Spectroscopy, Microscopy Allow Precise Imaging of Optical Frequencies

An imaging technique combining spectroscopy with high-resolution microscopy to produce rapid, precise measurements of quantum behavior in an atomic clock has been developed. Physicists believe the technique could improve atomic clock precision and provide a path toward measuring many-body interactions and testing fundamental physics.

This is artwork made with JILA’s new imaging technique, which rapidly and precisely measures quantum behavior in an atomic clock. The images are false-color representations of atoms detected in the ground state (blue) or excited state (red). The white region represents a fine mixture of atoms in the two states, which creates quantum noise in the image. This occurs because all the atoms were initially prepared in a quantum state of superposition, or both ground and excited states simultaneously, and the imaging measurement prompts a collapse into one of the two states. The imaging technique will help improve clock precision and add new atomic-level detail to studies of phenomena such as magnetism and superconductivity. Courtesy of Marti/JILA.

The technique, developed by scientists at JILA, a research institute operated jointly by NIST and the University of Colorado, makes spatial maps of energy shifts among the atoms in a 3D strontium lattice atomic clock and provides information about each atom’s quantum state. The atoms reside in a so-called quantum degenerate gas, in which large numbers of atoms will interact.

“This technique allows us to write a piece of beautiful ‘music’ with laser light and atoms, and then map that into a structure and freeze it like a stone so we can look at individual atoms listening to the different tones of the laser, read out directly as an image,” JILA/NIST Fellow Jun Ye said.

To prepare atoms for imaging, researchers used a laser pulse to drive about 10,000 strontium atoms from a ground state to an excited state. A blue laser was then positioned underneath the lattice and shined vertically up and through the atoms.

Researchers photographed the shadow cast by the atoms (the shadow is an indication of how much light the atoms absorb). The resulting images showed false-color representations of atoms in the ground state and excited state and a region representing atoms in a mixture of about 50 percent ground state and 50 percent excited state.

To demonstrate the technique, the JILA team created a series of images to map small frequency shifts of the atoms across different regions of the lattice. The researchers reported achieving a record precision in measuring frequency of 2.5 × 10⁻¹⁹ (error of just 0.25 parts per billion billion) in 6 hours.

Imaging spectroscopy provides information about the local environment of the atoms, similar to the resolution offered by scanning tunneling microscopy. So far, the technique has been used to produce 2D images, but researchers say it could be used to make 3D images, based on layer-by-layer measurements like those done in tomography.

The lattice of atoms could be used as a magnetic or gravitational sensor to test the interplay between different fields of physics. Ye is most excited about the future possibility of using the atoms in the clock as a gravity sensor, to see how quantum mechanics, which operates on very small spatial scales, interacts with general relativity, a macroscopic force.

“As the clock gets better in the next 20 years, this little crystal could not only map out how gravity affects frequency, but we could also start to see the interplay of gravity and quantum mechanics,” Ye said. “This is a physical effect that no experimental probe has ever measured. This imaging technique could become a very important tool.”

The research was published in Physical Review Letters (doi:10.1103/PhysRevLett.120.103201).