• Physics 16, 36
Researchers have attained a 100-fold improve within the accuracy of a molecular clock that would function a terahertz-frequency normal and as a platform for investigating new physics.
Molecules can rotate, vibrate, and twist. With every of these levels of freedom comes a ladder of quantized vitality ranges whose spacings usually occupy the terahertz band. Because the ladder’s steps are tiny, molecules are exquisitely delicate probes of inside and exterior fields. But the sensitivity complicates the 2 principal operations wanted to construct a working probe: cooling and trapping. In 2019 Tanya Zelevinsky of Columbia University and her collaborators overcame these hurdles to measure a vibrational transition in diatomic strontium to 1 half in . Now, after 4 years of enhancements to the experiment, the staff has attained an accuracy instances larger [1].
Zelevinsky and her staff selected for his or her molecular clock as a result of its constituent atoms could be readily cooled by diode lasers. The factor’s most ample isotope, , additionally lacks spin, which, if it have been current, would complicate each experimental dealing with and theoretical remedy.
To make the molecules, the researchers first cooled atoms in a magneto-optical lure. Illuminating the atoms with a laser pushed pairs of atoms out of their unbound state and into an excited molecular state that promptly decayed by spontaneous emission.
The clock transition that Zelevinsky and her staff measured is between the bottom state’s lowest vibrational stage, , and its highest sure vibrational stage, . The rotational state is zero in each circumstances. Because the transition is forbidden, the staff effected the transition in two steps by utilizing a laser to excite the molecules from the state right into a digital state that decayed into the state.
The central problem in precisely measuring a molecular transition is to attenuate the broadening of the transition linewidth as a result of molecules’ random movement (Doppler broadening). Trapping the molecules within the troughs of a standing wave of close to infrared laser mild—an optical lattice—holds the molecules nonetheless. However, the laser’s personal electrical area shifts the energies of the transition’s begin and finish states, a conduct often known as the Stark impact. The shifts could be mitigated by tuning the frequency of the trapping laser in order that the beginning and finish states share the identical polarizability. At that so-called magic frequency, the Stark impact disappears.
In the case of the and states, the specified cancellation could be accompanied by unwelcome scattering. To keep away from that detriment, Zelevinsky and her staff set the trapping laser’s frequency near a resonance between one of many two states and a better digital state. That workaround preserved the cancellation on the expense of making an escape route for molecules present process the transition. Still, sufficient of the molecules remained shuttling between the 2 ranges that the transition’s frequency might be measured to an accuracy of elements in .
The staff additionally compiled an uncertainty finances via adjusting varied experimental settings. This finances quantified and ranked sources of systematic error, with the Stark impact popping out on prime. One path to minimizing this error supply is to determine easy methods to scale back the depth of the trapping laser, and subsequently the dimensions of the Stark impact, with out liberating the molecules, Zelevinsky says.
By serving as a frequency normal, a strontium molecular clock might allow new functions in terahertz-frequency metrology. Zelevinsky says that she can also be curious about utilizing strontium clocks to seek for a hypothetical gravity-like interplay that is dependent upon particle mass. The telltale proof might present up as variations within the terahertz spectra of the three molecular isotopologues of strontium: , , and the used on this examine. David DeMille of the University of Chicago anticipates {that a} strontium molecular clock might additionally decide whether or not the proton–electron mass ratio is dependent upon time or gravity. “Such a signal could provide evidence for certain hypothesized types of dark matter and/or new scalar fields associated with particles of very low mass,” he says.
–Charles Day
Charles Day is a Senior Editor for Physics Magazine.
References
- Okay. H. Leung et al., “Terahertz vibrational molecular clock with systematic uncertainty at the 10−14 level,” Phys. Rev. X 13, 011047 (2023).