Some of the experiments we are pursuing are listed below.  For more information, take a look at my publications or look at the "students" page to read about the projects my students are working on.

Atom Interferometry

If you've taken a few physics courses, you are probably aware of the fact that light is really just an oscillating electric and magnetic field --- an electromagnetic wave.  Because light is a wave it can be made to interfere --- the simplest example is the canonical "two slit" experiment.  If you have taken a few semesters of physics, you should be familiar with the two slit experiment.  If not, take a look at the Physics 2000 web page for a simple introduction.  The interference of light has been used to make extremely sensitive instruments.  For example, optical interferometers can measure the shape of objects with nanometer precision.  They have been used to make precision devices which aid navigation by "feeling" how an airplane is rotating.  In our lab we have used optical interferometry to measure prism deflection angles with an accuracy of 0.0003 degrees.  Optical interferometers were also used to perform key experiments which led to Einstein's theory of relativity.

The story doesn't end with light.  Quantum mechanics tells us that everything in the universe is represented by a wave.  So just like light, atoms are also waves and can be made to interfere.  Due to their mass, however, atoms can have very small wavelengths (in our apparatus the atoms will have a wavelength of 0.01 nm, compared to the wavelength of hundreds of nm for visible light).  The shorter wavelength means that some devices based on optical interferometry could be made even more precise with interfering atoms.  One such device is the rotation sensor or "gyroscope" which I worked on as a postdoc (see "Long term stability of an area-reversible atom-interferometer Sagnac gyroscope").  The fact that atoms have inertial mass means that atom interferometers can measure some things that optical interferometers cannot.  An optical interferometer, for example, cannot measure the acceleration of gravity.  Atom interferometers can.

To make an atom interferometer you first need a source of atoms.  In our interferometer design, shown below, the atoms are produced in a dual-species oven.  This produces a beam of calcium and strontium atoms, shown as a green line in the diagram below.  The atoms streaming out of the oven are transversely laser cooled using a blue laser (the cooling laser beams are indicated with large blue arrows on the left side of the diagram).  The atomic beam is then collimated with a pair of small apertures to produce a thin, well defined beam.   Using two different elements at once will allow certain drifts and errors which are common to both elements to be cancelled out.

The next thing that you need in an interferometer is a way to split and recombine your beam.  In an optical interferometer you can use a partially reflective mirror to split the beam in two, fully reflective mirrors to redirect the two beams back together, and another partially reflective mirror to recombine the two beams and produce interference.  In our atom interferometer we will use laser beams to split and recombine the atomic beam.  These laser beams are shown as red lines on the diagram.  The atomic beam will pass through red laser beams four times.  During the first encounter with a laser beam the atom has a 50% chance of absorbing a photon of light and going into an excited state.  Quantum mechanically, this means that each atom is put into an equal superposition of ground and excited state.  The excited-state part of the wave gets a momentum kick due to the momentum of the absorbed light, causing it to recoil away, effectively splitting the atomic beam in two.  The next three passes through the laser beams cause the two split atomic beams to come back together and interfere.

The state of the atoms exiting the interferometer will be measured by shining a resonant blue laser at the atoms (indicated by the blue arrows at the right of the diagram) and detecting how much of the laser light is scattered by the atoms.  Using this information we will be able to perform several different types of experiments including fundamental studies of the validity of General and Special Relativity under extremely non-relativistic conditions and a search for time variation of fundamental constants.  We will also explore the possibility of using our device as an optical frequency standard (with expected stability comparable to the best atomic clocks), as a precision accelerometer, and as a gyroscope.  Our design has several novel features, including a dual species atomic beam for common-mode error cancellation and the use of precision prisms to reduce alignment drift.  In the process of building the device we have developed several interesting optical methods including a new external cavity diode laser design and a robust, inexpensive scheme to measure prism angles with microradian precision (for more information please go to the "students " page).

Below is a picture of the giant vacuum chamber that we are mounting all of the optics into.  The guy stabilizing the lid is Justin Paul, an undergraduate working on the project.

Laser Cooling and Trapping

The ability to cool and trap atoms using laser light makes possible many of the experiments done in our lab and in labs around the world.  If you've never heard of laser cooling, take a look at the simple description at the  1997 Nobel Prize in Physics page.  Calcium, the atom which we principally use in our experiments, has some peculiarities which make it more difficult to laser cool than many other atoms.  First of all, the wavelength of the principle transition is 423 nm --- which is blue.  And blue lasers are much more difficult to make than red lasers.  More importantly, since the ground state of calcium doesn't have any orbital or spin angular momentum, some of the laser cooling tricks which get your atoms to really low temperatures don't work.  As such, we are considering several alternative methods to reach colder temperatures and higher densities with our calcium atoms.  If these methods work, they will not only improve our experiments but should have an impact on the general progress of calcium-based experiments around the world (including several emerging optical frequency standards).  Some of the methods we've considered include optically pumping calcium atoms into metastable excited states which have angular momentum and trapping calcium atoms in the focus of intense laser beams.

We have recently had some success trapping calcium atoms in the focus of an off resonance laser beam.  To do this we first pre-cool the atoms in what is known as a magneto-optical trap (MOT).  The off resonance beam is focused through the center of the MOT, and generates local energy level shifts in the atoms.  Atoms which are slowed down enough no longer have enough kinetic energy to get out of the light.  This type of trap, commonly known as an optical dipole trap or a far-off-resonance trap (FORT) has been realized in other systems.  But to our knowledge it has never been done in calcium.  Dipole traps in calcium are complicated by the fact that it is harder to laser cool calcium to very low temperatures.  As such the dipole trap laser has to be more intense to produce greater energy level shifts.  Also, the complicated level structure of calcium (compared to the alkali atoms) results in the atoms passing through many energy levels as they are laser cooled in the presence of the dipole trap beam.  This creates the potential for atoms to be accelerated by the dipole trap beam before they end up in the state which we wish to trap them in.

Our recent work demonstrated an optical dipole trap for calcium atoms in an excited singlet D state.  This state has a lifetime of around 1 ms.  This is not long, but is long enough for us to see that the atoms are indeed trapped.  We generated this trap using an argon ion laser emission line.  The laser line we used is relatively close to a transition out of the D state, resulting in large energy level shifts to produce the deep trap we need.  Unfortunately, this also results in a significant rate of two-photon ionization out of our dipole trap.  The good news is that this ionization process allows us to use a charged particle detector to make very sensitive measurements of our dipole trap.

Studies of Ultracold Plasmas

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© Dallin S. Durfee 2004