Physics
Optical tweezing is a powerful technique for trapping and manipulating particles and can be applied to a broad range of size scales—from single cells and viruses to glass beads several micrometers in diameter. Tweezing can also be used to trap single atoms. However, the interaction between photons in the trapping light beam and the atom generally results in atom jitter. This motion of the atom tends to prevent the atoms from being cooled to the lowest temperatures, where interesting quantum effects can then be probed. Kaufman et al. used an optical trapping beam and a suite of laser pulses to lasso a single rubidium atom and exploited Raman transitions of the atom to cool it to the quantum ground state. Once trapped and cooled to remove all vibrations from the atom, they also show that they could coherently manipulate its quantum spin and motion. The generality and flexibility of the optical tweezing approach may allow more complex systems comprising arrays of overlapping and interacting trapped atoms or molecules to be designed to form quantum simulators (i.e., well-controlled, engineered quantum systems that can be used to model other less-well-understood condensed-matter systems).
Phys. Rev. X 2 , 041014 (2012).
Physics
Optical tweezing is a powerful technique for trapping and manipulating particles and can be applied to a broad range of size scales—from single cells and viruses to glass beads several micrometers in diameter. Tweezing can also be used to trap single atoms. However, the interaction between photons in the trapping light beam and the atom generally results in atom jitter. This motion of the atom tends to prevent the atoms from being cooled to the lowest temperatures, where interesting quantum effects can then be probed. Kaufman et al. used an optical trapping beam and a suite of laser pulses to lasso a single rubidium atom and exploited Raman transitions of the atom to cool it to the quantum ground state. Once trapped and cooled to remove all vibrations from the atom, they also show that they could coherently manipulate its quantum spin and motion. The generality and flexibility of the optical tweezing approach may allow more complex systems comprising arrays of overlapping and interacting trapped atoms or molecules to be designed to form quantum simulators (i.e., well-controlled, engineered quantum systems that can be used to model other less-well-understood condensed-matter systems).
Phys. Rev. X 2 , 041014 (2012).
This important volume contains selected papers and extensive commentaries on laser trapping and manipulation of neutral particles using radiation pressure forces. Such techniques apply to a variety of small particles, such as atoms, molecules, macroscopic dielectric particles, living cells, and organelles within cells. These optical methods have had a revolutionary impact on the fields of atomic and molecular physics, biophysics, and many aspects of nanotechnology.In atomic physics, the trapping and cooling of atoms down to nanokelvins and even picokelvin temperatures are possible. These are the lowest temperatures in the universe. This made possible the first demonstration of Bose–Einstein condensation of atomic and molecular vapors. Some of the applications are high precision atomic clocks, gyroscopes, the measurement of gravity, cryptology, atomic computers, cavity quantum electrodynamics and coherent atom lasers.A major application in biophysics is the study of the mechanical properties of the many types of motor molecules, mechanoenzymes, and other macromolecules responsible for the motion of organelles within cells and the locomotion of entire cells. Unique in vitro and in vivo assays study the driving forces, stepping motion, kinetics, and efficiency of these motors as they move along the cell's cytoskeleton. Positional and temporal resolutions have been achieved, making possible the study of RNA and DNA polymerases, as they undergo their various copying, backtracking, and error correcting functions on a single base pair basis.Many applications in nanotechnology involve particle and cell sorting, particle rotation, microfabrication of simple machines, microfluidics, and other micrometer devices. The number of applications continues to grow at a rapid rate.The author is the discoverer of optical trapping and optical tweezers. With his colleagues, he first demonstrated optical levitation, the trapping of atoms, and tweezer trapping and manipulation of living cells and biological particles.This is the only review volume covering the many fields of optical trapping and manipulation. The intention is to provide a selective guide to the literature and to teach how optical traps really work.
Molecules, with their rich spectrum of internal states as well as strong and tunable dipolar interactions, are promising candidates for a broad range of experiments including quantum information, quantum chemistry, precision measurement and probing physics beyond the Standard Model, and quantum simulation of interacting many-body systems. At the same time, the complexity that made them attractive also poses a significant challenge to achieve a high level of quantum control that is required in many of their potential applications. Multiple approaches are being pursued for full quantum control of molecules including association of atomic gases and direct cooling of molecules, yet it remains challenging to control molecules at single particle level due to the difficulty in tuning the atom-atom interaction and laser cooling of molecules. In this thesis, we achieved full control on a single weakly bound ground state molecule created by coherent optical association from single atoms. We first discuss the trapping and imaging of the atoms in the optical tweezer and the cooling into a single quantum state in order to achieve the quantum control of atoms. Combining this control with the resolution and flexibility of tweezers. we developed a versatile platform to study interactions between single atoms and properties of single molecules, using precise optical spectroscopy on pairs of atoms in the tweezer. Based on these studies, we proposed a promising pathway for coherent all-optical creation of molecules and experimentally demonstrated full control on the molecule including all degrees of freedom.
