As stated in the NIST Physics Laboratory report for 2008, the mission of the Quantum Physics Division is as follows : “[The division’s goal is] to make transformational advances at the frontiers of science, in partnership with the University of Colorado at JILA…. The strategy of the Quantum Physics Division is to help produce the next generation of scientists and to investigate new ways of precisely directing and controlling light, atoms and molecules; measuring electronic, chemical and biological processes at the nanoscale; and manipulating ultrashort light pulses.”10
The strategic elements of this division are as follows:
To develop measurement science tools and their applications to technology;
To exploit Bose-Einstein condensation, quantum degenerate Fermi gases, and cold molecules for metrology and ultralow-temperature physics;
To advance ultrafast science and apply it to physics and biophysics;
To apply cutting-edge measurement science to biological systems;
To apply laser spectroscopy to important problems in chemical physics and biophysics; and
To educate a supply of top-quality scientists for NIST and elsewhere.
JILA excels in the area of neutral atom cooling and trapping, and the Nobel Prize for physics was shared by two JILA Fellows and an MIT professor for realizing Bose-Einstein condensation in 2001. Work continues at the frontier of fundamental understanding of the condensed state as well as in applications of BEC. One example is the study of fundamental elementary excitations of BEC such as vortices. By confining ultracold atoms in an optical lattice, a phase-coherent quasi-two-dimensional array of Bose-Einstein condensed atoms has been produced. In two dimensions, an exotic phase transition known as the Berezinskii-Kosterlitz-Thouless (BKT) transition is expected, and this has been observed for the first time in BEC as vortex excitations appear with increasing temperature. Another example is the use of a BEC of rubidium atoms for a new sensitive measure of the Casimir-Polder force due to quantum interactions between the condensed atoms and a nearby surface of fused silica.
Ultracold clouds of Fermi atoms are also studied. In this system correlated pairs of atoms form, and the pairs could Bose condense similar to the formation of Cooper
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Quantum Physics Division DESCRIPTION OF THE DIVISION Mission As stated in the NIST Physics Laboratory report for 2008, the mission of the Quantum Physics Division is as follows : “[The division’s goal is] to make transformational advances at the frontiers of science, in partnership with the University of Colorado at JILA. . . . The strategy of the Quantum Physics Division is to help produce the next generation of scientists and to investigate new ways of precisely directing and controlling light, atoms and molecules; measuring electronic, chemical and biological processes at the nanoscale; and manipulating ultrashort light pulses.”10 Scope The strategic elements of this division are as follows: • To develop measurement science tools and their applications to technology; • To exploit Bose-Einstein condensation, quantum degenerate Fermi gases, and cold molecules for metrology and ultralow-temperature physics; • To advance ultrafast science and apply it to physics and biophysics; • To apply cutting-edge measurement science to biological systems; • To apply laser spectroscopy to important problems in chemical physics and biophysics; and • To educate a supply of top-quality scientists for NIST and elsewhere. Projects JILA excels in the area of neutral atom cooling and trapping, and the Nobel Prize for physics was shared by two JILA Fellows and an MIT professor for realizing Bose- Einstein condensation in 2001. Work continues at the frontier of fundamental understanding of the condensed state as well as in applications of BEC. One example is the study of fundamental elementary excitations of BEC such as vortices. By confining ultracold atoms in an optical lattice, a phase-coherent quasi-two-dimensional array of Bose-Einstein condensed atoms has been produced. In two dimensions, an exotic phase transition known as the Berezinskii-Kosterlitz-Thouless (BKT) transition is expected, and this has been observed for the first time in BEC as vortex excitations appear with increasing temperature. Another example is the use of a BEC of rubidium atoms for a new sensitive measure of the Casimir-Polder force due to quantum interactions between the condensed atoms and a nearby surface of fused silica. Ultracold clouds of Fermi atoms are also studied. In this system correlated pairs of atoms form, and the pairs could Bose condense similar to the formation of Cooper 10 National Institute of Standards and Technology Physics Laboratory, 2008, Physics Laboratory, Gaithersburg, Maryland: National Institute of Standards and Technology, p. 44. 41
