The Yost Research Group focuses on the spectroscopy of simple atoms and the development of UV lasers needed for these experiments. The study of simple atoms – namely hydrogen – has been an extremely fruitful endeavor, catalyzing the development of quantum mechanics, quantum electrodynamics and even some nuclear physics. All of these advances occurred when discrepancies were found between the physical theories describing the atomic energy levels and the experimental data. In fact, such discrepancies persist until today (http://arxiv.org/pdf/1301.0905v2.pdf), which may be evidence of physics beyond the standard model.
A major part of our research effort is the development of lasers with extremely precise frequencies. In fact, to perform competitive spectroscopic measurements, we will need to measure and control our laser frequency with 15 digits of precision. However, the precision of such an advanced laser source will not be transferred to an experimental measurement if the motion of the atoms is not well controlled. Therefore, we will also investigate laser cooling of atomic hydrogen, which will require somewhat less precise but high-power UV lasers. A description of some of the research projects currently underway to help accomplish this goal are described below.
Two-photon cooling of atomic hydrogen
A common limitation to hydrogen spectroscopy experiments is the non-zero atomic velocity of the atomic sample at finite temperature. This introduces relativistic Doppler shifts, and transit-time broadening, inhibiting the precision with which hydrogen transitions can be measured. These difficulties can be mitigated with laser cooling.
Laser cooling involves tuning the frequency of a laser so that an atom will absorb radiation, but only if the direction of the photon opposes the motion of the atom. This technique has proved incredibly versatile and has become ubiquitous for the cooling of certain, heavy atomic species – mostly alkali and alkaline-earth atoms. Unfortunately, for hydrogen and anti-hydrogen, laser cooling requires Lyman-alpha radiation with a wavelength of 121 nm.
Laser cooling can also proceed with a radiation source at 243 nm which requires photons with half the energy of a Lyman-alpha source. The two-photon cooling technique requires that a hydrogen atom absorb two photons from one direction simultaneously. A tremendous advantage is that radiation at 243 nm is much easier to produce and manipulate. Two-photon cooling techniques for hydrogen have been considered for some time but have yet to be demonstrated because of high average power requirements. Therefore, my research group has undertaken the development of a high power 243 nm laser source. To date, our 243 nm laser source produces 630 mW of average power and we estimate that this source is sufficient to cool a beam of atomic hydrogen down to 3 mK in about 20 ms. We are now actively pursuing this goal in the lab.
Free hydrogen readily forms molecules (H2) whereas our experiments are preformed with atomic hydrogen. In order to obtain atomic hydrogen, we use an RF discharge which breaks apart the molecules and produces a bright pink glow due to a combination of Balmer-alpha and Balmer-beta radiation. The atomic hydrogen then flows to a cold nozzle where its temperature is reduced to around 4.5 K. This hydrogen will then be magnetically guided to our intense 243 nm laser where the temperature will be decreased to only about 1/100th of a degree above absolute zero.
Measuring & Stabilizing Optical Frequencies
In order to measure and stabilize the cooling and spectroscopy lasers in our lab, we are building an optical frequency comb. Frequency combs are a relatively recent invention in optics. The output of a frequency comb consists of evenly-spaced “delta-function-like” frequencies. Given that the comb “teeth” are equally spaced, the frequency comb is like a ruler, albeit in frequency space. Frequency combs provide a direct link from optical frequencies (hundreds of THz) to radio frequencies (MHz-GHz). This allows us to measure frequencies in the optical domain and relate them back to the cesium microwave frequency standard.
As part of the ATRAP collaboration, our group is also assisting in laser cooling and spectroscopy of anti-hydrogen. Anti-hydrogen is the antimatter equivalent of normal hydrogen and consists of an anti-proton and a positron (anti-electron). The relationship between matter and antimatter is thought to obey CPT symmetry. The CPT theorem states that if we replace matter with antimatter, inverts spatial coordinates, and reverse time then our laws of physics will be identical.
To date, no experimental evidence has shown a definitive violation of CPT symmetry. However, the measurement of the antihydrogen 1S-2S transition planned by the ATRAP collaboration at CERN would offer one of the most sensitive searches of CPT violations yet. Such a measurement must be done with extremely cold anti-hydrogen which will require the development of hydrogen/anti-hydrogen laser cooling techniques.