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MT-MOT: a Hybrid Magnetic Trap / Magneto-Optical Trap

Paul Walker

Published Jan 1, 2018• 2 citations
OpenAlex

Abstract

Arrays of cold atoms in magnetic traps have been implemented for use in quantum simulation, metrology, and ultracold chemistry. The scope of all of these fields could be greatly enhanced with cold and ultracold molecules. Cooling and trapping molecules, however, poses unique challenges compared to atoms. Sympathetic cooling, whereby molecules are cooled via collisions with ultracold atoms, will be a key technique for achieving the required densities and temperatures.\\ Towards this end, the Carty group in Durham has constructed a moving-trap Zeeman decelerator (`MTZD'), based on the same operating principles as a conventional Zeeman decelerator but producing a truly dynamic three-dimensional trapping potential. We have begun to characterise the decelerator with metastable argon, showing 3D guiding and deceleration, and intend to move to molecules in the near future.\\ In the longer term the decelerator is intended to load a magnetic trap. Presented here are the findings from preliminary work to design a hybrid magnetic trap with magneto-optical trap capability, or `MT-MOT'. The MT-MOT comprises six toroidal permanent magnets plus switchable electromagnetic coils. We have performed simulations to determine the feasibility of co-loading lithium atoms and calcium hydride molecules. Some consideration of loss mechanisms and likely trap lifetimes are discussed.

Related papers

Development of a magneto-optical trap for CaF molecules

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Laser cooling and trapping in a magneto-optical trap (MOT) have been essential to the success of cold atom physics in the last decades. Recently, the application of the same techniques to molecules has begun. The complexity of even a simple diatomic molecule makes laser cooling difficult, but promises new applications in many areas of research. In this thesis I describe the development of the first three-dimensional MOT of calcium fluoride (CaF) molecules. First, a cryogenic buffer gas source was set up, producing a pulsed beam of 9.3*10^10 molecules per steradian per pulse with forward velocities around 170 m/s. A similar source for very large molecules was set up during a 5 month internship at the University of Vienna. Next, the molecular pulse was slowed down to the capture velocity of a MOT using chirped laser slowing, resulting in about 7*10^5 CaF molecules passing through the typical MOT volume of 1 cm^3 at velocities of 15+-5 m/s. A new deceleration method, called Zeeman-Sisyphus deceleration, was also investigated. In this method molecules move through a spatially varying magnetic field and are optically pumped between low- and high-field seeking states in such as a way that they are always losing kinetic energy. The method promises to deliver more slow molecules because the molecules are guided transversely as they are decelerated. A small prototype was built and the optical pumping step was tested successfully. Finally, 7.6*10^3 CaF molecules were trapped in a MOT and cooled to a temperature of 8.5 mK. The radial trap frequency is 2 pi*130 Hz and the damping constant is beta=9.5*10^2 s^-1. The lifetime is about 100 ms and depends strongly on the scattering rate. This MOT is an an ideal starting point for a wide range of new experiments with ultracold molecules.

