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Topic Name: Optical atomic clocks ; Frequency-stabilized lasers and precision optical frequency metrology

Category: Organic electronics

Research persons: John Hall, Jun Ye, and Steve Cundiff

Location: JILA,University of Colorado,440 UCB,Boulder, CO 80309-0440,303) 492-7789,FAX: 303-492-5235, United States

Details

JILA a world leader in the development of frequency-stabilized lasers and precision optical frequency metrology. They use ultrafast lasers, fiber optics, and optical cavities to create highly correlated light pulses. The pulses have a spectrum that consists of an evenly spaced frequency comb of thousands of sharp spectral lines. Each laser pulse consists of a unique carrier (wave)-envelope phase that can be described mathematically in a way that allows scientists to know the frequency of every single comb line with one or two measurements. The researchers are investigating exactly what femtosecond combs look like, the optical processes that create them, and how the combs evolve through a succession of pulses. Research on frequency combs has made it possible to accomplish precise optical frequency measurements in a matter of minutes. The combs also provide a direct bi-directional link between optical and microwave frequencies.

The optical-microwave link is opening up a new world of precision-measurement possibilities, including the ability (1) to synthesize combs of optical reference frequencies; (2) to create a comb synthesizer from reference lines derived from cold trapped ions, atoms, or molecules; (3) to use an atomic- or molecular-based comb synthesizer as the clockwork for the next generation of atomic clocks; (4) to develop counters that will measure optical frequencies as easily as today's devices measure radio frequencies; and (5) to develop coherent spectroscopy.

Optical atomic clocks are one of the most powerful new applications of optical frequency combs. Jun Ye and his group are collaborating with NIST-Boulder's Time and Frequency Division to explore and evaluate some promising new concepts. Ye's group is developing a clock based on transitions in ultracold strontium (Sr) atoms. Their clock holds Sr atoms in an optical lattice with "magic light," a carefully chosen wavelength of light that has no net effect on the critical atomic transition at the heart of the optical atomic clock. Thus far, the performance of this clock compares favorably with a NIST design based on a single mercury (Hg+) ion. Within five years, both clocks could surpass the performance of the NIST-F1 cesium fountain atomic clock (the nation's current primary time and frequency standard) by a hundredfold. Ye also studies the optical transitions of strontium atoms and develops increasingly stable femtosecond lasers, both of which will be part of future precision clocks.

Cundiff and Ye investigate unique ways to communicate time via ultrafast lasers and fiber-optic networks. Their interest in time communication has grown out of optical atomic clock research to measure time in intervals that are thousands of times smaller than is possible with today's microwave cesium clocks. Recently, researchers from both groups figured out how to send a frequency reference linked to an optical atomic clock in Ye's lab on a round trip over a fiber network that links JILA with NIST. When the signal completed its roundtrip, the researchers found it to be at least 10 times more stable than what had ever been achieved for transmitting clock signals over similar distances. The secret to their success was a noise cancellation device that removed fiber noise due to road sounds and temperature changes. Now they want to improve the stability of signal transmission by at least another tenfold to ensure that time can travel as accurately as Ye's optical atomic clock can measure it.

Jun Ye is investigating the precise control of femtosecond combs as part of his research on coherent spectroscopy. He wants to probe the interactions of ultracold atoms and molecules with ultrafast, precisely controlled lasers. Coherent spectroscopic techniques are expected to offer society tangible benefits such as the ability to detect molecules of explosives in luggage or clothing at airports and other high-security installations. In the laboratory, quantum control can now be achieved at unprecedented spectral resolution.

His group recently developed a powerful new comb-based method for identifying and investigating atoms and molecules. This method allows the researchers to study molecular vibrations, rotations, and collisions as well as temperature changes and chemical reactions. It can detect trace amounts of chemicals with exquisite sensitivity because it can precisely identify their characteristic patterns of laser light absorption, or molecular fingerprints. What sets this technology apart is its broad spectral bandwidth, which makes it possible to detect millions of parallel light channels simultaneously in real time.

The first step in molecular fingerprinting is the creation of a wide bandwidth optical frequency comb with an ultrafast laser. Second, the comb is coupled to an optical cavity containing a sample of one or more molecules. Third, chemicals inside the cavity absorb photons of particular frequencies, which are determined by each chemical's electronic, vibrational, and rotational structure. Finally, light exiting the cavity is analyzed to see not only what frequencies of light are missing photons, but also how many photons of a particular frequency have disappeared. Since the pattern of photon absorption is unique for every atom or molecule, the analysis reveals the identity of sample constituents.

Because precision femtosecond lasers are crucial for precision measurement, Hall, Ye, and Cundiff continue to collaborate on their development. In addition to their goal of developing the best possible atomic clock oscillators, they want to identify the devices that provide the best stability with the least complexity. In other words, they are working to create new technologies that offer the greatest benefit for the most reasonable cost. One exciting application of ultrafast precision lasers would be for their use in the next generation of GRACE satellites that measure global gravity variations, particularly those associated with ocean heights and climate change.

About researcher's:

Fellow of JILA
Lecturer, Department of Physics
jhall@jila.colorado.edu (303) 492-7843

Research Areas: Atomic & Molecular Physics, Optical Physics, Precision Measurement

Fellow of JILA
Associate Professor Adjoint, Department of Physics
ye@jila.colorado.edu (303) 735-3171

Research Areas: Atomic & Molecular Physics, Nanoscience, Optical Physics, Precision Measurement

Fellow of JILA
Associate Professor Adjoint, Departments of Physics and Electrical and Computer Engineering
cundiffs@jila.colorado.edu (303) 492-7858

Research Areas: Atomic & Molecular Physics, Nanoscience, Optical Physics, Precision Measurement

Funding:

JILA is jointly operated by the University of Colorado (CU) and the National Institute of Standards and Technology (NIST).

In picture:

1.Jun Ye

2.Optical atomic clocks

3.John Hall

4.Steve Cundiff