NIST Tech Beat - October 14, 2008

First Tunable, ‘Noiseless’ Amplifier May Boost Quantum Computing, Communications

Researchers at the National Institute of Standards and Technology (NIST) and JILA, a joint institute of NIST and the University of Colorado (CU) at Boulder, have made the first tunable “noiseless” amplifier. By significantly reducing the uncertainty in delicate measurements of microwave signals, the new amplifier could boost the speed and precision of quantum computing and communications systems.

Conventional amplifiers add unwanted “noise,” or random fluctuations, when they measure and boost electromagnetic signals. Amplifiers that theoretically add no noise have been demonstrated before, but the JILA/NIST technology, described in an Oct. 5, 2008, advance online publication of Nature Physics,* offers better performance and is the first to be tunable, operating between 4 and 8 gigahertz, according to JILA group leader Konrad Lehnert. It is also the first amplifier of any type ever to boost signals sufficiently to overcome noise generated by the next amplifier in a series along a signal path, Lehnert says, a valuable feature for building practical systems.

Noisy amplifiers force researchers to make repeated measurements of, for example, the delicate quantum states of microwave fields—that is, the shape of the waves as measured in amplitude (or power) and phase (or point in time when each wave begins). The rules of quantum mechanics say that the noise in amplitude and phase can’t both be zero, but the JILA/NIST amplifier exploits a loophole stipulating that if you measure and amplify only one of these parameters—amplitude, in this case—then the amplifier is theoretically capable of adding no noise. In reality, the JILA/NIST amplifier adds about half the noise that would be expected from measuring both amplitude and phase.

The JILA/NIST amplifier could enable faster, more precise measurements in certain types of quantum computers—which, if they can be built, could solve some problems considered intractable today—or quantum communications systems providing “unbreakable” encryption. It also offers the related and useful capability to “squeeze” microwave fields, trading reduced noise in the signal phase for increased noise in the signal amplitude. By combining two squeezed entities, scientists can “entangle” them, linking their properties in predictable ways that are useful in quantum computing and communications. Entanglement of microwave signals, as opposed to optical signals, offer some practical advantages in computing and communication such as relatively simple equipment requirements, Lehnert says.

The new amplifier is a 5-millimeter-long niobium cavity lined with 480 magnetic sensors called SQUIDs (superconducting quantum interference devices). The line of SQUIDs acts like a “metamaterial,” a structure not found in nature that has strange effects on electromagnetic energy. Microwaves ricochet back and forth inside the cavity like a skateboarder on a ramp. Scientists tune the wave velocity by manipulating the magnetic fields in the SQUIDs and the intensity of the microwaves. An injection of an intense pump tone at a particular frequency, like a skateboarder jumping at particular times to boost speed and height on a ramp, causes the microwave power to oscillate at twice the pump frequency. Only the portion of the signal which is synchronous with the pump is amplified.

Funding for the research was provided by NIST, the National Science Foundation, and a NIST-CU seed grant.

* M.A. Castellanos-Beltran, K.D. Irwin, G.C. Hilton, L.R. Vale and K.W. Lehnert. Amplification and squeezing of quantum noise with a tunable Josephson metamaterial. Nature Physics, published online: 5 Oct. 5 2008; doi:10.1038/nphys1090.

Using highly uniform samples of carbon nanotubes—sorted by centrifuge for length—materials scientists at the National Institute of Standards and Technology (NIST) have made some of the most precise measurements yet of the concentrations at which delicate mats of nanotubes become transparent, conducting sheets. Their recent experiments* point up the importance of using relatively homogeneous—not overly short, but uniform in length— nanotubes for making high performance conducting films.

Among their other qualities, single-wall carbon nanotubes (SWCNTs) have attracted much attention as tiny electrical conductors. Relatively small concentrations of nanotubes can change a normally insulating polymer film to a transparent electrical conductor. Potential applications range from transparent electrical shielding materials to futuristic flexible video displays, thin-film chemical sensors and other foldable electronics. One key design parameter for conductive films is the so-called “percolation threshold”—essentially the concentration at which random two- or three-dimensional networks of nanotubes first become electrically conducting.

To test theories on how both the conductance and optical properties of such nanotube-infused films depend on the length of the tubes, the NIST team made samples of “buckypaper” by mixing nanotubes in water and draining the water away through nanoscale filters to leave behind a delicate nanotube mat. The highly refined, length-sorted nanotube samples were produced by an efficient technique developed earlier by the NIST group (see “Spin Control: New Technique Sorts Nanotubes by Length”).

The NIST measurements validated one theory: buckypaper made of length-sorted carbon nanotubes closely follows the percolation theory for ideal two-dimensional sheets, with concentration threshold for conductivity getting lower as the tubes get longer. A sheet of 820 nanometer long nanotubes becomes conducting at an amazingly low 18 nanograms per square centimeter, the lowest yet reported. Interestingly, batches of short nanotubes or mixed-length batches form more three-dimensional networks that perform noticeably worse. On the other hand, predictions that optically the sheets would behave like thin metallic films turn out not to be the case. Optical properties are better predicted by the same general percolation theory, say the NIST researchers, which will provide a convenient theoretical framework for designing and engineering nanotech applications with these materials.

* D. Simien, J.A. Fagan, W. Luo, J.F. Douglas, K. Migler and J. Obrzut. Influence of nanotube length on the optical and conductivity properties of thin single-wall carbon nanotube networks. ACS Nano, Vol.. 2 , No. 9, 1879-1884.

