The IMPT’s ‘Quantum Technologies’ research area focuses, among other things, on the miniaturisation of quantum systems. In collaboration with the German Aerospace Centre (DLR) and the Institute of Quantum Optics at Leibniz University Hannover (IQO), the IMPT is developing atomic chips, which are the central components of a quantum gravimeter. A Bose-Einstein condensate is generated on the atom chip, enabling the measurement of Earth’s gravity with the highest accuracy.
The atom chip system
The atomic chip system itself consists of two silicon components with conductor tracks, which are manufactured using microtechnology in the institute’s own clean room by means of etching, coating, gold electroplating and polishing. Finally, a highly reflective mirror coating is deposited on the upper chip in order to reflect the laser beams with as little loss as possible.
The two chips must then be mounted on ceramic carriers that carry the necessary electrical connections. The conductor tracks on the silicon components generate current-induced magnetic fields which, in combination with external laser beams, result in a so-called magneto-optical trap. In this trap, an atomic cloud can be captured, compressed and finally cooled to a Bose-Einstein condensate in a multi-stage process.
In real-world use in a vacuum chamber, the atomic chip system is firmly integrated into a measuring device. Therefore, the operating limits of the system must be precisely known and determined in previous, extensive functional tests. The knowledge gained from this can then also be used to optimise the atom chip system and design the next generation of atom chips. The test series focus primarily on electrical, thermal, magnetic, mechanical, surface and ultra-high vacuum-related properties.
Atom chips extensively evaluated
Electrical | The conductor structures in the chips and ceramic carriers are powered by electrical current to generate the required magnetic field. Therefore, the electrical conductivity of the various materials used is a decisive factor for the operation of the atomic chip system. For verification purposes, the electrical resistance of the gold conductor tracks is precisely determined using a four-wire measurement. This technique offers the advantage that the small resistances of the supply lines can be measured precisely. The current in the gold structures significantly determines the strength of the magnetic field generated, so that with higher current, the Bose-Einstein condensate can be generated in a shorter time. The maximum current carrying capacity of the gold conductor tracks is also investigated until thermal overload occurs.
Thermal | During operation of the atomic chip system, electrical resistance causes heating, which results in intense blackbody radiation emission, impairing the function of the magneto-optical trap. Therefore, the operating temperature is determined. An infrared camera is used for this purpose, which detects the blackbody radiation emitted by the atomic chip system. The temperature of the system can be derived from the intensity and spectral distribution. A second method is used to precisely calculate the temperature in situ based on the change in resistance of the current-carrying gold structures and the thermal coefficient of gold. The advantage of this continuous method is that no additional sensors or wiring are required for temperature monitoring.
Magnetic | The conductor structures in chips and ceramics enable various magnetic field configurations that are necessary for the creation of a Bose-Einstein condensate. Therefore, precise magnetic field measurement is a central component of system characterisation. For the purpose of this characterisation, a 3D printer from the institute is used as a magnetic field mapper: the print head is replaced with a 3D Hall sensor that precisely measures the magnetic field strength in three spatial dimensions, while the stepper motors move the sensor through a defined measurement volume. The result is a precise three-dimensional representation of the magnetic field strength.
Mechanical | The silicon chips and ceramic substrates are permanently bonded together, meaning that failure of the joints under the environmental influences of the application area inevitably leads to complete system failure. Such stresses include, in particular, vibrations and mechanical stresses during handling and transport. An extreme case also occurs, for example, when the atomic chip system is transported into space on a research rocket. To simulate these conditions, a vibration test is performed in which the system is mounted on an electrodynamic shaker and subjected to defined vibration profiles (standard: ECSS-E-HB-32-26A). Afterwards, the natural frequencies of the system are measured before and after the test, as any change is undesirable, and the system is checked for functionality.
Surface quality | Laser beams form the optical part of the magneto-optical trap and are ideally reflected without loss on the upper silicon chip. This is made possible by the highly reflective mirror coating, which is specially adapted to the wavelength of the laser light. The surface quality of the chip and the highly reflective coating itself plays a decisive role in ensuring reflection with as little loss as possible. In order to achieve an approximation of an ideally smooth surface, the entire chip is treated in a chemical-mechanical polishing process using a diamond suspension. This produces an average surface roughness in the double-digit nanometre range, which is verified by confocal laser microscopy.
Ultra-high vacuum | The final application environment of the system is in an ultra-high vacuum, as this is where a Bose-Einstein condensate is generated for gravimetric measurements. In the DLR or IQO laboratories, experiments are carried out in a vacuum chamber that is placed in an ultra-high vacuum to minimise interference from gas molecules in the ambient atmosphere. The setup is also tested on the institute’s own vacuum test bench, but here with regard to the operating temperature and its difference from atmospheric conditions, as well as the suitability of all materials used for the ultra-high vacuum environment. For this purpose, it is essential to determine the outgassing rate of all components, which is done in collaboration with scientists from the Hannover Institute of Technology (HITec).
Future of the atom chip systems
As part of its comprehensive evaluation of the atom chip system, the IMPT identified key characteristics, potential weaknesses and limitations of use, which led to an optimised process plan for the atom chip system. The first improved versions have already been delivered to the DLR and IQO , which is continuing to test the atom chip systems intensively and will subsequently use them for quantum gravimetric experiments. The research clearly shows that a deep understanding of the limitations of use leads to a more reliable and at the same time more sophisticated design of future experiments.
A special aspect of future research work is the integration of the vacuum peripherals and the ongoing miniaturisation of the atom chip system. This development opens up new perspectives for mobile applications and improves the accessibility of quantum technology research.
