Evidence for sympathetic vibrational cooling of translationally cold molecules

• Published in: Nature (London) Vol. 495; no. 7442; pp. 490 - 494
• Main Authors: Rellergert, Wade G., Sullivan, Scott T., Schowalter, Steven J., Kotochigova, Svetlana, Chen, Kuang, Hudson, Eric R.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 28.03.2013; Nature Publishing Group
• Subjects: 639/766/36/1120; Atoms & subatomic particles; Calcium; Collisions (Physics); Cooling; Experiments; Humanities and Social Sciences; Lasers; letter; Methods; Molecular structure; multidisciplinary; Observations; Population; Properties; Science; Vibration; United States
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature11937

The vibrational motion of trapped BaCl + molecules can be quenched by collisions with ultracold calcium atoms at a rate comparable to the classical scattering rate; this method is over four orders of magnitude more efficient than traditional sympathetic cooling schemes and should be applicable to many different types of molecule. Collision course to vibrationally cooled molecules Cooling of physical systems is the starting point of many studies that aim to control and explore the quantum states of matter. Atoms are normally cooled using lasers, but this technique is not suitable for molecules, which have complex internal structures. This paper reports a novel technique for cooling the vibrational motion of trapped barium chloride (BaCl + ) molecules through collisions with ultracold calcium atoms. The method should be applicable to many different combinations of ultracold atoms and molecules for experiments in quantum chemistry, fundamental physics and quantum information applications. Compared with atoms, molecules have a rich internal structure that offers many opportunities for technological and scientific advancement. The study of this structure could yield critical insights into quantum chemistry 1 , 2 , 3 , new methods for manipulating quantum information 4 , 5 , and improved tests of discrete symmetry violation 6 , 7 and fundamental constant variation 8 , 9 , 10 . Harnessing this potential typically requires the preparation of cold molecules in their quantum rovibrational ground state. However, the molecular internal structure severely complicates efforts to produce such samples. Removal of energy stored in long-lived vibrational levels is particularly problematic because optical transitions between vibrational levels are not governed by strict selection rules, which makes laser cooling difficult. Additionally, traditional collisional, or sympathetic, cooling methods are inefficient at quenching molecular vibrational motion 11 . Here we experimentally demonstrate that the vibrational motion of trapped BaCl + molecules is quenched by collisions with ultracold calcium atoms at a rate comparable to the classical scattering, or Langevin, rate. This is over four orders of magnitude more efficient than traditional sympathetic cooling schemes 11 . The high cooling rate, a consequence of a strong interaction potential (due to the high polarizability of calcium), along with the low collision energies involved 12 , leads to molecular samples with a vibrational ground-state occupancy of at least 90 per cent. Our demonstration uses a novel thermometry technique that relies on relative photodissociation yields. Although the decrease in vibrational temperature is modest, with straightforward improvements it should be possible to produce molecular samples with a vibrational ground-state occupancy greater than 99 per cent in less than 100 milliseconds. Because sympathetic cooling of molecular rotational motion is much more efficient than vibrational cooling in traditional systems, we expect that the method also allows efficient cooling of the rotational motion of the molecules. Moreover, the technique should work for many different combinations of ultracold atoms and molecules.