Precision Measurements Using Rydberg Atoms

Raithel Lab, University of Michigan

Participating Students: Ryan Cardman, Andira Ramos '19, Kaitlin Moore '17

Rydberg atoms are great candidates for precision measurements. Their long lifetimes enable longer interaction times with probing fields. This leads to narrower peak linewidths, which, in precision measurements, is key for determining transition frequencies accurately. Moreover, their weakly bound electron makes it possible to trap Rydberg atoms and drive microwave transitions between these states using optical ponderomotive potentials. These lead to more efficient trapping and to Fourier-limited linewidths. To exploit the advantages of Rydberg atoms, we have built an ultra-high vacuum, cold-atom spectroscopy apparatus equipped with three dimensional electric and magnetic field controls that allows us to perform precision measurements (shown below). Additionally, the experiment is in thermal contact with a liquid-helium cryostat for shielding blackbody radiation and extending the radiative lifetimes of our Rydberg ensembles.

Our in-vacuum spectroscopy enclosure consists of six plate electrodes for electric-field control.

Rydberg Constant Measurement (RCM)

The Rydberg constant is a key physical constant which is employed in determining atomic energy levels of any atom and is related to other fundamental constants. The current value of the Rydberg constant started to be questioned with the appearance of the "proton radius puzzle". In order to help solve this puzzle, we have proposed performing a precision measurement of the Rydberg constant using cold circular Rydberg atoms. Circular atoms are in the highest l and m-state possible for a given n. This provides us with a measurement independent from the proton size and QED effects, which, in turn, should help in solving the proton radius puzzle. The atoms are trapped using a Rydberg-atom optical lattice and transitions are driven using a recently demonstrated lattice-modulation technique (explained below) to perform Doppler-free spectroscopy. The circular-state transition frequency yields the Rydberg constant.

Our apparatus for precision spectroscopy.

Spectroscopy and Coherent Control of Rydberg Atoms with Optical Lattices

When Rydberg atoms are trapped in optical lattices that are periodically modulated at resonant frequencies between two Rydberg levels, Fourier- limited, Doppler-free transitions can be driven, free of l selection rules. This effect comes from the fact that a Rydberg atom's electronic wave function sees a spatially-dependent potential within a standing wave of light. Therefore, this provides an angular-momentum coupling to a state with the same or opposite parity. Using an amplitude-modulated optical lattice, we have already demonstrated this technique and will use it for a Rydberg constant measurement. Furthermore, we have recently developed the theoretical framework for using one and two-dimensional time-dependent ponderomotive optical potentials to initialize Rydberg atoms in circular states for spectroscopy.

Optical molasses of 85Rb at sub-Doppler temperatures.

Recent Breakthroughs

As a demonstration of how well we can make measurements with our apparatus, we obtained the hyperfine coupling constant for the nS1/2 states of 85Rb, with a relative uncertainty of 0.5%, well surpassing the precision of all previous measurements. To do so, we performed mm-wave spectroscopy on Rydberg atoms excited from an optical molasses cooled by polarization gradients (shown above).

a) Fourier-limited 45S1/2 hyperfine peaks obtained by mm-wave spectroscopy.

b) Splitting of those peaks as a function of 1/n*3.

Publications

1. A. Ramos, K. Moore, and G. Raithel, Phys. Rev. A 96, 032513 (2017).

2. K.R. Moore and G. Raithel, Phys. Rev. Lett. 115, 163003 (2015).

3. K.R. Moore, S.E. Anderson, and G. Raithel, Nat. Commun. 6, 6090 (2015).

This work is supported by NASA, the NSF, and NIST.

Webpage author: Ryan Cardman