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pairs in a superconductor. The interactions between pairs are mediated by the so-called Feshbach resonance, in which the relative translational motion of the pairs is strongly coupled to a bound state with a different total spin. Because of the difference in spin, the strength of this interaction can be tuned with an external magnetic field, while the cold atoms are held in an optical dipole trap. The ability to tune the pair interaction has led to a wealth of phenomena. In one experiment, a sudden pulse of magnetic field puts pairs of atoms into a novel quantum superposition that oscillates in time between bound and unbound states. Another study explores the variation from the BEC regime, where localized pairs could form a Bose condensate to the Bardeen-Cooper-Schrieffer (BCS) regime, where pair formation is mediated by the surrounding condensate atoms and the size of the pair could become very large. This BEC-BCS crossover is important theoretically, and its understanding could impact the theory of High-Tc superconductivity. This work has been enabled by novel techniques developed for extracting information on the pair correlations from noise in absorption images of the cold atomic cloud. A third line of experimentation seeks to convert the highly interacting pair states into real bound states by removing the appropriate energy from the pairs with an optical Raman process. This has recently succeeded in producing bound diatomic molecules (though still highly excited relative to the molecular ground state) from these interacting cold Fermi pairs. Techniques for cooling small molecules are also being developed, with the goal of placing the molecules into a single chosen quantum state in order to study the fundamental quantum physics of cold molecules and their chemical interactions in single quantum states. To this end, OH molecules have been cooled with a novel Stark deceleration method and trapped in a magnetic trap. Work also continues on the development of new systems and techniques in frequency measurement. JILA has long been renowned as a center of excellence for laser frequency stabilization and for the development of methods for precision frequency measurement. One of the most important tools in recent years is the stabilized laser frequency comb, a technique that was brought from conception to fruition at JILA and for which a retired NIST JILA Fellow (now NIST Scientist Emeritus and JILA Fellow Adjoint) shared the Nobel Prize in physics in 2005. The laser frequency comb has produced a revolution in optical frequency measurement and is key to the development of optical atomic clocks. Current work in this division applies the frequency comb technique to a new atomic clock system composed of an optical lattice of laser-cooled strontium atoms. Scientists in this division realized that the energy states of strontium had characteristics that should lead to extremely small systematic frequency uncertainty and thus to potential clock accuracy and repeatability comparable with or better than the best previously identified clock systems. In a tour de force of atom cooling, trapping in an optical lattice and optical clock technology, this novel optical clock features a large number of atoms responding identically, thus providing the precision of a single atom with vastly enhanced signal-to-noise ratio. Fractional frequency instability of 3 × 10-15 has already been achieved, with the expectation that this can be improved by a factor of three. A cooperative effort with the NIST Time and Frequency Division in Boulder has compared the accuracy of this clock with the calcium optical clock at Time and Frequency. This was achieved using the capability for coherent optical phase transfer 42
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over a 3 km optical fiber with instability of 10-17 in 1 s, which allowed the overall systematic uncertainty of the strontium lattice clock to be evaluated as 1.5 × 10-16 in 1 s. The ability to transfer a stable optical frequency over considerable distance through optical fiber has broad potential applicability to precision metrology in the context of synchronized telescope arrays and long-distance interferometry, for example. The current performance represents a factor-of-100 improvement over previous techniques at 10s-of-kilometer distances. Further development of the optical frequency comb and extension of comb techniques to new applications are also underway. These include the Arbitrary Optical Waveform Generator, which is a joint competency program with the Time and Frequency Division, and the use of optical frequency combs for trace molecule detection and molecular fingerprinting. Another area of endeavor involves approaching quantum-limited nanoscale mechanical and electronic measurement. Studies of novel mesoscopic and nanoelectronic devices aim to explore the ultimate quantum limits of mechanical and microwave electronic measurements. Areas of application include quantum information, noiseless microwave amplification, and high-bandwidth detector arrays. Precision measurements on microfabricated mechanical oscillators explore the onset of quantum behavior as the dimensions approach the nanometer scale and low temperatures remove most of the thermal excitation. A sensitive scheme for the detection of the amplitude of a nanomechanical gold beam uses an atomic point contact and can detect motion at the quantum level as well as the effect on the oscillator of the quantum measurement noise imposed by the measurement of its position. The quantum limits to microwave measurement, amplification, and interferometry are tested in a series of elegant experiments that use novel microwave interferometers and nonlinear Josephson devices to explore the concept of microwave quantum optics. A microwave Fabry-Perot interferometer has been fabricated containing the microwave equivalent of a nonlinear Kerr medium composed of a metamaterial built from a series of 400 Josephson junctions. This device is a phase-sensitive parametric