Cooling of a levitated nanoparticle to the motional quantum ground state

Uroš Delić, Manuel Reisenbauer, Kahan Dare, David Grass, Vladan Vuletić, Nikolai Kiesel, Markus Aspelmeyer

Quantum control of complex objects in the regime of large size and mass provides opportunities for sensing applications and tests of fundamental physics. The realization of such extreme quantum states of matter remains a major challenge. We demonstrate a quantum interface that combines optical trapping of solids with cavity-mediated light matter interaction. Precise control over the frequency and position of the trap laser with respect to the optical cavity allows us to laser-cool an optically trapped nanoparticle into its quantum ground state of motion from room temperature. The particle comprises of 108 atoms, similar to current Bose-Einstein condensates, with the density of a solid object. Our cooling, in combination with optical trap manipulation, may enable otherwise unachievable superposition states involving large masses.

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Probing gravity by holding atoms for 20 seconds

Victoria Xu, Matt Jaffe, Cristian D. Panda, Sofus L. Kristensen, Logan W. Clark, Holger Müller

Atom interferometers are powerful tools for both measurements in fundamental physics and inertial sensing applications. Their performance, however, has been limited by the available interrogation time of freely falling atoms in a gravitational field. By suspending the spatially separated atomic wave packets in a lattice formed by the mode of an optical cavity, we realize an interrogation time of 20 seconds. Our approach allows gravitational potentials to be measured by holding, rather than dropping, atoms. After seconds of hold time, gravitational potential energy differences from as little as micrometers of vertical separation generate megaradians of interferometer phase. This trapped geometry suppresses the phase variance due to vibrations by three to four orders of magnitude, overcoming the dominant noise source in atom-interferometric gravimeters.

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Three-dimensional crystals of adaptive knots

Jung-Shen B. Tai, Ivan I. Smalyukh

Starting with Gauss and Kelvin, knots in fields were postulated to behave like particles, but experimentally they were found only as transient features or required complex boundary conditions to exist and could not self-assemble into three-dimensional crystals. We introduce energetically stable, micrometer-sized knots in helical fields of chiral liquid crystals. While spatially localized and freely diffusing in all directions, they resemble colloidal particles and atoms, self-assembling into crystalline lattices with open and closed structures. These knots are robust and topologically distinct from the host medium, though they can be morphed and reconfigured by weak stimuli under conditions such as those in displays. A combination of energy-minimizing numerical modeling and optical imaging uncovers the internal structure and topology of individual helical field knots and the various hierarchical crystalline organizations that they form.

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Seconds-scale coherence on an optical clock transition in a tweezer array

Matthew A. Norcia, Aaron W. Young, William J. Eckner, Eric Oelker, Jun Ye, Adam M. Kaufman

Coherent control of high–quality factor optical transitions in atoms has revolutionized precision frequency metrology. Leading optical atomic clocks rely on the interrogation of such transitions in either single ions or ensembles of neutral atoms to stabilize a laser frequency at high precision and accuracy. We demonstrate a platform that combines the key strengths of these two approaches, based on arrays of individual strontium atoms held within optical tweezers. We report coherence times of 3.4 seconds, single-ensemble duty cycles up to 96% through repeated interrogation, and frequency stability of 4.7 × 10−16 (τ/s)–1/2. These results establish optical tweezer arrays as a powerful tool for coherent control of optical transitions for metrology and quantum information science.

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Making perfectly controlled arrays of molecules at rest

Svetlana Kotochigova

Since their invention in the early 1970s, optical tweezers have evolved from enabling simple manipulation to applying calibrated forces on—and measuring nanometer-level displacements of—optically trapped objects. Optical tweezers use laser light to create a force trap that can hold nanometer- to micrometer-sized dielectric objects (1). They can noninvasively manipulate objects such as biological cells in water, as well as apply piconewton forces to single molecules in solution or in free space. Combining these optical traps with laser cooling, which stops atoms and small molecules from moving in free space at ultracold temperatures, allows for precision measurements. On page 1156 of this issue, Anderegg et al. (2) created an array of optical tweezers filled with ultracold calcium monofluoride (CaF) molecules. Individual molecules were brought together by tweezer traps, enabling their interaction. Such unprecedented control should allow high-precision observations of molecular collisions and could provide insight into specific chemical reactions.

