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Precision Optical Frequency Metrology

Precision optical frequency metrology relies on new laser technologies developed by JILA scientists. These technologies include one of the world’s most stable lasers, which is used with the strontium (Sr)-lattice optical atomic clocks and quantum simulator in the Ye labs, a super-radiant laser, (3) a mid-infrared (IR) frequency comb developed for molecular fingerprinting and control, and ongoing efforts to extend the range of precision optical frequency combs into new regions of the electromagnetic spectrum.

 

Optical Atomic Clock

The Jun Ye group has worked for more than 10 years on the design, implementation, and enhancement of the what is today the world’s most accurate, precise, and stable optical atomic clock. The strontium (Sr) lattice clock's precision timekeeping mechanism is based on a narrow electronic transition in Sr atoms held inside an optical lattice.

The new clock’s accuracy is a measure of how well it keeps time against a mythical perfect standard. Its precision reflects how close repeated time measurements are to one another. Its stability is a measure of how long it has to operate to achieve its optimal accuracy.

The Sr-lattice clock will neither gain nor lose a second in more than 200 million years of operation. It is approximately 30 times more accurate and 300 times more stable than the cesium-based atomic clocks currently used as time standards in national laboratories around the world. Because the Sr-lattice clock’s optical transition has a much higher frequency than the microwave clock transition in Cs, it has had the potential from its inception to be more accurate than the NIST-F1. It is now  the most accurate clock in the world.

The initial design for a Sr-based atomic clock used an electronic transition that was not ideal for a next-generation optical atomic clock. Thus, the first major step forward in the clock experiment was the identification of a much more stable electronic transition in Sr. This achievement soon gave rise to the design of a new clock system employing two lasers (and three atomic levels) to create a unique energy level transition that otherwise would not occur in nature. The new transition was very narrow and stable, an ideal combination for an atomic clock. The improved clock transition made it necessary to create a better optical lattice to confine confine the Sr atoms. The new lattice was created with a “magic” laser wavelength that had no net effect on the critical atomic transition at the heart of the device.

The final hurdle to assembling the first Sr-lattice clock in 2005 was the development of a superstable clock laser. With the new laser, the accuracy of the new Sr-lattice optical atomic clock was just shy of that of the NIST-F1 Cs fountain clock within its first year of operation. After one additional year of performance enhancement, the Sr-lattice clock was more accurate than the NIST-F1 Cs fountain clock.
 
As part of ongoing efforts to enhance the performance of the Sr-lattice clock, the Ye group discovered in 2008–2009 that colliding fermions were causing frequency shifts in the clock. Until this unexpected discovery, most physicists assumed that there would be no such collisions if Sr atoms were cooled to sufficiently low temperatures. This assumption was grounded in the laws of quantum mechanics, which mandate that identical fermions cannot get close enough to one another to actually collide.

After months of work to solve the mystery of the colliding fermions, the researchers discovered that their laser-based precision measurement technique was interacting with the optical lattice confining the Sr atoms to cause the measured frequency shifts. During measurements when a laser beam was focused on the (slightly curved) Sr atom-filled lattice, the atoms in the lattice transitioned to their excited states at slightly different rates. And, because fermions in different superpositions of their ground and excited states are no longer identical, they can (and do) collide. The Ye group devised strategies to reduce, though not entirely eliminate, the number of atom-atom collisions and the resulting frequency shifts.
 
Then in 2011, with help from the Ana Maria Rey theory group, the Ye group solved the problem of colliding fermions. The researchers created a new trap that held more atoms, thereby increasing the interaction strength enough to suppress most of the frequency shifts. In the process, the group reduced the inaccuracy of the clock due to atomic collisions by 50-fold. This achievement eliminated the need for compromise between precision and accuracy in the on-going development of the Sr-lattice clock.

Next, the group developed an even faster, more stable clock laser that further increased the clock’s accuracy and precision. The new ultrastable laser not only exhibits the world’s best performance for a clock laser, but also significantly reduces noise in the clock system. With the new laser, the dominant source of noise in the clock was suddenly the quantum mechanical behavior of the atoms in the clock!

