to develop precision
measurement tools and
applications.
INTENDED OUTCOME AND BACKGROUND
Measurement science is used for many industrial and scientific purposes and is central to many NIST activities. Applications
range from providing the length scale for mechanical measurements to providing a direct connection between optical and radio frequencies. The Quantum Physics Division continues a leadership role in developing new precision-measurement techniques and applications.
The highly stabilized laser is the workhorse in precision measurement. Traditionally, this meant continuous wave (CW) lasers that generate light with a very precise and stable single frequency. Recently, the Division became an international leader in adapting
CW techniques to the stabilization of mode-locked lasers, which generate a broad comb composed of sharp lines, each of which has a very precise and stable frequency.
Such a stable frequency comb not only simplifies measurement at any given optical frequency, but also facilitates establishing a direct connection between optical and radio frequencies. This connection, in turn, enables
optical atomic clocks and absolute optical
frequency metrology. The importance of femtosecond (10-15 s) comb techniques, and the Division’s contributions to them, was recognized by the 2005 Nobel Prize in Physics, shared by long-time Division member
and JILA pioneer John L. Hall.
Applying precision optical spectroscopic techniques to help improve our understanding
of molecular interactions is also proving fruitful. They can also be used for addressing
fundamental physical problems, such as determining the electron electric dipole moment.
These developments continue JILA’s tradition
of developing laser stabilization and associated precision measurement methods used today by NIST, the international standards
community, and leading universities worldwide. Our strong position in this new field assures NIST’s continued leadership in standards and measurement.
Accomplishments
Electron Electric Dipole Measurement
Figure 1. The Quantum Physics Division’s newest hire, James Thompson.
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The so-called Standard Model of particle physics is enormously successful in predicting
the behavior of the zoo of subatomic particles that make up the world around us. However, it can be shown mathematically
that this model cannot explain what happens to particles that collide at very high energies, nor can it describe the high-temperature conditions that must have been present at the very earliest moments
after the big bang. It is not possible to build an accelerator big enough to reconstruct those conditions, but theorists point out that the speculative models they construct to explain the highest energy physics also make predictions about the properties of everyday, room temperature particles, predictions that can be tested in a precision measurement lab.
One such prediction is that the electron should have a nonzero electric dipole moment (EDM). The assertion is that the electron possesses a tiny asymmetry such that its center-of-mass and its center-ofcharge
will be offset from one another. If the current experimental limit on this off set could be improved by a factor of 100, a large class of proposed extensions to the Standard Model could be either disproved or else provided with their first experimental
support.
The experiment is daunting: the current experimental limit says the offset is smaller than 10-14 femtometers. Put another way, if you were to scale the electron up to the size of the earth, its asymmetry would be smaller than a wavelength of light.
To go another two orders of magnitude further, Division scientists are making use of cold, trapped molecular ions to serve as high-electric-field laboratories for studying electrons. They are probing the electron-in-a-molecule system with a combination of frequency-comb-enabled optical spectroscopy
and atomic-clock-driven radio-frequency spectroscopy.
Optical Atomic Clocks
An optical atomic clock uses an optical frequency
transition as its quantum reference, giving it better frequency stability than a clock using the standard microwave-frequency transition. An optical clock produces a phase-coherent RF signal from the optical standard by using an optical frequency comb that is precisely phase stabilized to the optical standard.
An optical atomic clock based on ultracold strontium atoms confined in an optical lattice has demonstrated a world-record spectral resolution, reaching a resonance quality factor of 2.4 × 1014. The fractional frequency instability has already reached 3 fHz/Hz at 1 s. We have characterized the systematic uncertainty in fractional frequency to 0.15 fHz/Hz, which has surpassed
the current best evaluations of the NIST cesium primary fountain standard. Future progress on this atomic clock is expected to be as fruitful as in the past. We expect to push this system to an accuracy
level reaching 0.01 fHz/Hz and instability lower than 1 fHz/Hz at 1 s.
The stability of such a clock can be evaluated
only through comparison to another high-stability optical clock. Ideally, comparison
to a third clock is needed to determine
the performance of all of the clocks involved. The need for comparison has hindered atomic clock development because the timing/frequency signals are degraded by transmission. Recent efforts at JILA and NIST have demonstrated coherent optical phase transfer over a 32 km optical fiber with fractional frequency instability of
0.01 fHz/Hz at 1 s.
This work constitutes a major advance in the ability to distribute extremely precise and accurate frequency and timing signals over long distances through fiber networks. In fact, we have recently taken advantage of this capability to remotely compare the Sr lattice clock at JILA against the Ca optical clock in the Time and Frequency Division at NIST. The short-term stability of the Ca clock permitted the evaluation of the overall systematic uncertainty of the Sr clock at the level of 0.15 fHz/Hz.
These fiber stabilization results are a twoorders-
of-magnitude improvement in the stability of frequency distribution, and demonstrate the lowest level of timing jitter
and phase noise for a tens of kilometer long-distance timing distribution system. This technology will be extremely valuable for particle accelerator facilities, synchronized
radio or optical telescope arrays, remote calibration of length standards, and long-distance interferometry.
Fundamental Limits to Femtosecond Combs
Femtosecond optical frequency combs generated
by mode-locked lasers have revolutionized
optical frequency metrology and enabled optical atomic clocks. Measurements
have shown that the intrinsic stability
of femtosecond combs is remarkable, better than 10-18 Hz/Hz.
At some level, quantum fluctuations will set a lower limit for the stability, in a manner
analogous to the well-known Schawlow-
Townes linewidth of a CW laser. Just as for the Schawlow-Townes linewidth, some photons spontaneously emitted by the gain medium will be incorporated into the lasing mode, but they will have the wrong phase, timing, and wavelength. A mode-locked laser is intrinsically a nonlinear
system, thus the analysis is much more complex than for a CW laser, although there are similarities to noise processes in an amplified fiber optic telecommunications
system.
Accurate calculation of the effect of quantum
fluctuations on a mode-locked laser requires a good understanding of the laser’s dynamic response to a small perturbation.
We have experimentally characterized
the dynamics by recording how the pulse energy, center frequency, phase, and timing respond to small changes in pump power. The measurements showed that gain dynamics, which had been neglected previously, must also be included. The full characterization is now being used to predict the phase fluctuations driven by spontaneously emitted photons.
First strategic focus |
Second strategic focus |
Third strategic focus |
Fourth strategic focus
"Technical Activities 2005-2007" - Table of Contents |