amplifier, capable of near-noiseless amplification of one quadrature of a microwave signal. It can also be used to generate squeezed states of the microwave field, and as much as 85 percent phase-dependent noise suppression has been observed. Using related technology, a low-noise superconducting quantum interference device multiplexer has been developed for the readout of arrays of low-temperature astronomical sensors. These have the potential to read out an array of thousands of detectors in a single channel with gigahertz bandwidth and could have application to novel detector arrays used in nuclear and particle physics, materials science, and astronomy. A group is also studying ultrashort, femtosecond laser pulse interactions with matter. There is significant overlap in work between this group with work on the dynamics of chromophores in proteins. This area is divided into three subareas: interactions between alkali atoms in the gas phase, condensed-matter systems, and spintronics dynamics. In the gas dynamics work, the effort is primarily on the study of the collision dynamics of potassium atoms. Potassium atoms are illuminated with a sequence of two pulses. The first excites the atom to a specific state, and the second has a frequency that is 43
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absorbed only by the excited atoms and thus induces emission, as long as the excited atoms have not lost their energy by collision. The intensity variation as a function of delay provides a measure of collision dynamics. The condensed-phase work involves cross-correlation two-dimensional near-IR spectroscopy of excitons in semiconductors using three femtosecond, near-IR pulses to generate excitons in semiconductors such as gallium arsenide (GaAs). The correlation between absorption and emission was determined in a plot of wave patterns and frequency. This two-dimensional spectroscopy method is capable of determining the correlated oscillation of two groups in a molecule or lattice. Using this rather new two- dimensional spectroscopic method, the structure of large biological molecules and also the electronic properties of semiconductors may be revealed, phenomena impossible to observe when only the normal one-dimensional spectroscopic methods are used. The knowledge obtained on GaAs and other semiconductors by this method may lead to better electronic devices. The spintronics work involves developing the capability of maintaining spin direction for several nanoseconds while being transported micron distances; it is of paramount importance for high-capacity storage and other devices. Researchers at JILA have determined that the spin direction is more stable when confined in the defects of semiconductors and that it loses alignment fast when embedded in a perfect single-crystal environment. It was determined that the place of highest spin stability is located in a crossover magic point somewhere between perfect crystal and defect. This research group confined electrons in quantum wells. An IR light pulse induces spin on the electron while the light of a second polarized pulse reflected by the quantum wells is rotated by the electrons. The magnitude of the rotation gives a measure of the number of electrons with the same spin. With the application of a magnetic field the spins are flipped, and consequently the reflected polarized light oscillates. From these oscillation patterns the spin disorder and spin stability-retardation time are calculated. Spintronic materials, where the spins of electrons can be manipulated, and their properties provide a wealth of basic scientific knowledge in addition to holding the promise of wide use in electronic devices and other areas of industry. In summation, the femtosecond laser work involves elegant research in several cutting-edge scientific areas that also have strong industrial applications. Another group is examining laser spectroscopy kinetics and dynamics of organic molecular and nanoparticle systems. As is true of the ultrashort, femtosecond laser pulse interactions with matter, there is a connection with biological physics, which is described below. This group determines basic chemical-reaction dynamics by means of laser spectroscopy of jet-cooled molecular ions, intermolecular energy surfaces, and low- temperature radical and ion kinetics. In addition, a large effort is made in the study of single-molecule kinetics and microscopy. The systems that have been studied in this area include single quantum dot emission kinetics of cadmium sulfide (CdS) and silver (Ag) nanoparticle lithography. This group has been very active and successful in the design of new equipment and devices and the improvement of standard equipment for utilization in specific experiments. For example, this group has developed, designed, and built a novel, near- field apertureless scanning optical microscope capable of less than 10 nm resolution that 44
