Atomic Clock With a 3D Optical Lattice

A quantum dege­nerate Fermi gas of Sr atoms confined in a three-dimen­sional optical lat­tice demon­strates measure­ment precision at the 19th decimal point for atomic clocks. (Source: Ye et al., S. Burrows, JILA)

JILA physicists have created an entirely new design for an atomic clock, in which strontium atoms are packed into a tiny three-dimen­sional cube at 1,000 times the density of previous one-dimen­sional clocks. In doing so, they are the first to harness the ultra-con­trolled behavior of a quantum gas to make a practical measure­ment device. With so many atoms completely immo­bilized in place, JILA’s cubic quantum gas clock sets a record for the quality factor and the resulting measure­ment pre­cision. A large quality factor trans­lates into a high level of synchro­nization between the atoms and the lasers used to probe them, and makes the clock’s “ticks” pure and stable for an unusually long time, thus achieving higher precision.

Until now, each of the thousands of “ticking” atoms in advanced clocks behave and are measured largely indepen­dently. In contrast, the new cubic quantum gas clock uses a globally inter­acting collec­tion of atoms to constrain collisions and improve measure­ments. The new approach promises to usher in an era of drama­tically improved measure­ments and tech­nologies across many areas based on controlled quantum systems. “We are entering a really exciting time when we can quantum engineer a state of matter for a particular measure­ment purpose,” said physicist Jun Ye of the National Institute of Standards and Tech­nology NIST. Ye works at JILA, which is jointly operated by NIST and the Uni­versity of Colorado Boulder.

The clock’s center­piece is a dege­nerate Fermi gas, first created in 1999 by Ye’s late colleague Deborah Jin. All prior atomic clocks have used thermal gases. The use of a quantum gas enables all of the atoms’ properties to be quantized, or restricted to specific values, for the first time. “The most impor­tant potential of the 3D quantum gas clock is the ability to scale up the atom numbers, which will lead to a huge gain in stability,” Ye said. “Also, we could reach the ideal condi­tion of running the clock with its full coherence time, which refers to how long a series of ticks can remain stable. The ability to scale up both the atom number and coherence time will make this new-genera­tion clock quali­tatively different from the previous gene­ration.”

Until now, atomic clocks have treated each atom as a separate quantum particle, and inter­actions among the atoms posed measurement problems. But an engi­neered and controlled collec­tion, a quantum many-body system, arranges all its atoms in a parti­cular pattern, or correlation, to create the lowest overall energy state. The atoms then avoid each other, regardless of how many atoms are added to the clock. The gas of atoms effectively turns itself into an insulator, which blocks inter­actions between consti­tuents.

The result is an atomic clock that can out­perform all pre­decessors. For example, stability can be thought of as how precisely the duration of each tick matches every other tick, which is directly linked to the clock’s measure­ment precision. Compared with Ye’s previous 1D clocks, the new 3D quantum gas clock can reach the same level of precision more than 20 times faster due to the large number of atoms and longer coherence times. The experimental data show the 3D quantum gas clock achieved a precision of just 3.5 parts error in 10 quin­tillion in about 2 hours, making it the first atomic clock to ever reach that threshold. “This represents a signi­ficant improve­ment over any previous demon­strations,” Ye said.

The older, 1D version of the JILA clock was, until now, the world’s most precise clock. This clock holds strontium atoms in a linear array of pancake-shaped traps formed by laser beams, an optical lattice. The new 3D quantum gas clock uses addi­tional lasers to trap atoms along three axes so that the atoms are held in a cubic arrange­ment. This clock can maintain stable ticks for nearly 10 seconds with 10,000 strontium atoms trapped at a density above 10 trillion atoms per cubic centi­meter. In the future, the clock may be able to probe millions of atoms for more than 100 seconds at a time.

Optical lattice clocks, despite their high levels of performance in 1D, have to deal with a tradeoff. Clock stabi­lity could be improved further by increasing the number of atoms, but a higher density of atoms also encourages colli­sions, shifting the fre­quencies at which the atoms tick and reducing clock accuracy. Coherence times are also limited by colli­sions. This is where the benefits of the many-body corre­lation can help.

The 3D lattice design eliminates that tradeoff by holding the atoms in place. The atoms are fermions, a class of particles that cannot be in the same quantum state and location at once. For a Fermi quantum gas under this clock’s operating conditions, quantum mechanics favors a confi­guration where each individual lattice site is occupied by only one atom, which prevents the frequency shifts induced by atomic inter­actions in the 1D version of the clock.

JILA researchers used an ultra-stable laser to achieve a record level of synchroni­zation between the atoms and lasers, reaching a record-high quality factor of 5.2 quadrillion. Quality factor refers to how long an oscil­lation or waveform can persist without dissi­pating. The researchers found that atom collisions were reduced such that their contri­bution to frequency shifts in the clock was much less than in previous experi­ments.

“This new strontium clock using a quantum gas is an early and astounding success in the practical appli­cation of the ‘new quantum revolution,’ sometimes called ‘quantum 2.0’,” said Thomas O’Brian, chief of the NIST Quantum Physics Division and Ye’s supervisor. “This approach holds enormous promise for NIST and JILA to harness quantum corre­lations for a broad range of measure­ments and new tech­nologies, far beyond timing.”

Depending on measure­ment goals and appli­cations, JILA researchers can optimize the clock’s parameters such as opera­tional tempera­ture (10 to 50 nanokelvins), atom number (10,000 to 100,000), and physical size of the cube (20 to 60 micrometers, or millionths of a meter). Atomic clocks have long been advancing the frontier of measure­ment science, not only in time­keeping and navi­gation but also in defi­nitions of other measure­ment units and other areas of research such as in tabletop searches for the missing dark matter in the universe. (Source: NIST)

Reference: S. L. Campbell et al.: A Fermi-degenerate three-dimensional optical lattice clock, Science 358, 90 (2017); DOI: 10.1126/science.aam5538

Link: Amo Physics and Precision Measurements (J. Ye), JILA, National Institute of Standards and Technology NIST, Boulder, USA

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