Introduction to Our Research


Cold Molecules

Isolated molecules are ideal systems to test our understanding of fundamental physics. Due to their relative simplicity, we can calculate some of their properties to extreme levels of accuracy. If such predictions agree with the results of experimental measurements, we must conclude that our understanding is correct---at least to that level of accuracy. Determining molecular structures or the distributions of electronic charge in molecules, for example, is important for the development of chemistry. However, molecules are also good systems to test our understanding of nature in a much broader sense. Some properties of the world are fixed by physics derived from mathematical symmetries, while others seem to be selected from an ensemble of possibilities. On the one hand, it is interesting to know whether our theories are correct and which mathematical symmetries should hold. For example, time reversal symmetry is when the laws of quantum mechanics are left unchanged if time runs backwards. One of the consequences of this symmetry is that fundamental particles cannot show an electric dipole moment. So, finding a non-zero electric dipole moment in, for example, an electron would force us to review our theories. It turns out that the accurate measurement of some molecular states is one of the best ways to determine the magnitude of such a dipole moment. On the other hand, the values of some fundamental physical constants are extremely critical in shaping the world as we know it. A small change in some values and life on our planet would not be possible as we know it. It is natural to wonder about the reasons for these specific values and how constant these constants really are. Since the comparison of two measurements separated by a million years is unpractical, it is better to improve the accuracy of a single measurement by one million times and repeat it after one year. Again, molecules are ideal systems for these kinds of experiments.

The kinds of things we are good at calculating are, for instance, the relative energies of the different quantum states of a molecule. In experimental terms, molecular spectroscopy allows for a measurement of the energy difference between two energy levels, which corresponds to the frequency of the light that is absorbed or emitted by the molecule. The precision of a frequency measurement is proportional to the duration of the measurement itself. However, as molecules in the gas phase move typically at the speed of sound, achieving a long measurement time is not trivial. Pinning them down on a clean surface or keeping them in the liquid phase, either pure or in solution, would induce large changes in the molecules themselves, thus destroying the simplicity of our isolated system. Of course, one might be specifically interested in the interactions of the molecules with their environment, which is a fascinating research field---and in that case the aforementioned solutions are viable. However, for high-resolution spectroscopy, our laboratory focuses on isolated, small molecular systems, and hence we specialize in controlling the motion of molecules in free space. One of the main tools we use for this is the molecule chip.

Molecule Chips

The idea of integrating various parts of a laboratory on a microchip is now over twenty years old and was motivated by a desire for simplification, reduction of costs, portability, speed of measurement, and ease of reproducibility. A reduced size allows for shorter transport times and strong confinement forces. For physics, the atom chip and ion chip have been employed in fields as diverse as quantum computation, many-body non-equilibrium physics, and gravitational sensing. For chemistry, the lab-on-a-chip shrinks the pipettes, beakers and test tubes of a modern lab onto a microchip-sized substrate, with applications from the international space station to anti-terrorism. The molecule chip, however, is currently in its infancy, but promises a marriage between fundamental quantum physics and the richness of the chemical world. A particular advantage of using molecules instead of atoms on a chip is that they can be coupled to photons over a wider range of frequencies by their rotational and vibrational degrees of freedom. Moreover, for chemists the molecule chip offers the prospect of extending the control of molecular concentrations and interactions to the level of single molecules with the accuracy in interaction energy enhanced to the mK level or beyond. In our case, the molecule chip is a microfabricated array of electrodes on a substrate.


The minimal requirements for a integrated laboratory on a chip are that molecules are loaded, manipulated, and detected on the chip. The loading of the molecule chip is complicated by the fact that there is no handy source of cold molecules as there is for cold atoms (for example, magneto-optical traps). While supersonic beams offer a possible approach, the cold molecules they provide travel at the speed of sound. Some researchers have therefore tried to move a supersonic source backward with this very high speed, but shooting pieces of equipment around the lab at the speed of sound can bring more problems than it solves. Our solution using the molecule chip consists of creating electric-field traps that can be easily moved at supersonic speeds. Once the molecules are captured in the microtraps, they can be decelerated to a standstill within a few of centimeters.

Besides the mechanical manipulation of molecules, we have also shown that all their internal degrees of freedom can be manipulated using light of the appropriate wavelength while the molecules are on the chip. Light from the UV to the IR to microwaves can be coupled to the molecules on the chip to induce electronic, vibrational, and rotational transitions. As molecules are typically loaded on our molecule chip in a single quantum state, being able to selectively pump them into another particular state of our choice is a demonstration of the extreme level of control we can achieve.

Molecules are much harder to detect than atoms for the same reason for which they are difficult to laser cool. Laser cooling and imaging work by scattering millions of photons of the same frequency, driving the system continuously in a closed two-level cycle. Molecules have a much richer spectrum than atoms and therefore, after scattering a few photons, they are very likely to be found in a different state. This precludes the many-million-fold cycling that makes these events measurable. Therefore, in order to detect the molecules while they are on the chip, we ionize them and then guide the ions to an external detector using ion optics. In so doing we are even able to preserve the original spatial distribution of the molecules and obtain direct images at a time of our choice. By taking snapshots at different times, we can follow the molecular movements and study the molecular distributions in the phase-space.