INTENDED OUTCOME AND BACKGROUND

This strategic element focuses on the physics of laser cooling and electromagnetic trapping of neutral particles, the manipulation of Bose-Einstein condensates (BECs), and the use of optical dipole forces as a new tool for analyzing of microscopic objects in biochemistry. It includes both fundamental and applied studies, such as developing measurement techniques for biomolecular systems and developing a quantum information processor. A strong theoretical-experimental collaboration is aimed at interpreting experimental results and providing guidance for new experiments.

The development of laser cooling and trapping techniques allows exquisite control over the motion of atoms. Such control has been exploited to build more precise atomic clocks and gravity gradiometers. These techniques also enable the study and manipulation of atoms and molecules under conditions in which their quantum or wave behavior dominates. This research has revolutionized the field of matter-wave optics.

Our research includes theoretical and experimental projects that contribute to the understanding and exploitation of Bose-Einstein condensation of neutral atoms, matter-wave optics, optical and magnetic control of trapped-atom collisions, advanced laser cooling and collision studies for atomic clocks, ultracold plasmas and Rydberg atoms, the study of the superfluid to Mott-insulator quantum phase transition, quantum information processing, quantum-computing architectures, and optical characterization and manipulation of single molecules, biomolecules, and biomembranes.

Accomplishments

• From an Atomic BEC to Mott-Insulator to a Molecular BEC

  Figure 1. Schematic showing two-color formation of molecules in their ground electronic state by Raman Photoassociation. In a BEC this process is initially coherent and leads to a wavefunction that is both atoms and molecules.

  Recent theoretical calculations show how to obtain a quantum phase transition that takes the superfluid state appropriate to a BEC in a shallow, three-dimensional, optical lattice and transforms it to a Mott-Insulator state appropriate to a deep optical lattice. In the superfluid state all the atoms are identical, whereas in the deep lattice, Mott-Insulator state the atoms are distinct since they are individually labeled by their lattice position.

  We have shown that if we start with an average of two atoms per optical lattice site and increase the lattice depth to obtain a Mott-Insulator state with exactly two atoms per well site, we can then convert the atom pairs into ground state molecules using laser light. Finally, after molecular formation, the Mott-Insulator can be "melted" to yield a molecular BEC. Specific calculations have been done for the homonuclear species ⁸⁷Rb₂ and the heteronuclear species KRb.

  Studies of atomic BEC systems have proven to be extraordinarily fruitful, with connections to a number of disciplines, including atomic, molecular, and optical physics, quantum optics, condensed-matter physics, solid-state physics, quantum field theory, and quantum information and computing. One of the primary purposes of the Mott-Insulator transition is the initialization of a neutral-atom quantum register for quantum computing. The production of molecular BECs will extend applications to molecular species. Experiments along some of these lines are being planned.

  CONTACT: Dr. Carl Williams (301) 975-3531 carl.williams@nist.gov

  
  
  

• Photoassociation in a Bose-Einstein Condensate

  We have investigated the photoassociation of atoms (two colliding atoms absorbing a photon, forming a molecule) in a trapped, sodium BEC. We measured a rate coefficient that exceeds the classical limit by more than four orders of magnitude. The measured rate coefficient is, however, in good agreement with results from a quantum-mechanical two-body scattering theory. Classically, atoms have to be next to each other to form a molecule, but quantum mechanically, the BEC has a single wavefunction for all the atoms extending over the entire trapped gas.

  This is another example of how the quantum world can give remarkably different results than the classical world. Such studies are important for developing theories that describe the BEC. The theories can then be used to exploit the BEC as a source of atoms analogous to the source of photons from a laser for use in precision measuring devices, such as atom interferometers.

  CONTACT: Dr. Paul Lett (301) 975-6559 paul.lett@nist.gov

  
  
  

• Real-Time Measurements of Antigen-Antibody Binding

  Adhesion is an ubiquitous process in biological systems. We have developed a new technique to study the adhesion of biomolecules in real time under biologically relevant conditions, similar to the situation when two cells collide and adhere.

  Using optical tweezers, we trap a pair of microspheres, one coated with an antigen and the other coated with the corresponding antibody, and bring them close enough to each other that they repeatedly collide due to thermally driven motion. By monitoring the position of the trapped, antigen-coated microsphere, we can observe single antigen-to-antibody binding events in real time. We also measure the single molecule, spontaneous dissociation rate and the average rate at which antigen-antibody pairs unbind due to thermal fluctuations. By varying the number of antigen-to-antibody bonds that can form in a collision, we can observe cooperativity in the binding. We observe not only positive cooperativity, but also negative cooperativity (which is rarer in nature) depending on how rigidly the antigen molecule is attached to the microsphere surface.

  CONTACT: Dr. Kristian Helmerson (301) 975-4266 kristian.helmerson@nist.gov

  
  
  

• Patterned Loading of Atoms into an Optical Lattice

  Figure 2. Diffraction pattern showing the contrast between atoms coherently loading into every third lattice site, versus every lattice site.

  Quantum systems, such as individual atoms, can be used as bits of information. The processing of such information, governed by the rules of quantum mechanics, is called quantum computing. There is currently great interest in realizing a quantum computer, which is predicted to require exponentially less effort than a classical computer to solve certain large-scale problems, such as factoring large numbers.

  We are developing a processor for quantum information, using neutral atoms trapped in an optical lattice as the quantum information register. In an optical lattice, atoms are trapped in the periodic intensity pattern formed from the interference of intersecting laser beams.

  In order to achieve the best performance for quantum information processing, we would like atoms tightly confined, which can be achieved with a short-period optical lattice. However, to initialize and read out the quantum register, we would like atoms in sites spaced more than an optical wavelength apart. We have taken a major step towards achieving this goal by loading every third site of a one-dimensional, short-period optical lattice with atoms from a rubidium Bose-Einstein condensate.

  CONTACT: Dr. James (Trey) Porto (301) 975-3238 trey.porto@nist.gov

First strategic focus   |   Second strategic focus   |   Third strategic focus   |   Fourth strategic focus

"Technical Activities 2002" - Table of Contents