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Polyatomic Molecules

We use polyatomic molecules to realize quantum control via laser cooling and trapping while maintaining large polarizability and properties favorable for precision measurements.

Cold and Ultracold Molecules

As discussed in the research overview (which is required reading for this page...),  molecules are powerful tools to search for new physics through fundamental symmetry violations.  That is the primary focus of our current research, but molecules have many diverse and exciting applications:
  • Molecules have strong, long-range, anisotropic, and tunable electric dipole-dipole interactions.  These interactions can be used to realize strongly-interacting quantum many-body systems, and novel quantum phases of matter.
  • The dipole-dipole interaction can be used to perform qubit operations to realize quantum information/computation systems, where the internal state of the molecule serves as the qubit.
  • Molecules have chemical bonds, so they can be used to study chemical processes at the completely fundamental level.
A very good overview of the many applications of molecules can be found in this review.  Realization of these applications requires a large degree of quantum control, which in turn requires very low temperatures - typically less than around 1 milliKelvin, so that internal and external degrees of freedom cannot be thermally excited, which leads to decoherence.

Searching for New Physics at the PeV Scale

Searches for new physics with molecules typically involve measuring phases that accumulate as the molecule precesses in electromagnetic fields.  This means that the sensitivity of these experiments is linear in the time that the molecules spends precessing, which is called the precession time or coherence time.  It turns out that the longest precession that that you can reasonably achieve with a molecular beam, even a cryogenic beam like a CBGB, is a few milliseconds.  In the ultracold world, coherence times of seconds can be realized with cold and controlled quantum samples.  Ultracold molecules therefore have the potential to increase sensitivity to new physics by many orders of magnitude, into the PeV regime.

Reaching the few milliKelvin regime requires more than buffer gas cooling, which inevitably fails around a few hundred milliKelvin.  There are several proposed routes, but an extremely promising route is to use what atomic physicists have been using for decades, and has been the workhorse of the ultracold field since then - laser cooling.

Laser-Cooling Molecules

Laser cooling works by using repeatedly scattering photons to apply forces on atoms or molecules.  Since photons have momentum, if an atom or molecule absorbs and then re-emits a photon, it will receive a momentum kick - in other words, it experiences a force.  However, the momentum kick is very tiny, so typically hundreds of thousands of photon scatters are needed to reduce the velocity of a typical gas-phase atom to the ultracold regime.  If you want to learn more about laser cooling, consider taking Physics 103!

The problem with molecules is that they have internal, mechanical modes - rotation, and vibration - that can be excited when a molecule receives a momentum kick, which will cause the molecule to start rotating or vibrating (or both).  Because laser cooling schemes generally address only a single quantum state per laser, exciting these internal modes is effectively loss of molecules.  This is a serious problem because if you naively start scattering photons off of a molecule, you about as likely to excite one of these internal modes as you are likely to not excite one - meaning that you can scatter "a few" photons before the molecule is effectively gone.  This is so short of "a few hundred thousand" that the naive approach simply doesn't work.

There are a number of very clever tricks that enable laser cooling for certain molecules, which was first demonstrated in 2010.  So far, only molecules with very particular electronic structure can be laser-cooled, though there are several examples of laser-coolable molecules that are also sensitive to CP violation.  However, there are no diatomic molecules that offer both laser cooling and strong rejection of systematic effects via internal co-magnetometers.  Internal co-magnetometers have already shown themselves to be extremely useful for precision measurements in molecules, so it is worth expanding our horizons to find species that offer all of these desirable features.

Polyatomic Molecules

Laser cooling and internal co-magnetometers conflict in diatomics because both features have to come from very specific electronic structure on the heavy atom, and these requirements are at odds.  On the other hand, polyatomic molecules, with at least three atoms, offer laser cooling and internal co-magnetometers.  Like in diatomics, a heavy metal atom with suitable electronic structure and bonding can provide photon cycling and laser cooling; unlike diatomics, internal co-magnetometers can arise from degenerate vibrational or rotational motions that exist only in molecules with at least three atoms.

Degenerate vibrational motions give rise to highly polarizable states
Figure: degenerate bending modes in a linear triatomic molecule give rise to states with vibrational angular momentum that can be easily polarized in electric fields.  Here the orange atom is a heavy metal center with new physics sensitivity and favorable laser-cooling properties, such as Yb.

Co-magnetometer states that can be easily polarized in electric fields are a generic feature of linear and symmetric top polyatomic molecules that do not depend on the electronic properties of their constituent atoms.  Similarly, the laser cooling properties only depend on the electronic properties, and in general not the bonding partners (for suitably chosen ligands.)  We can therefore construct molecules with laser-coolable metal centers, such as YbOH or YbOCH3, that can be laser-cooled, have co-magnetometers, and are sensitive to a wide range of new symmetry violating physics, such as nuclear magnetic quadrupole moments and permanent EDMs.

Figure: The metal center (orange) can cycle photons without exciting internal modes, independent of the bonding partners, under a wide range of conditions.  Here we show a symmetric top molecule, where the polarizable states arise from low energy rotation of the molecule about the symmetry axis.

Want to know more?

Please be in touch if you have any questions!  Here is some suggested reading for more information as well.

Our proposal:
Precision Measurement of Time-Reversal Symmetry Violation with Laser-Cooled Polyatomic Molecules
Ivan Kozyryev and Nicholas R. Hutzler

A selection of papers regarding laser-cooling molecules:

Laser-cooling molecules
M. D. Di Rosa

Magneto-optical Trap for Polar Molecules
B. K. Stuhl, B. C. Sawyer, D. Wang, and J. Ye

Laser cooling of a diatomic molecule
E. S. Shuman, J. F. Barry & D. DeMille

Magneto-optical trapping of a diatomic molecule
J. F. Barry, D. J. McCarron, E. B. Norrgard, M. H. Steinecker & D. DeMille

Radio Frequency Magneto-Optical Trapping of CaF with High Density
L. Anderegg, B. L. Augenbraun, E. Chae, B. Hemmerling, N. R. Hutzler, A. Ravi, A. Collopy, J. Ye, W. Ketterle, and J.M. Doyle

Molecules cooled below the Doppler limit
S. Truppe, H. J. Williams, M. Hambach, L. Caldwell, N. J. Fitch, E. A. Hinds, B. E. Sauer & M. R. Tarbutt

Sisyphus Laser Cooling of a Polyatomic Molecule
Ivan Kozyryev, Louis Baum, Kyle Matsuda, Benjamin L. Augenbraun, Loic Anderegg, Alexander P. Sedlack, and John M. Doyle