New technologies
We move cavity-enhanced spectroscopy to the deep cryogenic regime, below 5 K [80, 81]. At low temperature, molecules move slowly and spectral lines become narrower. This improves the accuracy of precision measurements. Compared with our room-temperature spectrometer [43], the accuracy is better by over an order of magnitude [81].
The first challenge is the cavity length. To scan a cavity, we must change its length by at least half of the laser wavelength. In the mid-infrared this means several microns, and most piezoelectric actuators lose much of their range when cold. We built a mirror actuator that works from room temperature down to 10 K and reaches a tuning range of over 22 μm at 10 K [59]. The mirror sits on a spring plate, and the piezo stack only pushes it. A self-positioning mechanism keeps the mirror on the optical axis, even though the piezo ceramic and the copper housing shrink by very different amounts [59]. Flexible straps made of high-purity copper keep the mirror at the same temperature as the frame [59].
The second challenge is the cryogenic platform. A cavity needs a stable temperature and very low vibrations, but a cryocooler gives neither by default. We built a platform with a copper chamber of nearly 9 l and a length of 78 cm [80]. Thick walls keep the temperature uniform across the chamber. The chamber stands two metres from the cryocooler, and heavy copper rods with flexible copper straps link them. This chain works as a low-pass filter for temperature fluctuations and vibrations [80]. A 60 K shield blocks thermal radiation, and the whole system hangs on thin wires, which limit heat leakage [80].
Together, these two elements form a cavity-enhanced spectrometer that works fully at 4 K [81]. We cool the gas sample and the whole cavity including the mirrors. With this instrument we made an accurate test of quantum electrodynamics for molecules, and we realized optical standards of temperature, concentration and pressure at the deep cryogenic conditions [81]. We also measured the H₂ phase diagram and the ortho–para conversion rate [81]. We now use the same technology in our project on trapping cold molecular hydrogen.
New laboratory: trapping cold molecular hydrogen
We are building a new 90 m² laboratory at Nicolaus Copernicus University in Toruń for the H2TRAP project, funded by the European Research Council (grant no. 101075678). Construction began in 2022. The project's main goal is to trap a cold sample of molecular hydrogen (H₂) for the first time.
H₂ is the simplest neutral molecule, so we can calculate its properties from first principles. It also has several hundred very long-lived rovibrational states. In principle, this allows testing fundamental physics with H₂ at a relative accuracy of 10−24. Current experiments are far from this potential. The reason is that H₂ in its ground electronic state interacts very weakly with electric and magnetic fields. For this reason, standard methods of decelerating, cooling and trapping do not work well for H₂.
We will use an ultra-strong optical dipole trap, about 1 mK deep. We will reach 5 K with standard cooling technology, and the trap will be filled in situ from the thermal distribution of a 5 K sample of H₂. No technology can cool H₂ gas from 5 K down to 1 mK, so the only option is to catch the coldest fraction of the molecules directly. Most molecules that first fill the trap region will be lost. However, the high initial density of H₂ will allow us to confine up to 600,000 molecules.
In the trap, we will perform double-resonance spectroscopy of H₂ in the near-infrared and ultraviolet. We will measure the frequencies against an optical frequency comb and a primary frequency standard. Spectroscopy on a cold, trapped sample removes the uncertainty sources that limited our best earlier approaches. We expect to improve the accuracy by at least two orders of magnitude.
In the new laboratory we will combine the trap with our cavity-enhanced spectroscopy technologies [43, 81]. Our cryogenic platform already reaches temperatures below 5 K [80], and our spectrometer works fully at 4 K [81]. H₂ traps will open a new way to further long-term progress in the metrology of H₂ rovibrational lines.