•Jul 1, 2017•0 citations

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•Jan 1, 2018•0 citations

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Measurement Science and Technology•Jul 14, 2009•17 citations
Arrays of cold atoms in magnetic traps have been implemented for use in quantum simulation, metrology, and ultracold chemistry. The scope of all of these fields could be greatly enhanced with cold and ultracold molecules. Cooling and trapping molecules, however, poses unique challenges compared to atoms. Sympathetic cooling, whereby molecules are cooled via collisions with ultracold atoms, will be a key technique for achieving the required densities and temperatures.\ Towards this end, the Carty group in Durham has constructed a moving-trap Zeeman decelerator (MTZD'), based on the same operating principles as a conventional Zeeman decelerator but producing a truly dynamic three-dimensional trapping potential. We have begun to characterise the decelerator with metastable argon, showing 3D guiding and deceleration, and intend to move to molecules in the near future.\\ In the longer term the decelerator is intended to load a magnetic trap. Presented here are the findings from preliminary work to design a hybrid magnetic trap with magneto-optical trap capability, or MT-MOT'. The MT-MOT comprises six toroidal permanent magnets plus switchable electromagnetic coils. We have performed simulations to determine the feasibility of co-loading lithium atoms and calcium hydride molecules. Some consideration of loss mechanisms and likely trap lifetimes are discussed.
Laser cooling and trapping in a magneto-optical trap (MOT) have been essential to the success of cold atom physics in the last decades. Recently, the application of the same techniques to molecules has begun. The complexity of even a simple diatomic molecule makes laser cooling difficult, but promises new applications in many areas of research. In this thesis I describe the development of the first three-dimensional MOT of calcium fluoride (CaF) molecules. First, a cryogenic buffer gas source was set up, producing a pulsed beam of 9.3*10^10 molecules per steradian per pulse with forward velocities around 170 m/s. A similar source for very large molecules was set up during a 5 month internship at the University of Vienna. Next, the molecular pulse was slowed down to the capture velocity of a MOT using chirped laser slowing, resulting in about 7*10^5 CaF molecules passing through the typical MOT volume of 1 cm^3 at velocities of 15+-5 m/s. A new deceleration method, called Zeeman-Sisyphus deceleration, was also investigated. In this method molecules move through a spatially varying magnetic field and are optically pumped between low- and high-field seeking states in such as a way that they are always losing kinetic energy. The method promises to deliver more slow molecules because the molecules are guided transversely as they are decelerated. A small prototype was built and the optical pumping step was tested successfully. Finally, 7.6*10^3 CaF molecules were trapped in a MOT and cooled to a temperature of 8.5 mK. The radial trap frequency is 2 pi*130 Hz and the damping constant is beta=9.5*10^2 s^-1. The lifetime is about 100 ms and depends strongly on the scattering rate. This MOT is an an ideal starting point for a wide range of new experiments with ultracold molecules.
The aim of our experiment is to explore two methods of creating an ultracold dipolar gas which can subsequently be used to simulate quantum phenomena. The first method is to sympathetically cool polar molecules. In this case, the molecules are overlapped with ultracold lithium atoms, thus allowing the two clouds to thermalise through elastic collisions. The second method is to electrically polarise ultracold lithium atoms using an electric field of approximately 1MV/cm. This involves placing the atoms between two high voltage electrodes. This thesis describes and characterises the setup used to produce, trap and transport a cloud of lithium-7 atoms. The setup consists of a lithium oven, Zeeman slower, magneto-optical trap (MOT) and magnetic trap. Up to 2.3×10^8 atoms are loaded into the MOT with an initial temperature of 1.3 mK. By implementing a compressed MOT phase the temperature is reduced to 0.75 mK. Before transport, 23% of the MOT atoms are transferred into the magnetic trap, which has a lifetime of 1.53±0.01 s in the MOT chamber. Using a motorised translation stage to move the magnetic trapping coils, atomic transport over a distance of 44 cm from the MOT chamber to the science chamber has been demonstrated. The transport efficiency is 41%. In the science chamber the lifetime of the magnetic trap has been measured as 18.5±0.7 s. Experiments to optimise the absorption imaging system have also been carried out, highlighting the fact that a time and position dependent magnetic field is present after the trapping coils switch off. The feasibility of producing a 1MV/cm electric field has been investigated. By using indium tin oxide coated glass electrodes in an adjustable electrode mount, an electric field of approximately 0.2MV/cm has been generated. These electrodes were subsequently replaced with super-polished stainless steel electrodes which generated a field of 0.38MV/cm.