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An optical tweezer array of ultracold molecules

Loïc Anderegg, Lawrence W. Cheuk, Yicheng Bao, Sean Burchesky, Wolfgang Ketterle, Kang-Kuen Ni, John M. Doyle

Ultracold molecules have important applications that range from quantum simulation and computation to precision measurements probing physics beyond the Standard Model. Optical tweezer arrays of laser-cooled molecules, which allow control of individual particles, offer a platform for realizing this full potential. In this work, we report on creating an optical tweezer array of laser-cooled calcium monofluoride molecules. This platform has also allowed us to observe ground-state collisions of laser-cooled molecules both in the presence and absence of near-resonant light.

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Observation of a symmetry-protected topological phase of interacting bosons with Rydberg atoms

Sylvain de Léséleuc, Vincent Lienhard, Pascal Scholl, Daniel Barredo, Sebastian Weber, Nicolai Lang, Hans Peter Büchler, Thierry Lahaye, Antoine Browaeys

The concept of topological phases is a powerful framework for characterizing ground states of quantum many-body systems that goes beyond the paradigm of symmetry breaking. Topological phases can appear in condensed-matter systems naturally, whereas the implementation and study of such quantum many-body ground states in artificial matter require careful engineering. Here, we report the experimental realization of a symmetry-protected topological phase of interacting bosons in a one-dimensional lattice and demonstrate a robust ground state degeneracy attributed to protected zero-energy edge states. The experimental setup is based on atoms trapped in an array of optical tweezers and excited into Rydberg levels, which gives rise to hard-core bosons with an effective hopping generated by dipolar exchange interaction.

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Stable Casimir equilibria and quantum trapping

Rongkuo Zhao, Lin Li, Sui Yang, Wei Bao, Yang Xia, Paul Ashby, Yuan Wang, Xiang Zhang

The Casimir interaction between two parallel metal plates in close proximity is usually thought of as an attractive interaction. By coating one object with a low–refractive index thin film, we show that the Casimir interaction between two objects of the same material can be reversed at short distances and preserved at long distances so that two objects can remain without contact at a specific distance. With such a stable Casimir equilibrium, we experimentally demonstrate passive Casimir trapping of an object in the vicinity of another at the nanometer scale, without requiring any external energy input. This stable Casimir equilibrium and quantum trapping can be used as a platform for a variety of applications such as contact-free nanomachines, ultrasensitive force sensors, and nanoscale manipulations.

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Building one molecule from a reservoir of two atoms

L. R. Liu, J. D. Hood, Y. Yu, J. T. Zhang, N. R. Hutzler, T. Rosenband

Chemical reactions typically proceed via stochastic encounters between reactants. Going beyond this paradigm, we combine exactly two atoms into a single, controlled reaction. The experimental apparatus traps two individual laser-cooled atoms (one sodium and one cesium) in separate optical tweezers and then merges them into one optical dipole trap. Subsequently, photo-association forms an excited-state NaCs molecule. The discovery of previously unseen resonances near the molecular dissociation threshold and measurement of collision rates are enabled by the tightly trapped ultracold sample of atoms. As laser-cooling and trapping capabilities are extended to more elements, the technique will enable the study of more diverse, and eventually more complex, molecules in an isolated environment, as well as synthesis of designer molecules for qubits.

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Arbitrary spin-to-orbital angular momentum conversion of light

Robert C. Devlin, Antonio Ambrosio, Noah A. Rubin, J. P. Balthasar Mueller, Federico Capasso

Optical elements that convert the spin angular momentum (SAM) of light into vortex beams have found applications in classical and quantum optics. These elements, SAM to orbital angular momentum (OAM) converters, are based on the geometric phase and only permit the conversion of left- and right-circular polarizations (spin states) into states with opposite OAM. We present a method for converting arbitrary SAM states into total angular momentum states characterized by a superposition of independent OAM. We design a metasurface that converts left- and right-circular polarizations into states with independent values of OAM, and another device that performs this operation for elliptically polarized states. These results illustrate a general material-mediated connection between SAM and OAM of light and may find applications in producing complex structured light and in optical communication.

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Vinculin forms a directionally asymmetric catch bond with F-actin

Derek L. Huang, Nicolas A. Bax, Craig D. Buckley, William I. Weis, Alexander R. Dunn

Vinculin is an actin-binding protein thought to reinforce cell-cell and cell-matrix adhesions. However, how mechanical load affects the vinculin–F-actin bond is unclear. Using a single-molecule optical trap assay, we found that vinculin forms a force-dependent catch bond with F-actin through its tail domain, but with lifetimes that depend strongly on the direction of the applied force. Force toward the pointed (–) end of the actin filament resulted in a bond that was maximally stable at 8 piconewtons, with a mean lifetime (12 seconds) 10 times as long as the mean lifetime when force was applied toward the barbed (+) end. A computational model of lamellipodial actin dynamics suggests that the directionality of the vinculin–F-actin bond could establish long-range order in the actin cytoskeleton. The directional and force-stabilized binding of vinculin to F-actin may be a mechanism by which adhesion complexes maintain front-rear asymmetry in migrating cells.

