A growing subfield of atomic physics is focused on the miniaturization of Bose-Einstein condensate (BEC)-producing apparatus. Perhaps the largest step forward in this area was the development of the so-called “atom chip.” These microfabricated devices can create tight traps for neutral atoms. In 2018, NASA’s Cold Atom Laboratory (CAL) was launched and installed aboard the International Space Station (ISS). It utilizes a modified miniature atom chip apparatus produced by ColdQuanta called the RuBECi. In this thesis, we present progress towards achieving BEC in a Bates-owned RuBECi, particularly focusing on the laser cooling and magneto-optical trapping stages of the BEC process.
A growing subfield of atomic physics is focused on the miniaturization of Bose-Einstein condensate (BEC)-producing apparatus. Perhaps the largest step forward in this area was the development of the so-called “atom chip.” These microfabricated devices can create tight traps for neutral atoms. In 2018, NASA’s Cold Atom Laboratory (CAL) was launched and installed aboard the International Space Station (ISS). It utilizes a modified miniature atom chip apparatus produced by ColdQuanta called the RuBECi. In this thesis, we present progress towards achieving BEC in a Bates-owned RuBECi, particularly focusing on the laser cooling and magneto-optical trapping stages of the BEC process.
Since their initial production in 1995, the study of Bose-Einstein Condensates (BECs) has offered exciting new insights into many fields including quantum mechanics and solid state physics. Atom chips, non-conducting surfaces with conducting lithographic traces printed on them, have proven successful in generating lower power magnetic traps for the formation of BECs. The combination of their lower power consumption, small form factor, and dense traps makes atom chips a promising way to face a ubiquitous challenge in the study of BECs: the effect of the earth’s gravitational field. To avoid these effects the NASA Cold Atom Laboratory aims to create BECs aboard the International Space Station (ISS) using an atom chip. This thesis presents the development of a similar atom chip based BEC experiment at Bates College, which will serve as a functional testbed for future research aboard the ISS. Both a theoretical background and a description of the experimental process by which a BEC is formed on an atom chip are provided. This process is primarily driven by laser cooling and magnetic trapping techniques. A 2D+ Magneto-Optical Trap (MOT), loaded from vapor pressure, is used as a slowed source of 87Rb in order to create a 3D MOT. We present measurements of the performance of both MOTs, and outline a path forward to condensation on the chip-based magnetic trap through use of radiofrequency evaporative cooling.
To simplify applications that rely on optical trapping of cold and ultracold atoms, ColdQuanta is developing techniques to incorporate miniature optical components onto in-vacuum atom chips. The result is a hybrid atom chip that combines an in-vacuum micro-optical bench for optical control with an atom chip for magnetic control. Placing optical components on a chip inside of the vacuum system produces a compact system that can be targeted to specific experiments, in this case the generation of optical lattices. Applications that can benefit from this technology include timekeeping, inertial sensing, gravimetry, quantum information, and emulation of quantum many-body systems. ColdQuanta’s GlasSi atom chip technology incorporates glass windows in the plane of a silicon atom chip. In conjunction with the in-vacuum micro-optical bench, optical lattices can be generated within a few hundred microns of an atom chip window through which single atomic lattice sites can be imaged with sub-micron spatial resolution. The result is a quantum gas microscope that allows optical lattices to be studied at the level of single lattice sites. Similar to what ColdQuanta has achieved with magneto-optical traps (MOTs) in its miniMOT system and with Bose- Einstein condensates (BECs) in its RuBECi(R) system, ColdQuanta seeks to apply the on-chip optical bench technology to studies of optical lattices in a commercially available, turnkey system. These techniques are currently being considered for lattice experiments in NASA’s Cold Atom Laboratory (CAL) slated for flight on the International Space Station.
Atom chips are an excellent tool for studying ultracold degenerate quantum gases, due to the high degree of controllability afforded by the precise potentials generated from the current-carrying microfabricated wires on the chip surface. The geometries of the trapping potentials are inherently capable of realising extreme aspect ratios, and therefore creating model systems with effectively reduced dimensionality, particularly the theoretically-tractable one-dimensional Bose gas. In addition, the temporal tunability makes it possible to impart non-adiabatic changes on the trapping potentials, allowing experimental investigation of samples which have been brought out of equilibrium - a situation which is not fully theoretically understood. This thesis describes the implementation, development and characterisation of an experimental system for producing the first Bose-Einstein condensates of atomic rubidium 87 gas trapped on the surface of an atom chip in Nottingham. Such an apparatus is very complex and requires careful characterisation in order to run in a stable and reliable way. Details of the experimental setup are thoroughly outlined, including the vacuum system, lasers, electronics, computer control and timing, and the optical imaging system. A newly installed compact two-dimensional magneto-optical trap provides an loading rate of 5e7 atoms per second for loading a three-dimensional mirror-magneto-optical trap with 1.5e8 atoms, at a temperature of 300uK within 10s. The cloud is then sub-Doppler cooled to 50uK, and spin-polarised with 96% purity into the |F=2,mF=+2> ground state within 5ms, in preparation for loading a purely magnetic trap. A millimeter sized copper Z-shaped conductor located beneath the atom chip surface creates a Ioffe-Pritchard magnetic trap, into which the laser cooled cloud is loaded with 70% efficiency, and can be held with a vacuum-limited lifetime of 40s. Evaporative cooling then pre-cools the sample to below 20uK within 10s, to allow the subsequent loading into potentials created by the atom chip with 100% efficiency. A final evaporation stage then cools the cloud below the phase transition temperature of 800nK, resulting finally in pure BECs with 105 atoms confined using the atom chip. Key measurements of various properties of the trapped condensates are presented, which are important in order to characterise the system fully, and to compare with theoretical expectations. In particular, included are the variation of condensate fraction with temperature, the BEC expansion dynamics, and the condensate lifetime in the trap, for example. Finally, it is demonstrated how BECs can be produced on the atom chip without the use of external macroscopic coils, achieved by using novel, integrated sheet structures located beneath the chip surface - unique to this experimental system - to create the necessary bias fields.
