Skip to main content
U.S. flag

An official website of the United States government

Dot gov

Official websites use .gov
A .gov website belongs to an official government organization in the United States.


Secure .gov websites use HTTPS
A lock ( ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.

Ion Optical Clocks and Precision Measurements


Our research focuses on the use of trapped ions for precision measurements.  In particular, we are interested in optical frequency metrology, which provides the basis for diverse applications from optical clocks to tests of fundamental physics and relativistic geodesy.  

Below are descriptions of several of our experiments and links to some key publications.  A more complete list of publications can be found here.  Contact information for any current member of the group is available in the Time and Frequency Division Staff Listing.


Al+ Optical Clocks  

Image shows an ion trap assembly used in the aluminum-ion optical clock.
Linear RF Paul trap used in an aluminum ion optical clock.

This project uses techniques from quantum information science to enable precision metrology.  We use the dipole-forbidden 1S0 - 3P0 transition in singly-ionized aluminum as an stable frequency reference (natural linewidth ~8 mHz), which we detect using quantum logic spectroscopy with a second ion held in the same trap [1, 2].  Current efforts focus on reducing systematic effects, such as relativistic shifts due to time dilation [3], and increasing clock stability by use of quantum entanglement and classical correlations.  These clocks have demonstrated record accuracy for optical clocks worldwide, with the current generation reaching fractional uncertainty below 1x10-18 [4].

[1] P. O. Schmidt et al., "Spectroscopy using quantum logic", Science 309, 749-752 (2005) 

[2] C. W. Chou et al.,  "Frequency Comparison of Two High-Accuracy Al+ Optical Clocks", Phys. Rev. Lett. 104, 070802 (2010) 

[3] J.-S. Chen et al., "Sympathetic Ground State Cooling and Time Dilation Shifts in a 27Al+ Optical Clock", Phys. Rev. Lett. 118, 053002 (2017)

[4] S. M. Brewer et al., "An Al+ quantum-logic clock with systematic uncertainty below 10-18", arXiv:1902.07694 (2019)

Hg+ Optical Clock  

This image shows the electrodes of an ion trap used to trap mercury ions.  The electrodes are placed on a US penny to show scale, and are about the size of the text "E Pluribus Unum".
Electrode structure for the spherical Paul trap used in the Hg+ optical clock.

The mercury ion optical clock, operating in a cryogenic environment, was the first to demonstrate performance exceeding  that of the microwave clock standards and continues to be one of the best-characterized optical clocks [1, 2].  It is particularly interesting for the  strong dependence of its frequency on the fine-structure constant, which can be exploited to test for drifts of the fundamental "constants" [3].

[1] S. A. Diddams et al., "An Optical Clock Based on a Single Trapped 199Hg+ Ion", Science 293, 825 (2001) 

[2] W. H. Oskay et al., "Single-atom optical clock with high accuracy", Phys. Rev. Lett. 97, 020801 (2006) 

[3] T. Rosenband et al., "Frequency ratio of Al+ and Hg+ single-ion optical clocks; Metrology at the 17th decimal place", Science 319, 1808 (2008) 

Molecular Spectroscopy  

Cartoon image shows a depiction of two ions, CaH+ and Ca+, trapped in an ion trap.
Cartoon image of CaH+ trapped with Ca+

Molecules, in comparison to atoms, exhibit  more complicated internal structure, which presents both experimental challenges and great opportunities for exploring new physics.  In this project, the tools of quantum information processing are applied to performing  precision measurement and quantum control of a single molecular ion [1].

[1] D. Leibfried, "Quantum state preparation and control of single molecular ions", New J. Phys. 14, 023029 (2012) 

[2] C. W. Chou et al.  "Preparation and Coherent Manipulation of Pure Quantum States of a Single Molecular Ion", Nature 545, 203 (2017) 

[3] Data for "Frequency-comb spectroscopy on pure quantum states of a single molecular ion"

Optical Frequency Stabilization

Image shows the housing for the two cryogenically cooled crystals used in a spectral hole burning experiment.
Cryogenic housing for the spectral hole burning experiment.  Inside the copper chamber are two Eu3+:Y2SiO5 crystals, which are used for frequency stabilizing a laser via absorption spectroscopy.

As a key enabling technology for many high-resolution spectroscopy experiments, we are developing state-of-the-art frequency-stabilized lasers with linewidths in the mHz regime [1].  One project currently underway investigates laser stabilization using a technique called spectral hole burning [2, 3].  Another project pursues laser stabilization using a cryogenically cooled optical cavity.  

[1] B. C. Young et al., "Visible lasers with subhertz linewidths", Phys. Rev. Lett. 82, 3799 (1999) 

[2] M. J. Thorpe et al., "Frequency-stabilization to 6x10^-16 via spectral-hole burning", Nat. Phot. 10, 1038 (2011)

[3] S. Cook et al., "Laser-Frequency Stabilization Based on Steady-State Spectral-Hole Burning in Eu3+∶Y2SiO5", Phys. Rev. Lett. 114, 253902 (2015)


Created October 29, 2016, Updated March 4, 2020