Novel nanoparticles being tested at the National Institute of Standards and Technology (NIST) have researchers seeing stars. In a recent paper,* NIST scientists used surface-enhanced Raman spectroscopy (SERS) to demonstrate that gold nanostars exhibit optical qualities that make them superior for chemical and biological sensing and imaging. These uniquely shaped nanoparticles may one day be used in a range of applications from disease diagnostics to contraband identification.

SERS relies on metallic nanoparticles, most commonly gold and silver, to amplify signals from molecules present in only trace quantities. For these types of experiments, scientists shine laser light on an aqueous solution containing the nanoparticles and the molecule of interest and monitor the scattered light. The detailed characteristics of both the molecule and the nanoparticle affect the strength of scattered light, which contains an identifying fingerprint for the molecule known as its vibrational signature. With nanoparticles amplifying the signature, it is possible to detect a very low concentration of molecules in a solution.

The NIST team tested the optical properties of the nanostars using two target molecules, 2-mercaptopyridine and crystal violet. These molecules were selected because of their structural similarity to biological molecules and their large number of delocalized electrons, a characteristic that lends itself to SERS. NIST researchers found that the Raman signal of 2-mercaptopyridine was 100,000 stronger when nanostars were present in the solution. The stars were also shown to be particularly capable of enhancing the signature of crystal violet, delivering a signal about 10 times stronger than the previous winner, nanorods. Both the nanostars and the nanorods outperformed the nanospheres commonly used for Raman enhancement.

NIST physicist Angela Hight Walker and her team perfected the process for making gold nanostars, building them from the bottom-up using surface alterations to manipulate their growth and control their shape. Once suspended in a solution, the team guided the nanostars to gather together to form multiple “hot spots,” where the enhancement is dramatically larger than for a single nanostar.

According to Hight Walker, the fact that they can now be created en masse and have desirable optical properties should prompt researchers to examine their possible applications, perhaps eventually making them the stars of the nanoworld.

National Institute of Standards and Technology (NIST) scientists have released an updated technical specification for Personal Identity Verification (PIV) cards that are being phased in by federal departments and agencies for use by their employees and contractors. The technical specification updates the specifications issued in 2006 and will assist federal departments and agencies that are implementing the PIV system and the vendors and system integrators that supply PIV system components and services.

All federal government employees and contractors will soon be required to use PIV cards to access federal facilities and information systems, according to Homeland Security Presidential Directive 12. NIST is responsible for providing the technical specification for the PIV cards—smart cards that securely store data such as fingerprint templates and a facial image that are used to verify the cardholder’s identity.

NIST Special Publication 800-73-2, Interfaces for Personal Identity Verification, details what data objects are stored on the PIV card, how they are encoded and how to retrieve and use the data objects from the PIV card. SP 800-73-2 incorporates errata from the previous version, SP 800-73-1, and aligns the card’s cryptographic capabilities with the cryptographic specifications issued in SP 800-78-1, Cryptographic Algorithms and Key Sizes for Personal Identity Verification, published in 2007.

For convenience, SP 800-73-2 is being issued in four parts to align with different segments of the industry. These are:

A new report describes tests carried out by the National Institute of Standards and Technology (NIST) of the wireless environment in automotive manufacturing facilities. The measurements, carried out through a joint collaboration between NIST and USCAR, indicate that these facilities are highly reflective, reverberant environments that can complicate reliable performance of wireless technology. The measured results provide key parameters that describe the wireless propagation environment, which will be useful for assessing current and future deployment of wireless technology in industrial manufacturing, for standards development and for qualifying the performance of wireless equipment used in highly reflective environments. Using the measured data, NIST is currently developing laboratory-based methods for qualifying wireless equipment for use in highly reflective environments.

Katharine B. Gebbie, director of the Physics Laboratory at the National Institute of Standards and Technology (NIST), was inducted as a member of the 228th Class of Fellows of the American Academy of Arts and Sciences on Oct. 11, 2008, at a ceremony in Cambridge, Mass. As a member of the 2008 class, Gebbie is among 212 scholars, scientists, artists, civic, corporate and philanthropic leaders representing 20 states and 15 countries and ranging in age from 37 to 86.

Gebbie graduated from Bryn Mawr College with a B.A. in physics. She subsequently earned a B.S. in astronomy and a Ph.D. in physics from University College London. She joined NIST in 1968 as a physicist in the Quantum Physics Division of JILA, a cooperative enterprise between NIST and the University of Colorado in Boulder. Before being appointed director of the newly formed Physics Laboratory in 1991, she served as chief of the Quantum Physics Division and acting director of the Center for Atomic Molecular and Optical Physics. Gebbie is a fellow of the American Physical Society, a fellow of the American Association for the Advancement of Science and a member of several professional societies including Sigma Xi and American Women in Science.

At the Academy, Gebbie will serve in the Educational, Scientific, Cultural & Philanthropic Administration.

Founded in 1780, the American Academy of Arts and Sciences is an independent research center that conducts multidisciplinary studies of complex and emerging problems. Current Academy research focuses on science and global security, social policy, the humanities and culture, and education.

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Editor: Michael Baum

Date created: October 14, 2008
Date updated: October 15, 2008
Contact: inquiries@nist.gov