We present a moving-trap Zeeman decelerator (MTZD) for use in molecular beam manipulation and magnetic-trapping experiments of paramagnetic atoms and molecules. The 0.49 m MTZD consists of a combination of a 2D magnetic quadrupole guide and deceleration coils, which together produce an array of 3D magnetic traps. The design is based on that of Trimeche et al. with significant modifications. The 2D quadrupole is driven by fast rising and falling square pulses of current (up to 700 A) of arbitrary lengths of time. The traps can be made to move at velocities from ca. 370 m/s down to zero, which is done by driving through the deceleration coils a sinusoidal current with frequencies ranging from zero to ca. 9 kHz and peak currents up to 1000 A. The trap velocity is proportional to the current frequency. The MTZD manipulates the velocities of molecular beams by traveling initially at the same velocity as the beam. 3D guiding is achieved with a constant current frequency and deceleration is achieved with a downward chirp of the current frequency at a rate corresponding to the desired deceleration. We explain the technical design and operation principles of the MTZD and we detail the custom power electronics that drive the 2D quadrupole guide and the decelerator coils. We present extensive Monte-Carlo trajectory simulations to demonstrate the properties of the MTZD and we conclude that decelerations should be kept below 30000 m/s/s to maintain a good 6D phase-space acceptance. In proof-of-principle experiments, with the deceleration coils operating with a reduced current in the range 100-200 A, we demonstrate the 3D guiding of a beam of metastable argon atoms at 373 m/s and deceleration at 25000 m/s/s from 342 m/s to 304 m/s. The deceleration corresponds to the removal of 21% of the kinetic energy of the beam.
Laser cooling and magneto-optical trapping of neutral atoms has been a driving force in physics over the last few decades by providing an efficient method to produce cold and dense samples of cold atoms. Polar molecules which can have strong, long-range dipolar interactions and possess more rich internal structure promise a wide range of new physics and chemistry studies if they can be controlled efficiently. Until recently, the only way to achieve sub-milliKelvin level temperatures in molecules was to associate two precooled atomic species, which while very effective, has a firm bound on the chemical diversity accessible. While laser cooling of molecules relies on fortuitous molecular properties, it will significantly increase the choices of species available for molecular cooling.In this thesis I present the improved optical deceleration of the molecule yttrium monoxide (YO) and the loading of molecules into a 3-D magneto-optical trap (MOT). By making an improved choice of repumping scheme from our groups’ previous work, our photon scattering rate has been increased. By also improving our buffer gas cell source, we produce a much larger flux (2000 times higher) of slowed molecules at 5 m/s to reach the trapping region. I have designed and implemented 3-D MOT field coils operating at 5 MHz, and demonstrated a successful operation of a 3-D MOT. This is the first 3-D laser trapping of an oxide and the first of a molecule with an electronic state intermediate to the main cycling transition states.I have also performed a study of the transition from the ground state to the intermediate electronic state, which has a narrower linewidth of 150 kHz, in contrast to the main cycling transition’s linewidth of 5 MHz. This transition is well suited to second-stage narrow line cooling after loading molecules with the main MOT transition, which will allow direct laser cooling of a molecule to the 10 uK regime.
To prepare cold lithium atoms for atomic interferometry experiments, we have carried out experimental researches on Zeeman deceleration and magneto-optical trap (MOT) of lithium atoms. We have also designed and implemented a compact adjustable Zeeman slower with an inner water cooling chamber, to decelerate the velocity of the 7Li atom beam from 600 m/s down to 60 m/s, and load them into the MOT. The loading rate is 5108 /s, the total trapped atom number is 1109 , and the lowest temperature of the atom cloud is 220 30 K. Then we investigate the dependence of lifetime of 7Li atoms in optical molasses on the detuning of trapping laser beams. The above results lay a foundation for further sub-Doppler cooling, optical trap based on evaporative cooling, and atomic interferometry experiments.
We present a compact apparatus for laser cooling and trapping of atomic calcium. The set-up consisting of a Zeeman-slowed thermal beam and a magneto-optical trap (MOT) has potential use for studying optical lattices, optical clocks, sensitive trace analysis, ultra-cold collisions and the feasibility of achieving quantum degeneracy in an alkaline earth element. Construction of the apparatus is described as well as the characterization of the trap in terms of atom number and lifetime. The lifetime results are compared with a rate equation model for the system.