Atomic clock breaks precision records


Rebecca Pool

Wednesday, March 7, 2018 - 15:45
Image: 'Artwork' made with new imaging technique, which rapidly and precisely measures quantum behavior in an atomic clock. 
US-based researchers have used high spatial resolution imaging and spectroscopy to rapidly and precisely measure quantum behaviour in an atomic clock.
Dr Jun Ye from JILA, National Institute of Standards and Technology, and the University of Colorado at Boulder, and colleagues, have set a new record amongst optical atomic clocks by demonstrating a strontium clock with a relative precision of 2.5×10−19.
The method uses thousands of ultracold Sr atoms confined in a 3D optical lattice and beats the previous 'best' optical clock by a factor of 1.4 in precision. 
"We implement imaging spectroscopy of the optical clock transition of lattice-trapped degenerate fermionic stronitum in the Mott-insulating regime," writes lead researcher, Dr Jun Ye from JILA, National Institute of Standards and Technology, and the University of Colorado at Boulder, in Physical Review Letters.
"[We have] combined micron spatial resolution with submillihertz spectral precision," he adds. "We have used these tools to demonstrate atomic coherence for up to 15 s on the clock transition and reach a record frequency precision of  2.5×10−19."
The resulting images are false-colour representations of atoms in the ground state (blue) and excited state (red). The white region represents atoms in a fine mixture of about 50 percent red and 50 percent blue, creating a dappled effect. This occurs because these 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, which creates “noise” in the image.
Researchers use a laser pulse to drive about 10,000 strontium atoms from a low-energy ground state to a high-energy, excited state.
Then, a blue laser positioned underneath the lattice is directed upward vertically through the atoms, with a camera capturing the shadow the atoms cast, which is a function of how much light they absorb. 
The method generates 'near-instant visual art', spatial maps of energy shifts amongst the atoms in the a 3D strontium lattice atomic clock, and provides information about each atom’s location and energy level, or quantum state.
Using the new method, the researchers have created a series of images to map small frequency shifts, or fractions of atoms in the excited state, across different regions of the lattice.
The researchers reported achieving a record precision in measuring frequency of 2.5 x 10-19 in six hours. 
In the future, the method may allow scientists to finally see new physics such as the connection between quantum physics and gravity.
“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,” says Ye.
Imaging spectroscopy provides information about the local environment of the atoms, similar to the incredible resolution offered by scanning tunneling microscopy.
So far, the method has been used to produce 2D images, but Ye reckons it could be used to generate 3D images based on layer-by-layer measurements as is done in tomography.
“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.”
Research is published in Physical Review Letters.
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