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finds use in many areas of physics, chemistry, and biology. This group has sufficient space, equipment, and funding to continue to generate a large quantity of excellent theoretical and experimental research data in diverse, yet very fruitful fields of chemistry, physics, and biology. Biological physics is a growing area of expertise at JILA, and increasing collaborations are developing among the various groups there. It is very encouraging to see the application of techniques coming out of seemingly esoteric areas such as frequency combs to very applied biological problems. Several groups of this division have some biological projects, but that is not the core of their efforts. There are two primary biological physics principal investigators. An outstanding example of the cross-fertilization of technologies developed at JILA for the core mission of the development of measurement science tools is the extension of the Nobel Prize-winning work on optical combs to the application of this technology to biomolecule detection. The latter group invented cavity-enhanced direct frequency comb spectroscopy and has shown that it can perform ultrasensitive detection of unknown chemicals. This group has recently developed a more sensitive, smaller, and less costly fiber laser for this system, which can make ultrasensitive and fast detection of organic molecules using a simple charge-coupled device camera, opening the door for using this technique for medical and homeland security applications. Another group has built on its expertise in single-molecule optical detection to elucidate the conformational kinetics of single ribonucleic acid (RNA) molecules and single deoxyribonucleic acid (DNA) molecules in electrophoresis. This group has used fluorescence energy transfer techniques at the single molecule level to measure distances of 2 to 8 nm between specifically labeled sites on the RNA. This information is crucial to understanding RNA-based enzymes, or ribozymes. In the future, these techniques should make it possible to probe the folding and unfolding of biomolecules in chemically active states. The two main biological physics groups have quite different areas of expertise and interact with different groups within JILA and at the University of Colorado. One group studies single molecules as does the other, but while one uses fluorescence energy transfer techniques the other is a renowned expert in the use of optical tweezers and lately atomic force microscopes in the incredibly precise measurements of subnanometer motions of biological molecules. This work on the molecular motor RecBCD (also known as Exonuclease V, a protein of the Escherichia coli bacterium) is an example of how much there is to learn about how nanomotors move along biological polymers. The questions of the step sizes of these motors, the role of thermal noise, and the basic physics of very small displacement measurements all come into play here. This group is working to improve the resolution of optical tweezers by increasing sensitivity while reducing system noise, which is clearly connected to the JILA tradition of precision measurements. This group has explored the use of a more stable optical design (with improved laser-pointing stability and reduced microscope-stage drift), active sample stabilization, and the introduction of a grid of nanofabricated fiducial marks. Some work overlaps with the work on protein dynamics. The researchers here are interested in performing experiments that provide data for the understanding of the motion and thermodynamics of proteins and the influence that motion has in the chemistry of these and other large biological molecules. This group is also exerting an 45
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effort to measure accurately the motion of proteins, and from this motion data to calculate several of the important thermodynamic values including entropy. This group employs several cutting-edge techniques and equipment such as femtosecond lasers for the study of ultrafast spectroscopy and kinetics of these molecules, and because the laser pulse duration is much shorter than the protein motion, a sequence of such pulses separated by a pre-selected period of time may freeze motion and thus record snapshots of the protein at a given time. When a train of snapshots is put together in a movie format, a histogram of the protein motion will be obtained, from which the thermodynamic values may be calculated as a function of time and process such as coiling. A time-resolved extended x-ray absorption fine structure (EXAFS) system that is aimed at revealing the transient structure of proteins in motion is under consideration for being constructed in collaboration with a high-power-laser JILA group. Another active area is microfluidics where chemical concentrations have been mapped in three- dimensional form inside a microfluidic cell. This has been achieved by employing two- photon fluorescence imaging techniques. Some of these projects are still in the development stage; therefore, this group has not yet had the number of publications and impact that other established research groups of the Quantum Physics Division have had. The research, however, is of good quality, and it is expected that the group will grow in size and generate a high research output of excellent quality. Staffing The Quantum Physics Division consists of nine principal investigators who are NIST employees, in addition to three administrative staff members. Recent years have seen the retirement of two senior JILA Fellows and the hiring of a new principal investigator in the areas of atom cooling, spin squeezing, measurement science; and theory of ultracold atoms and optical lattices, respectively. The supporting research staff, technicians, postdoctoral fellows, and graduate students are of high quality and are sufficient in numbers. Major Equipment, Facilities, Ancillary Support, and Resources The Quantum Physics Division is housed in the JILA building