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Revealing the subfemtosecond dynamics of orbital angular momentum in nanoplasmonic vortices

G. Spektor, D. Kilbane, A. K. Mahro, B. Frank, S. Ristok, L. Gal, P. Kahl, D. Podbiel, S. Mathias, H. Giessen, F.-J. Meyer zu Heringdorf, M. Orenstein, M. Aeschlimann

The ability of light to carry and deliver orbital angular momentum (OAM) in the form of optical vortices has attracted much interest. The physical properties of light with a helical wavefront can be confined onto two-dimensional surfaces with subwavelength dimensions in the form of plasmonic vortices, opening avenues for thus far unknown light-matter interactions. Because of their extreme rotational velocity, the ultrafast dynamics of such vortices remained unexplored. Here we show the detailed spatiotemporal evolution of nanovortices using time-resolved two-photon photoemission electron microscopy. We observe both long- and short-range plasmonic vortices confined to deep subwavelength dimensions on the scale of 100 nanometers with nanometer spatial resolution and subfemtosecond time-step resolution. Finally, by measuring the angular velocity of the vortex, we directly extract the OAM magnitude of light.

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Experimental measurement of binding energy, selectivity, and allostery using fluctuation theorems

Joan Camunas-Soler, Anna Alemany, Felix Ritort

Thermodynamic bulk measurements of binding reactions rely on the validity of the law of mass action and the assumption of a dilute solution. Yet, important biological systems such as allosteric ligand-receptor binding, macromolecular crowding, or misfolded molecules may not follow these assumptions and may require a particular reaction model. Here we introduce a fluctuation theorem for ligand binding and an experimental approach using single-molecule force spectroscopy to determine binding energies, selectivity, and allostery of nucleic acids and peptides in a model-independent fashion. A similar approach could be used for proteins. This work extends the use of fluctuation theorems beyond unimolecular folding reactions, bridging the thermodynamics of small systems and the basic laws of chemical equilibrium.

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Atom-by-atom assembly of defect-free one-dimensional cold atom arrays

Manuel Endres, Hannes Bernien, Alexander Keesling, Harry Levine, Eric R. Anschuetz, Alexandre Krajenbrink, Crystal Senko, Vladan Vuletic, Markus Greiner, Mikhail D. Lukin

The realization of large-scale fully controllable quantum systems is an exciting frontier in modern physical science. We use atom-by-atom assembly to implement a platform for the deterministic preparation of regular one-dimensional arrays of individually controlled cold atoms. In our approach, a measurement and feedback procedure eliminates the entropy associated with probabilistic trap occupation and results in defect-free arrays of over 50 atoms in less than 400 milliseconds. The technique is based on fast, real-time control of 100 optical tweezers, which we use to arrange atoms in desired geometric patterns and to maintain these configurations by replacing lost atoms with surplus atoms from a reservoir. This bottom-up approach may enable controlled engineering of scalable many-body systems for quantum information processing, quantum simulations, and precision measurements.

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Single-molecule dissection of stacking forces in DNA

Fabian Kilchherr, Christian Wachauf, Benjamin Pelz, Matthias Rief, Martin Zacharias, Hendrik Dietz

We directly measured at the single-molecule level the forces and lifetimes of DNA base-pair stacking interactions for all stack sequence combinations. Our experimental approach combined dual-beam optical tweezers with DNA origami components to allow positioning of blunt-end DNA helices so that the weak stacking force could be isolated. Base-pair stack arrays that lacked a covalent backbone connection spontaneously dissociated at average rates ranging from 0.02 to 500 per second, depending on the sequence combination and stack array size. Forces in the range from 2 to 8 piconewtons that act along the helical direction only mildly accelerated the stochastic unstacking process. The free-energy increments per stack that we estimate from the measured forward and backward kinetic rates ranged from –0.8 to –3.4 kilocalories per mole, depending on the sequence combination. Our data contributes to understanding the mechanics of DNA processing in biology, and it is helpful for designing the kinetics of DNA-based nanoscale devices according to user specifications.

