PRIMUS - precision interferometry with matter waves in zero gravity
PRIMUS is a pathfinder experiment that aims to perform first atom interferometric measurements at extended free evoultion times available in zero gravity. To that end PRIMUS will use a Bose Einstein Condensate as a matter wave source, made available by the drop tower compatible QUANTUS setup.
basic description
Atom interferometers constitute the matter wave analogon to optical interferometers. They often use ultra-cold atoms as a source of coherent matter waves and laser pulses to further control and manipulate the atoms. Matter wave interferometers have been used for extremely sensitive measurements of inertial forces such as accelerations or rotations in many laboratory experiments already. They have also been used in precision experiments of fundamental physics that have determined e.g. the local gravitational acceleration g, Newton´s gravitational constant G or the fine structure constant a. A typical implementation of these matterwave interferometers is shown in Fig.1 which depicts a so called Mach-Zehnder configuration. This configuration applies a sequence of beam splitters and mirrors which are implemented by laser pulses that coherently transfer two photon momenta (or multiples thereof) to the atoms, at either 50% or 100% probability.
In many applications the phase shift between matter waves in the two interferometer arms scales as a function of external potentials or inertial forces and increases with T2, where T is the free evolution time between subsequent laser pulses. For example in a Mach Zehnder interferometer operated as an accelerometer, a residual acceleration a of the atoms a relative to the laser beams of wave vector k leads to an overall phase shift of
Φ = ka T2.
benefits of a drop tower experiment
To guarantee free evolution of the matter waves freely falling atoms are required such as are available in atomic fountain setups. However in a typical earthbound atomic fountain the limited size of the apparatus restricts the time of free fall to few hundreds of miliseconds. Only in a space based experiment i.e. in a zero gravity environment largely extended free evolution times on the order of seconds will become available, which should ultimately allow for matter wave interferometric precision measurements of unprecedented precision.
The PRIMUS experiment aims to evaluate this potential by using the drop tower compatible QUANTUS experiment as a test bed to perform atom interferometric measurements at extend free evoution times on the order of 1 second. The low level of resiudal vibrations that is provided in the drop tower (10-6 g, without additional measures of isolation) is another substantial benefit of the drop tower over other microgravity platforms (ISS, parabolic flights, sounding rockets). Obviously, the limited number of measurements (3 drops per day) imposes a limit on the ability to improve sensitivity by integrating over a large number of measurements. Thus PRIMUS is mostly a pathfinder experiment towards future space born measurements.
towards an atom interferometric test of the universality of free fall
One of the major scientific incentives of PRIMUS is to prepare a test of the Universality of Free Fall, to be carried out by comparing the free fall of two species of atoms from a differential interferometric measurement. This will be done in collaboration with QUANTUS-II by preparing an ultra cold mixture of Rb and K atoms (QUANTUS-II) and by implementing two Mach Zehnder type matter wave interferometers to determine any residual differential acceleration. The drop tower experiments will allow to asses the potential sensitivity increase due to extended free evolution and the degree to which residual vibrations are supressed in the differential measurement. Ultimately this might pave the way for a space based measurement campaign that could achieve a sensitivity at the 10-15 level comparable to the level of accuracy targeted by the MICROSCOPE mission, which uses macroscopic test masses.
application of an optical frequency comb in a dual species atom interferometer
Since the dual interferometer will operate with beam splitter lasers at different wavelengths of 780 nm and 767 nm it will be necessary to provide a phase link between the two laser systems, to obtain an accurate estimate of the differential phase. This will be achieved by means of a femtosecond frequency comb. To that end we have acquired a drop tower compatible frequency comb from Menlo Systems which we use to implement a phase link between our Raman lasers.