Single atoms are promising candidates for physically implementing quantum bits, the fundamental unit of quantum information. We have built an apparatus for cooling, trapping and imaging single rubidium atoms in microscopic optical tweezers. The traps are formed from a tightly focused off-resonant laser beam, which traps atoms using the optical dipole force. The traps have a diameter of ~1 μm and a depth of ~1 mK. The novelty of our approach is the use a digital mirror device (DMD) to generate multiple independently movable tweezers from a single laser beam. The DMD consists of an array of micro-mirrors that can be switched on and off, thus acting as a binary amplitude modulator. We use the DMD to imprint a computer-generated hologram on the laser beam, which is converted in to the desired arrangement of traps in the focal plane of a lens. We have developed fast algorithms for calculating binary holograms suitable for the DMD. In addition, we use this method to measure and correct for errors in the phase of the wavefront caused by optical aberrations, which is necessary for producing diffraction-limited focal spots. Using this apparatus, we have trapped arrays of up to 20 atoms with arbitrary geometrical arrangements. We exploit light-assisted collisions between atoms to ensure there is at most one atom per trapping site. We measure the temperature of the atoms in the traps to be 12 μK, and their lifetime to be 1.4 s. Finally, we demonstrate the ability to select individual atoms from an array and transport them over a distance of 14μm with laser cooling, and 5 μm without.
A variety of powerful techniques to control the position and velocity of neutral particles has been developed. As examples of this new ability, lasers have been used to construct a variety of traps, to cool atoms to temperatures below 3 x 10(-6) kelvin, and to create atomic fountains that may give us a hundredfold increase in the accuracy of atomic clocks. Bacteria can be held with laser traps while they are being viewed in an optical microscope, and organelles within a cell can be manipulated without puncturing the cell wall. Single molecules of DNA can now be stretched out and pinned down in a water solution with optical traps. These new capabilities may soon be applied to a wide variety of scientific questions as diverse as precision measurements of fundamental symmetries in physics and the study of biochemistry on a single molecule basis.
Tweezing cold molecules Arrays of optical tweezers have been used to trap atoms, but trapping and laser-cooling molecules in this setting is tricky. Such an approach would, however, be generalizable to many molecular species. Anderegg et al. created an optical tweezer array of calcium monofluoride molecules, which were laser cooled to their ground state (see the Perspective by Kotochigova). By distinguishing between single and multiple molecules in the tweezers, the researchers were able to observe molecular collisions. Boasting exquisite control over individual molecules, the optical tweezer array platform holds much promise for extending the applications of ultracold molecules. Science , this issue p. 1156 ; see also p. 1079
Lasers are increasingly being used to impart mechanical forces on materialparticles ranging from atoms to biological cells. I present here experimentsinvolving such forces, concentrating on the formation and excitation of Bose-Einstein condensates (BEC) in ultra-cold rubidium 87 vapours. Also reportedare experiments in laser locking using hyperfine spectroscopy of 87Rb andexperiments in the optical manipulation of particles much larger than atoms.The cooling and imaging techniques used in BEC experiments dependcrucially on lasers with frequencies stabilised relative to the hyperfine structureof atoms. This structure is typically probed using saturated absorptionspectroscopy or polarisation spectroscopy, but the reliability of both is compromisedby the narrowness of the spectral lines involved. An extension ofpolarisation spectroscopy is presented which overcomes this by measuringa combination of Stokes parameters, the ratio of which provides a usefulfeedback signal over many times the line-width.Particles up to the size of biological cells can also be trapped and relocatedusing tightly focused laser beams called optical tweezers. Slight displacementsof particles within tweezer traps are used here to measure opticalforces applied by a tapered optical fiber lens that delivers light to a trappedpolystyrene bead. The optical force on the bead was mapped as it was movedrelative to the taper, showing that the taper provides a long and well colli-mated force-field that might be used as an optical “conveyor belt”.A BEC is a macroscopic collection of particles which collectively displayquantum coherence. Condensates are created by trapping and cooling particlesto temperatures below the critical temperature Tc for Bose condensation.These experiments produce BECs of 87Rb atoms with an atom chip apparatusthat confines atoms to a cigar shaped region using a nearly harmonicmagnetic potential. One axis of this trap is much weaker than the others andis bisected by a Gaussian sheet of laser light, to which atoms are attracted.The beam thus provides a narrow potential well (dimple) of variable depthsuperimposed on the magnetic trap.This trap was used to form condensates by mechanical compression (whichraises Tc) rather than cooling. Thermodynamic properties of vapours wereobserved before and after the dimple potential was turned on and comparedto detailed theoretical calculations based on semi-classical Hartree-Fock (mean-field) theory, which is often presumed to be accurate for Bosegases. Good agreement was found when the dimple was switched on slowlyin a reversible process. Rapid turn-on however produced much less heating(and hence higher final condensate fractions) than predicted.The dynamics of condensation formation were studied by measuring theproperties of vapours at various times after suddenly switching on the dimple.This contrasts with experiments in which high energy atoms are rapidly removedfrom a cloud. Condensates form in such systems after a very differentkind of rethermilsation process.The dimple is also used to create overdensities which propagate throughthe BEC after the optical potential is suddenly turned off. Analytical calculationsof classical hydrodynamics and numerical simulations of the condensatebased on the Gross-Pitaevskii wave equation both show that the leading edgeof such pulses should form shock waves in finite time. The regime in whichthis occurs is experimentally accessible and data are shown which are compatiblewith theoretical predictions. However the existence of shock waves isnot demonstrated because the available imaging resolution is insufficient. Aproposal is made to repeat this experiment using much larger condensates,which calculations show should make shock waves apparent.