on the campus of the University of Colorado. Laboratory and office space continues to be insufficient to house current programs adequately. The lack of space is detrimental to productivity, creates potentially unsafe working conditions, and could affect the ability to attract and hire top-class scientists in the future. This should be considered a top-priority item for this division. Funding to build additional space is now in the current proposed federal budget; it is important that this funding be provided so that expansion can proceed in a timely fashion. The Quantum Physics Division has access to a very good pool of graduate students by virtue of its association in JILA with the University of Colorado. The excellence of the scientific work at JILA attracts some of the very best of these students, and a large number of JILA graduates have gone on to employment within NIST. JILA also has top-notch mechanical and electronic shops, and this infrastructure is vital to the productivity and success of the division. The JILA Fellows were uniformly enthusiastic about the excellent quality of the mechanical and electronic shops; it is 46
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critical that this valuable resource be maintained at its present level of first-rate quality. ASSESSMENT OF THE DIVISION The major accomplishments of the Quantum Physics Division include the following: • Nobel Prizes in Bose-Einstein condensates and femtosecond laser frequency combs, continued excellence in extending work in ultracold Bose and Fermi atoms and molecules, and applications of optical frequency combs; • State-of-the-art measurement of piconewton forces at subnanometer length scales with optical tweezers; • The development of an innovative strontium optical lattice clock with high signal-to-noise ratio; • Ultrafast two-dimensional Fourier transform spectroscopy that reveals new insight into many-body effects in semiconductor charge and spin dynamics; • Pioneering work in microwave quantum optics; and • Conformational dynamics of single RNA and DNA molecules studied with fluorescence resonant energy transfer. The standard of technical research in the Quantum Physics Division is very high. The research is very productive and explores the frontiers of the areas that are investigated. In areas of its traditional core competencies—that is, laser stabilization, spectroscopy and precision frequency measurement, and trapping of ultracold atoms and molecules—the work of the division ranks among the best. The division has also attracted top applicants in areas identified for expansion such as biophysics and nanoscale physics. It is critical that funding for the new JILA building be put in place and that the plans for design and construction move forward, both to relieve the space crunch at JILA and to maintain the high morale and productivity that are key in making JILA an attractive place to continue to work for highly talented staff members who are actively recruited by other institutions. The increased emphasis on nanotechnology needs to be supported by upgrades to some instrumentation, especially an improved scanning electron microscope with a state-of-the-art field emission source. The impact of the Quantum Physics Division is outstanding, as measured against its stated mission of making important advances at the frontiers of science that enable future precision measurement technology and in producing graduates that form a talented pool of scientists who are now spread through the NIST laboratories and elsewhere. Collaborations between the Quantum Physics Division and the nearby Time and Frequency Division are strong, particularly in the further development of frequency combs and their use in high-precision remote frequency comparisons over an optical fiber connecting the two sites. They are also having significant impact through applications of their technology outside NIST, for example in sensitive high-resolution frequency comb spectroscopy for trace detection and molecular fingerprinting, and the development of technology for multiplexed low-temperature detector arrays for astronomy. 47
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The America COMPETES Act of 2007 has provided support ($100,000) to the area of mesoscopic physics, bridging the gap between quantum and classical physics, and electronic measurement work under the umbrella of quantum information. The funding aided in the development of the pioneering field of microwave quantum optics, in particular the noiseless amplification of microwave signals with innovative phase- sensitive amplifiers. Technical Merit Relative to State of the Art The standard of technical research in the Quantum Physics Division is very high. Without exception, the research reviewed is very productive and explores the frontiers of the areas that are being investigated. The association of the division with the faculty of the University of Colorado through JILA has resulted in a very collaborative and open environment, leading to a free exchange of ideas and a great deal of cross-fertilization between the research groups. The scientific work at JILA has resulted in many awards, including a sharing of Nobel Prizes in physics for Bose-Einstein condensation of cold neutral atoms in 2001 and for the development of the stabilized laser frequency comb in 2005. Work in both of these areas continues to be advanced and remains among the best. Other work that has earned notable awards includes