The precision of a molecular clock has increased 100 times, enabling researchers to use it as a terahertz frequency standard and a platform for new physics research.
Molecules can bend, vibrate and rotate. Associated with each of these degrees of freedom is a ladder of quantized energy levels, usually falling in the terahertz region. The tiny rungs of the ladder make the molecules incredibly sensitive sensors for both interior and exterior spaces. However, this sensitivity makes the two main processes required to create a functional probe (cooling and trapping) more difficult. Tanya Zelevinsky of Columbia University, with the help of her collaborators, demonstrated the vibrational transition in diatomic strontium in 2019.12 He was able to measure a percentage.
The experiment has been in development for four years now, and the team has achieved a 100x increase in accuracy.
Sr2 was the material of choice for Zelevinsky and his team's molecular clock because diode lasers could easily cool the atoms that made it up. The most common isotope of the element 88Sr is similarly devoid of spin and, if present, would make experiments and theoretical treatment more difficult to administer.
Before creating the molecules, the scientists cooled the Sr atoms in a magneto-optical trap. When atoms were exposed to laser light, their pairs were pushed from their unbonded state to an excited molecular state, which quickly underwent spontaneous emission decay.
The lowest vibrational level of the ground state, ν=0, and the highest bound vibrational level, ν=62, are where Zelevinsky and his team saw the clock shift. The rotation status is zero in all scenarios. The scientists used a laser to excite molecules from a state of ν=0 to a virtual state that decays to a state of ν=62 to accomplish the forbidden transition in two steps.
Limiting the transition line width resulting from the random movement of molecules is the main issue in precisely measuring a chemical transition. (Doppler expansion). Molecules are held still by being trapped in pits of a standing wave of near-infrared laser light or an optical lattice. The Stark effect, on the other hand, causes the laser's own electric field to change the energies of the transition's start and end states. Shifts can be minimized by adjusting the frequency of the capture laser so that the start and end states have the same polarizability. The Stark effect disappears at this so-called magic frequency.
Desired cancellation will be followed by undesired scatter in the case of v=0 and v=62. To overcome this disadvantage, Zelevinsky and his team tuned the frequency of the capture laser to be close to the resonance between one of the two states and a higher electronic state. This workaround continued the cancellation by providing an escape route for molecules traveling through the ν=0 → 62 transition. However, there were still enough molecules going back and forth between the two levels, and the frequency of the transition was 10.14 te could be determined with an accuracy of 5 parts.
In addition, the group created an uncertainty budget by changing various experimental parameters. The Stark effect peaked after measuring and ranking 11 sources of systematic error. According to Zelevinsky, finding a way to reduce the power of the trapping laser without releasing the molecules, and thus the amount of the Stark effect, is one way to minimize this source of error.
A strontium molecular clock could serve as a reference for frequency, opening up new opportunities for terahertz frequency metrology. According to Zelevinsky, he is also interested in using strontium clocks to look for an imaginary gravity-like interaction that depends on mass. Of the three molecular isotopeologies of strontium – 84Sr., 86Sr and used in this study 88Differences in the Sr – terahertz spectra could be a sign. A strontium molecular clock could potentially reveal whether the proton-electron mass ratio is affected by gravity or time, according to David DeMille of the University of Chicago. According to him, such a signal “could provide evidence for some putative types of dark matter and/or new scalar fields associated with very low-mass particles.”
Günceleme: 29/03/2023 10:23