and efficiently collects photons from an emitter placed near the beam waist into the fundamental guided mode. Numerical simulations show that the coupling efficiency reaches about 25%. We have constructed a confocal microscope with such a lensed fiber. The measurements are in good agreement with the numerical simulation. The monolithic structure with a high-photon-collection efficiency will provide a flexible substitute for a conventional lens system in various experiments such as single-atom trapping with a tightly focused optical trap.

Single-atom transfer in a strongly coupled cavity quantum electrodynamics: experiment and Monte Carlo simulation

The process of single-atom transfer in strongly coupled cavity quantum electrodynamics (QED) with free falling atoms is investigated by experiment and Monte Carlo simulation. We conduct the simulation of the whole physical process and give the corresponding experimental results. In experiment, a high finesse optical cavity is used for real-time detection of the single-atom transits from which the interaction information between single atoms and cavity can be extracted, including the transmission spectra of the cavity strongly coupled to single atoms, the interaction duration of the single atoms in the mode, the probability distribution of atom arrival time and the atomic kinetic energy distribution when arriving at the mode. All these can be completely derived from the transmission spectra of the different initial status. An intracavity far-off resonance trap (FORT) has been established and the single-atom trapping time inside the cavity is about 5 ms which is about 30 times longer than that without FORT. This study gives the detailed analysis of the whole procedure of free-falling atom transfer in cavity QED system and is helpful for optimizing the experimental parameters and design.

Real-time sub-Ångstrom imaging of reversible and irreversible conformations in rhodium catalysts and graphene

The dynamic responses of a rhodium catalyst and a graphene sheet are investigated upon random excitation with 80 kV electrons. An extraordinary electron microscope stability and resolution allow studying temporary atom displacements from their equilibrium lattice sites into metastable sites across projected distances as short as 60 pm. In the rhodium catalyst, directed and reversible atom displacements emerge from excitations into metastable interstitial sites and surface states that can be explained by single atom trajectories. Calculated energy barriers of 0.13 eV and 1.05 eV allow capturing single atom trapping events at video rates that are stabilized by the Rh [110] surface corrugation. Molecular dynamics simulations reveal that randomly delivered electrons can also reversibly enhance the $s{p}^{3}$ and the $s{p}^{1}$ characters of the $s{p}^{2}$-bonded carbon atoms in graphene. The underlying collective atom motion can dynamically stabilize characteristic atom displacements that are unpredictable by single atom trajectories. We detect three specific displacements and use two of them to propose a path for the irreversible phase transformation of a graphene nanoribbon into carbene. Collectively stabilized atom displacements greatly exceed the thermal vibration amplitudes described by Debye-Waller factors and their measured dose rate dependence is attributed to tunable phonon contributions to the internal energy of the systems. Our experiments suggest operating electron microscopes with beam currents as small as zepto-amperes/nm${}^{2}$ in a weak-excitation approach to improve on sample integrity and allow for time-resolved studies of conformational object changes that probe for functional behavior of catalytic surfaces or molecules.

Two-dimensional lattice of blue-detuned atom traps using a projected Gaussian beam array

We describe a blue-detuned optical lattice for atom trapping which is intrinsically two-dimensional, while providing three-dimensional atom localization. The lattice is insensitive to optical phase fluctuations since it does not depend on field interference between distinct optical beams. The array is created using an arrangement of weakly overlapping Gaussian beams that creates a two-dimensional array of dark traps which are suitable for magic trapping of ground and Rydberg states. We analyze the spatial localization that can be achieved and demonstrate trapping and detection of single Cs atoms in 6- and 49-site two-dimensional arrays.

Nondestructive light-shift measurements of single atoms in optical dipole traps

We measure the AC-Stark shifts of the 5S{1/2}, F=2 to 5P{3/2}, F'=3 transitions of individual optically trapped 87Rb atoms using a non-destructive detection technique that allows us to measure the fluorescent signal of one-and-the-same atom for over 60 seconds. These measurements allow efficient and rapid characterization of single atom traps that is required for many coherent quantum information protocols. Although this method is demonstrated using a single atom trap, the concept is readily extended to resolvable atomic arrays.

