Direct Cooling of Molecules

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1 Direct Cooling of Molecules An overview of the current state of the field

2 Why Cool (Polar) Molecules? Ultracold Chemistry Electron EDM Long-range Interactions The ACME Collaboration, /science J. Hutson, /science B. Yan et al., /nature

3 Difficulties in Cooling Molecules Difficulty lies in the capture of cold (>>1K) molecules Standard techniques from the field of ultracold atoms (MOT, Zeeman slower) generally fail due to lack of cycling transition Many Laser frequencies even for quasi-cycling 3

4 Indirect Creation of Cold Molecules The coldest polar molecules to date are formed through STIRAP using magnetoassociated ultracold atoms L.D. Carr et al., New J. Phys. 11 (2009)

5 Why Cool Molecules Directly? Adiabatic association is limited to molecules with laser-coolable constituents Direct cooling could produce ultracold molecules with richer structure or larger EDM 5

6 Ingredients necessary for direct cooling Just like the case of ultracold atoms, producing ultracold molecules will require a few key steps Source of molecules Cooling of molecules Trapping and possible further cooling 6

7 Molecular Sources Cooling of molecules seems really hard Translational AND internal temperatures must be cold Start with a source that's already cold: Supersonic expansion Cryogenic buffer gas cooled source 7

8 Supersonic Expansion Typical beam temperature of 3K Typical forward velocities of 300m/s M. Barr et al., Meas. Sci. Technol. 23 (2012) S. Y. T. van de Meerakker et al., Nature Physics 4, (2008) 8

9 Cryogenic buffer gas cooled source Typical beam temperature of 3K Typical forward velocities of 100m/s J. Barry, Thesis 9

10 Cold but Fast Molecules... Now What! By using supersonic or cryogenic buffer gas sources the molecules are internally (lowest ro-vibrational state) and externally (narrow velocity spread) cold Low temperatures come at the cost of boosted forward velocities Need to slow the molecules down 10

11 Molecular Slowing Techniques There exist a wide variety of slowing techniques Stark deceleration (the standard) Traveling wave deceleration Radiation pressure slowing Centrifuge deceleration 11

12 The Stark Decelerator Meerakker group, Berlin Energy of molecule in electric field: E V = d ~1cm-1 hill for 100kV/cm ~100 hills needed L.D. Carr et al., New J. Phys. 11 (2009)

13 Travelling Wave Deceleration Trap atoms in moving, decelerating traps Better transverse stability J.E. van den Berg et al., arxiv: v1 13

14 J.E. van den Berg et al., arxiv: v1 14

15 Disadvantages of Stark Deceleration Switching 10kV at 10kHz is demanding Smaller phase space acceptance for higher deceleration Inherently pulse mode (~0.1% duty cycle) more suitable for pulsed sources 15

16 Direct Slowing Through Laser Light Need quasi-cycling 3+ lasers modulated at the HF splitting, even with highly diagonal FCFs 3+ sharks expensive dangerous J. Barry, Thesis 16

17 Direct Slowing Through Laser Light Decrease in number due to transverse divergence/ heating Zeeman slowing prevented due to Zeeman dark states Would need to chirp laser J. Barry, Thesis 17

18 The Centrifuge Decelerator Continuous slowing of (guidable) molecules S. Chervenkov et al., Phys. Rev. Lett. 112, (2014) A Osterwalder., Physics 7, 2 (2014) 18

19 The Centrifuge Decelerator S. Chervenkov et al., Phys. Rev. Lett. 112, (2014) 19

20 The Centrifuge Decelerator S. Chervenkov et al., Phys. Rev. Lett. 112, (2014) 20

21 Molecular Cooling / Trapping Once molecules are decelerated, the particles need to be cooled further: Laser cooling (quasi-closed transitions, MOT) Evaporative cooling, Magnetic trapping Opto-electrical cooling and trapping 21

22 Laser Cooling of YO M.T. Hummon et al., Phys. Rev. Lett. 110, (2013) 22

23 Evaporative Cooling of OH B.K. Stuhl et al., Nature 492, (2012) 23

24 Opto-Electrical Cooling of CH3F M. Zeppenfeld et al., Nature 491, (2012) 24

25 Possible future directions It seems that current cooling techniques are on the verge of being good enough but could use another order of magnitude Stark deceleration suffers from low yield due to pulsed operation and reduced phase-space acceptance Direct laser cooling demonstrated so far only as proof-of-principle 25

26 Adiabatic Opto-electrical Slowing? The main problem with slowing molecules using laser light arise from the small momentum imparted by a single photon: Need many photon scattering events to slow down molecule Transverse heating and pumping into dark states How to combine laser slowing with transverse confinement? 26

27 Adiabatic slowing Slowing is not cooling: no need for dissipation Stimulate atom back down before it can emit spontaneously: no ro-vibrational branching E.R. Hudson, PHYSICAL REVIEW A 79, R (2009) 27

28 Using Adiabatic Microwave Transitions The noodle method: 28