This noise, called quantum projection noise, comes from not being able to measure the exact quantum states of the atoms at the heart of the clock. A certain amount of fluctuation in quantum measurements is intrinsic to any system governed by the laws of quantum mechanics. The stability of the Sr-lattice clock is now within a factor of two of this quantum limit. This improved stability is facilitating the group’s continuing efforts to improve the performance of the Sr-lattice clock. 

The new ability to probe quantum fluctuations in the Sr-lattice clock has opened up a whole new field of research: quantum simulation.  The clock’s new ultrastable laser is already making it possible to use the clock as quantum simulator to probe the quantum behavior of its Sr atoms. And, since work to further increase the stability of the clock laser—most recently with the use of a single-crystal silicon optical cavity—is ongoing, rapid advances in quantum simulation are likely to follow.

The Ye group’s wildly successful implementation of the Sr lattice clock has motivated theorist Ana Maria Rey to explore additional applications of Sr and other alkaline earth atoms in optical lattices.

A Stable Clock Laser

For nearly 50 years, JILA has been renowned for its development of stable lasers, primarily by Nobel Laureate John L. (Jan) Hall. Recently, the Jun Ye group has become a strong player in this field, with Hall’s blessing. In 2009, the group began a multiyear project to build the world’s most stable laser to use with the Sr-lattice optical atomic clock. Hall donated the space for the new laser, and the group enlisted help from stable-laser specialist Lisheng Chen and Todd Asnicar, head of JILA’s instrument shop.

Innovations for the new laser include (1) an actively stabilized optics table sitting atop a regular table with short legs, (2) an air purification system, (3) two nested vacuum chambers to lower the pressure to at least 10-8 Torr, (4) isolation and insulation of the laser to cool it as low as 200 nK, and (5) an optics cavity that uses a spacer made of ultraslow expansion glass and mirror substrates made from fused-silica glass. The cavity is located inside a copper box, which provides a passive temperature shield.

The First Superradiant Laser

In 2012, the James Thompson group built the world’s first superradiant laser based on the quantum synchronization of a million rubidium atoms (87Rb). The laser emits “light” at microwave frequencies and has, on average, less than one photon inside of it at any given moment. Inside the nearly lightless laser, chains of atoms oscillate together between their ground and excited states. Because at least one photon communicates between crystals of light holding the stacks of atoms, all the atoms in the laser oscillate together and form a “super spring” that emits coherent microwave light.

Thompson is currently building a superradiant laser based on the quantum synchronization of strontium (Sr) atoms.

 

Mid-IR Frequency Comb

In 2009, the Jun Ye group developed a frequency comb that produces a wide range of coherent IR light with adjustable wavelengths between 3 and 5 µm. Many fundamental molecular vibration frequencies, such as those of the molecules found in human breath, are located in the mid IR. Consequently this innovative frequency comb was critical to the development of molecular fingerprinting, a comb-based spectroscopy.
 
The production of a mid-IR comb requires an optical parametric oscillator, or OPO. In this device, photons from a near-IR frequency comb are down-converted to pairs of lower-energy photons that maintain the comb structure. The group has now combined the OPO-based laser with Fourier-transform infrared spectroscopy (FTIR) to come up with a system that works much faster than conventional FTIR spectrometers. The new technique can identify and measure the concentrations of many molecules in less than one minute—as compared to hours.  The speed is crucial for many practical applications, including molecular fingerprinting.
 
The group is now developing a frequency comb to reach longer wavelengths of up to 10 µm.

Optical Frequency Comb Extensions

In addition to extending the optical frequency comb technology into the mid-IR, as described above, in 2012 the Jun Ye group created a frequency comb in the extreme ultraviolet (XUV) region of the electromagnetic spectrum. The wavelengths of the new XUV comb lines, or colors, range from about 120 nm down to about 50 nm. These colors are detectable only with special laboratory instruments. In 2014, the group built a second XUV frequency comb and figured out how to "beat" them together to produce measurements that can be read by ordinary laboratory electronics.

The two XUV combs are being used as rulers to precisely measure the electronic transitions between different quantum states in atoms and molecules. They will soon make it possible to explore the internal quantum states of different atoms and molecules. XUV combs one day lead allow researchers to change nuclear states and design new clocks based on the behavior of atomic nuclei.