the production of ultracold degenerate Fermi gases and quantum-limited electromechanical measurements. Four of the 30 NIST Fellows are in this division. In the areas of their core competencies—laser stabilization, spectroscopy and precision frequency measurement, atom cooling and trapping, and Bose-Einstein condensation—the work of this division is outstanding and ranks with that of the top research groups in the field. When the research scope has been expanded with new hires in biophysics, mesoscopic physics, and nanotechnology, the strong scientific reputation of JILA has attracted top-notch applicants, and these new areas of research have kept pace with the overall level of excellence exhibited by this division as a whole. Adequacy of Infrastructure As noted above, it is critical that funding for the new JILA building be provided and that the plans for design and construction move forward, both to relieve the space crunch at JILA and to maintain the high morale and productivity of the highly talented JILA staff members. The division’s very impressive work could have broad applications on astrophysics detection capabilities. JILA would benefit from the hiring of an astrophysicist/instrumentalist to complement the division’s ongoing work. There is a strong emphasis in many of the projects on nanotechnology—for which JILA needs in-house a very high quality scanning electron microscope for the imaging of the nanostructures and to do nanolithography. The present SEM is not state of the art, and a newer-generation, higher-resolution SEM is necessary to support this growing area. Achievement of Objectives and Impact The work of the Quantum Physics Division is outstanding, as measured against 48
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the strategy that it has put forth. The division has been very successful at educating new generations of measurement scientists who have gone on to find employment within NIST as well as elsewhere and thus have a large impact on the field of precision measurement and metrology. The division’s scientific contributions in its traditionally core areas of emphasis continue to have significant impact in the overall physics community. This division has successfully expanded its expertise into new areas identified as being of importance to the future NIST mission. These include nanotechnology, mesoscopic physics, and biophysics. This division is the NIST part of JILA, a joint institute at the University of Colorado made up of principal investigators (called JILA Fellows) who are NIST staff members or university faculty and who manage groups of graduate students and postdoctoral fellows in an academic environment. They serve a key role in educating graduate students, thus creating a pool of talent in areas of measurement science of importance to NIST. Their stated mission is to do new science that will form the foundation for future advancements in the overall NIST mission, including developing new ways of controlling light, atoms, and molecules and their interactions, measuring nanoscale processes, and manipulating ultrashort light pulses. The relationship between NIST and the University of Colorado has a history of producing groundbreaking and excellent, high-impact scientific research that is being successfully continued today, resulting in two Nobel Prizes in recent history. Previous difficulties related to the NIST-university relationship appear to have been successfully solved or mitigated; the current relationship appears to be running smoothly. NIST plays a vital role in fostering a NIST-university collaborative environment with a free exchange of ideas; this panel had no opportunity to review or interview research groups in the university half of JILA, and this report reflects a NIST-centric view of this joint institute. CONCLUSIONS The research performed by the scientists in the Quantum Physics Division is outstanding. The output is significant in numbers and excellent in quality, and the research topics selected are at the forefront of related science and technology. The scientific equipment and technical support, such as electronic and machine shops, are sufficient for the high performance that has been demonstrated and is expected from this division. The supporting research staff, technicians, postdoctoral fellows, and graduate students are of high quality and sufficient in numbers. The purchase of a modern, high-resolution scanning electron microscope is needed to support the increasing efforts in nanotechnology. The interaction with the astrophysics group at the University of Colorado is of some concern; it should be enhanced, and the number of researchers of this group should be increased. Space, which is of concern, is expected to be increased by the construction of the new building, a project that is essential to the continued productivity of the division. The work of the division is of high caliber. There is a high and collegial spirit among the scientists and staff; productivity is high, and the quality of the basic science and its application to industry are outstanding. The Quantum Physics Division and JILA are national assets, and every effort 49
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should be exerted to sustain and, if possible, increase their support and funding. The most critical items that need to be addressed are funding for the JILA building expansion and funding for improvements in instrumentation relevant to nanotechnology research. 50