Cold Atom Physics Controlled by ADwin

Our experiment produces ultacold quantum gases containing either bosons (87Rb) or fermions (40K).  The experiment takes place under high vacuum inside a glass cell.  We collect the atoms in a magneto-optical trap (MOT,) which consists of 6 laser beams for each atomic species and a magnetic trap produced by two external coils with counter propagating current.  While the MOT is on, bright purple LEDs cause light assisted desorption of atoms from the walls of the cell so they can be captured in the MOT.  After a brief moment of optical molasses (lasers on but no magnetic field,) the atoms are gently transported vertically about 5 cm to within 200μm of a magnetic chip trap by changing the shape of the magnetic field with more coils.  Current passing through a wire on the chip, along with external coils, is used to tightly trap the atoms.  Radio frequency signals pass through another wire on the chip, which changes the shape of the trap and allows the hottest atoms to escape thus lowering the average energy of the atoms.  This evaporative cooling can produce quantum degenerate gases, either Bose-Einstein condensates or degenerate fermions.

For some experiments we choose to transfer these cold atoms from the magnetic chip trap to an optical dipole trap – crossed laser beams which catch the atoms as the chip trap is turned off.  This purely optical trap gives us the freedom to adjust the external magnetic field as we please, giving us the ability to address Feshbachresonances and tune the interactions between the atoms.  To image the atoms at the end of an experiment, the trap is turned off, the cloud expands and a pulse of laser light casts a shadow of the atoms onto a CCD camera.  From the size, shape and density of the shadow we can determine the physical properties of the cloud.

Every aspect of our experiment requires precise control.  Voltage controlled acousto-optical modulators alter the frequency and amplitude of the laser light.  Seven external coils as well as wires on our chip create the magnetic field.  The way these magnetic fields switch on and off are important to keeping the atoms cold.  We use voltage-controlled power supplies to control the current through these coils and wires.  Other equipment including radio and microwave frequency sources, shutters and cameras require well timed triggers.

We use an ADwin Real-Time Control System to provide us with precise timing and deterministic control of the processes in our experiment.  The analog channels of the ADwin are used to control current in our coils or wires on the chip and the frequencies and amplitudes of our lasers.  The analog output is programmed to step, ramp or follow an S-shaped curve as desired.  Easy programming of the ADwin’s analog channels provides a simple way to control many devices in our lab.  The digital channels are used to open shutters, trigger frequency sources, flip polarity of current sources and trigger cameras.  Several digital channels are also used to serially program frequency sources.  Signals from the digital channels go to a digital buffer consisting of optical isolators to prevent ground loops from forming in the system.

Looking into the vacuum system, we can see a small glowing MOT cloud. The cloud floats inside a pyrex cell at ultra-high vacuum. Blue (400nm) light-emitting diodes illuminate the cell and create a vapor of rubidium atoms. Immediately around the square glass cell, there are several magnetic trap coils, wound with square white wires. Polarization optics for the laser beams can be seen in the foreground

The Chip as seen from outside the vacuum cell.

The Vacuum System

The ADwin System (Blue case) with the digital isolator above it in the rack