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Direct observation of transition paths during the folding of proteins and nucleic acids

Krishna Neupane, Daniel A. N. Foster, Derek R. Dee, Hao Yu, Feng Wang, Michael T. Woodside

Transition paths, the fleeting trajectories through the transition states that dominate the dynamics of biomolecular folding reactions, encapsulate the critical information about how structure forms. Owing to their brief duration, however, they have not previously been observed directly. We measured transition paths for both nucleic acid and protein folding, using optical tweezers to observe the microscopic diffusive motion of single molecules traversing energy barriers. The average transit times and the shapes of the transit-time distributions agreed well with theoretical expectations for motion over the one-dimensional energy landscapes reconstructed for the same molecules, validating the physical theory of folding reactions. These measurements provide a first look at the critical microscopic events that occur during folding, opening exciting new avenues for investigating folding phenomena.

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Microtubule detyrosination guides chromosomes during mitosis

Marin Barisic, Ricardo Silva e Sousa, Suvranta K. Tripathy, Maria M. Magiera, Anatoly V. Zaytsev, Ana L. Pereira, Carsten Janke, Ekaterina L. Grishchuk, Helder Maiato

Before chromosomes segregate into daughter cells, they align at the mitotic spindle equator, a process known as chromosome congression. Centromere-associated protein E (CENP-E)/Kinesin-7 is a microtubule plus-end–directed kinetochore motor required for congression of pole-proximal chromosomes. Because the plus-ends of many astral microtubules in the spindle point to the cell cortex, it remains unknown how CENP-E guides pole-proximal chromosomes specifically toward the equator. We found that congression of pole-proximal chromosomes depended on specific posttranslational detyrosination of spindle microtubules that point to the equator. In vitro reconstitution experiments demonstrated that CENP-E–dependent transport was strongly enhanced on detyrosinated microtubules. Blocking tubulin tyrosination in cells caused ubiquitous detyrosination of spindle microtubules, and CENP-E transported chromosomes away from spindle poles in random directions. Thus, CENP-E–driven chromosome congression is guided by microtubule detyrosination.

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Factor-dependent processivity in human eIF4A DEAD-box helicase

Cuauhtémoc García-García, Kirsten L. Frieda, Kateryna Feoktistova, Christopher S. Fraser, Steven M. Block

During eukaryotic translation initiation, the small ribosomal subunit, assisted by initiation factors, locates the messenger RNA start codon by scanning from the 5′ cap. This process is powered by the eukaryotic initiation factor 4A (eIF4A), a DEAD-box helicase. eIF4A has been thought to unwind structures formed in the untranslated 5′ region via a nonprocessive mechanism. Using a single-molecule assay, we found that eIF4A functions instead as an adenosine triphosphate–dependent processive helicase when complexed with two accessory proteins, eIF4G and eIF4B. Translocation occurred in discrete steps of 11 ± 2 base pairs, irrespective of the accessory factor combination. Our findings support a memory-less stepwise mechanism for translation initiation and suggest that similar factor-dependent processivity may be shared by other members of the DEAD-box helicase family.

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Mechanical force releases nascent chain–mediated ribosome arrest in vitro and in vivo

Daniel H. Goldman, Christian M. Kaiser, Anthony Milin, Maurizio Righini, Ignacio Tinoco Jr., Carlos Bustamante

Protein synthesis rates can affect gene expression and the folding and activity of the translation product. Interactions between the nascent polypeptide and the ribosome exit tunnel represent one mode of regulating synthesis rates. The SecM protein arrests its own translation, and release of arrest at the translocon has been proposed to occur by mechanical force. Using optical tweezers, we demonstrate that arrest of SecM-stalled ribosomes can indeed be rescued by force alone and that the force needed to release stalling can be generated in vivo by a nascent chain folding near the ribosome tunnel exit. We formulate a kinetic model describing how a protein can regulate its own synthesis by the force generated during folding, tuning ribosome activity to structure acquisition by a nascent polypeptide.

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Engineering of a superhelicase through conformational control

Sinan Arslan, Rustem Khafizov, Christopher D. Thomas, Yann R. Chemla, Taekjip Ha

Conformational control of biomolecular activities can reveal functional insights and enable the engineering of novel activities. Here we show that conformational control through intramolecular cross-linking of a helicase monomer with undetectable unwinding activity converts it into a superhelicase that can unwind thousands of base pairs processively, even against a large opposing force. A natural partner that enhances the helicase activity is shown to achieve its stimulating role also by selectively stabilizing the active conformation. Our work provides insight into the regulation of nucleic acid unwinding activity and introduces a monomeric superhelicase without nuclease activities, which may be useful for biotechnological applications.

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