Entanglement of two movable mirrors and two-mode cavity fields generated by a single four-level atom

We present a scheme using a single four-level atom to entangle two mesoscopic mirrors. If the coupling between the cavity fields and the atom is suitable, then our study shows entanglement between the two movable mirrors, between the two-mode fields, and also between the mirror and the distant cavity mode. Because the energy difference between the two lowest levels of the four-level atom is not negligible, the entanglement of the mirrors as well as the squeezing spectrum of the output cavity fields display a splitting phenomenon. In the present scheme, only a single trapped atom is used, which sheds new light on the use of microscopic atomic coherence for the production of mesoscopic entanglement.

Quantum optics and cavity QED Quantum network with individual atoms and photons

Quantum physics allows a new approach to information processing. A grand challenge is the realization of a quantum network for long-distance quantum communication and large-scale quantum simulation. This paper highlights a first implementation of an elementary quantum network with two fibre-linked high-finesse optical resonators, each containing a single quasi-permanently trapped atom as a stationary quantum node. Reversible quantum state transfer between the two atoms and entanglement of the two atoms are achieved by the controlled exchange of a time-symmetric single photon. This approach to quantum networking is efficient and offers a clear perspective for scalability. It allows for arbitrary topologies and features controlled connectivity as well as, in principle, infinite-range interactions. Our system constitutes the largest man-made material quantum system to date and is an ideal test bed for fundamental investigations, e.g. quantum non-locality.

Fast transport, atom sample splitting and single-atom qubit supply in two-dimensional arrays of optical microtraps

Two-dimensional arrays of optical microtraps created by micro-optical elements present a versatile and scalable architecture for neutral atom quantum information processing, quantum simulation and the manipulation of ultra-cold quantum gases. In this paper, we demonstrate the advanced capabilities of this approach by introducing novel techniques and functionalities as well as the combined operation of previous separately implemented functions. We introduce piezo-actuator-based transport of atom ensembles over distances of more than one trap separation, examine the capabilities of rapid atom transport provided by acousto-optical beam steering and analyse the adiabaticity limit for atom transport in these configurations. We implement a spatial light modulator with 8 bit transmission control for the per-site adjustment of the trap depth and the number of atoms loaded. We combine single-site addressing, trap depth control and atom transport in one configuration for demonstrating the splitting of atom ensembles with variable ratio at predefined register sites. Finally, we use controlled sub-poissonian preparation of single trapped atoms from such an ensemble to show that our approach allows for the implementation of a continuous supply of single-atom qubits with high fidelity. These novel implementations and their combined operation significantly extend available techniques for the dynamical and reconfigurable manipulation of ultra-cold atoms in dipole traps.

The trapping and detection of single atoms using a spherical mirror

We fabricate a miniature spherical mirror for tightly focusing an optical dipole trap for neutral atoms. The mirror formation process is modelled to predict the dimensions for particular fabrication parameters. We integrate the spherical mirror with a neutral atom experiment to trap and detect a single atom with high efficiency. The mirror serves the dual purpose of focusing the dipole trap as well as collecting the atomic fluorescence into an optical fibre.

Local degradation of electroluminescent emission centers in ZnS:Cu,Cl phosphors

ZnS:Cu contains needle-shaped, CuS conducting nano-precipitates along the [111] planes which amplify the applied electric fields near the tips when the field is reversed, enabling AC electroluminescence for ∼50μm thick devices at AC voltages of ∼100V. Unfortunately, the degradation process is poorly understood; previous studies using photodiode measurements were only able to determine the overall device degradation. Here, we analyze many individual emission centers using time-lapse microscope images of several degrading devices. We find that most of the individual emission centers have large, sudden, discrete drops in luminosity and many have a luminosity that actually increases significantly, minutes to hours after a drop. This suggests that there are two primary degradation mechanisms: gradual multi-step diffusion of single-atom trap states away from the high-field region (causing the gradual decrease in luminosity) and the sudden break-up and reformation of complex trap states within the high field region (causing the sudden decreases and increases, respectively).