The development of an experimental apparatus to produce Bose-Einstein condensates (BECs) of ⁸⁷Rb atoms and their application to magnetometry are discussed. Optical detection of atomic Larmor precession is a widely explored method for high-sensitivity measurements of magnetic fields. In this context, short laser/atom interaction time, atomic thermal diffusion and decoherence effects are among the main limitations. In this thesis, we overcome such problems by using spin-polarised ⁸⁷Rb ultra-cold atoms as the sensing element. After the atoms are polarised, a resonant pulse of radio-frequency excites Larmor precession, which is sensitive to external magnetic fields. By measuring the perturbations of the radio-frequency induced spin precession, information on the magnetic fields of interest. This is achieved by monitoring the polarisation plane’s rotation of a linearly polarised resonant laser probe. In the first part of this thesis, the building and optimisation of a laser-cooling set up to obtain a BEC in a hybrid trap is reported. In order to achieve the Phase Space Density (PSD) required for BEC, several different stages of trapping and cooling are necessary. Each phase has been implemented and optimised. The first step consists in the magneto-optical trap (MOT). Here a velocity dependent damping force and a spatially dependent confining force give the largest changes in PSD. Then atoms are loaded into a hybrid trap obtained by overlapping a quadrupole magnetic potential and a far detuned optical crossed dipole trap. The final stage for the condensation consists of forced evaporative cooling, both via magnetic and optical evaporation. In the second part of the thesis, a general overview of the principles of optical atomic magnetometry is provided and the advantages of using ultra-cold atoms with respect to conventional thermal vapours are discussed. The implementation, operation and a preliminary characterisation of the ultra-cold atom magnetometer are described along with the preliminary results collected. Finally, a plan for future improvements of its sensitivity is presented.
Bose-Einstein condensates (BECs) have been proposed for many applications in atom interferometry, as their coherence over long evolution times promises unprecedented sensitivity. To date, BECs can be efficiently created in devices using atom chips, but these are still complex and place high demands on size, weight and power. To further simplify these setups, we equipped an atom chip with a nano-structured diffraction-grating to derive all beams for the magneto-optical trap (MOT) from a single laser beam. Moreover, using a 2D+-MOT as an atomic source and a beam with uniform intensity for the grating illumination, we capture 1×109 atoms in one second, cool them to 14μK, and demonstrate magnetic trapping using the atom chip. This is a major step towards the simplification of portable BEC devices for quantum sensing on earth and in space.
We construct a compact experimental apparatus for producing high-repetition-rate ultra cold 87Rb atom based on an external atom chip. Initially, we produce an atomic flux from a 2D+ MOT (magneto-optical trap) in a nearly isolated 2D+ MOT chamber and improve the flux by adjusting the cooling beam detuning of the 2D+ MOT. The flux is trapped in the 3D MOT for 1.2 s and is further cooled by performing compressing the MOT for 20 ms and using polarization gradient cooling for 6.2 ms. After optical pumping into the 5S F = 2 (m F = 2) ground state, we transport atoms toward the chip surface by performing external Z coil trapping. At 1 mm below the chip surface, the external Z coil trap is completely switched to an atom chip trap by using a Z wire and a dimple wire on the chip with external bias fields. The transport and the switching take totally 520 ms. The number of atoms trapped on the atom chip is measured to be about 2 × 107. After the atoms has been compressed for 100 ms to increase trap frequencies, we perform RF evaporative cooling in the RF frequency range from 32 MHz to 6.82 MHz for 2.4 seconds. The final number of atoms in the atomic cloud after RF cooling is 1.2 × 105 atoms, and we are able to observe some Bose-Einstein condensation. The repetition rate for producing the condensation is 0.234 Hz. In this paper, we describe our experimental apparatus and processes.