The Magnesium experiment includes a wide range of physical research fields. Following are the main research projects undertaken by us.
Laser Cooling And Trapping of Magnesium
Alkaline earth atoms have a versatile spectrum of optical transitions reaching from very fast cycling transitions to extremely narrow intercombination lines between the two different spin systems. Fig. 1 shows a simplified level scheme of 24Mg including the singlet system and the triplet system, respectively. These intercombination lines can be used for high resolution spectroscopy in the optical domain. In 24Mg we use the fast 1S0-1P1 transition for cooling and trapping the atoms, which are then probed on the 1S0-3P1 transition at 457 nm with a linewidth of only 31 Hz.
Cold atomic ensembles with well defined properties are starting point for many modern experiments in atom optics. Magneto-optical traps (MOT) thus became a standard technique to pre-cool an atomic cloud to the Doppler-limit.
24Mg possesses two valence electrons which creates a singlet and a triplet level system (see Fig. 1). The 1S0 -1P1 transition is the closed transition on which the MOT operates. In order to address this transition, we use a dye laser operating at 570 nm followed by a second-harmonic generation (SHG) in a nonlinear crystal which creates laser light at 285 nm (see Fig. 2). The singlet MOT cools an ensemble of about 109 Magnesium atoms to a temperature of 3 mK. The cold ensemble can be probed on the narrow intercombination line 1S0 -3P1 at 457 nm with a narrow linewidth laser that is stabilized to an ultrastable cavity. The atoms excited to the 3P1 state are detected by another MOT operating between the triplet states 3P2 -3D3. The 3P1 and 3P0 states are repumped to the cycling transition by independent repump beams. A diode laser system at 766 nm followed by a second harmonic generation stage is used to produce light at 383 nm. The triplet MOT allows a background-free fluorescence detection of even a small number of atoms excited to the triplet manifold. This is useful to detect small atom numbers in our interferometry sequence (see below).
The temperature of atoms in the triplet MOT is 1 mK and contains 108 atoms. This lower temperature and a smaller spatial size allow us to load 105 atoms into an optical dipole trap created by a single focused laser beam at 1064 nm generated from a fiber laser with 50 W output power. The ensemble in the dipole trap has a temperature of 100 µK.
In the future, the dipole trap will be used to study the molecular properties of Magnesium and also as an intermediate step to load atoms in a magic wavelength lattice to create an optical lattice clock with Magnesium.
Cold Atom Magnesium Optical Frequency Standard
We are investigating a frequency standard based on cold free-falling 24Mg atoms implemented in a Ramsey-Bordé-interferometer geometry.
The interferometer operates between the 1S0 ground state and the 3P1 metastable state. The interferometer consists of four π/2-pulses with two pairs of counter-propagating beams (see Fig. 4). The first pulse creates a superposition state of the ground and excited state. The momentum transferred by the light pulse during this process leads to a spatial separation of these states. The interferometer is closed by another pair of pulses which provide a momentum kick in the opposite direction. At the end of the interferometer sequence, the fraction of the atoms in the excited state is detected by fluorescence after capturing the atoms in a triplet MOT. This gives the absolute number of atoms in the excited state.
The sequence is repeated for different detunings of the clock laser around central frequency of the transition. The result is an oscillating signal in the Lamb dip called the “Ramsey-fringes”, as can see in Fig. 5. During steady operation, the laser stabilized to the central fringe.
In atom optics, compared to a classical interferometer, the roles of light and matter are exchanged. The deBroglie waves associated with quanta play the role of the light rays of classical optics, while laser light fields serve as optical elements like beam splitters. Effects associated with classical light waves can be observed in the matter wave regime, e.g. interference and diffraction. Atom optics, in contrary to classical optics, exploits the potential of the inner structure of the atoms. Unlike classical particles, atoms exhibit a complex and discrete spectrum of inner states. These can be probed and manipulated by interaction with external fields. The extreme sensitivity of the involved effects facilitates a rich diversity of high-precision measurement methods at the quantum limit.
Ultrastable Optical Resonators
For interrogation of the narrow intercombination transitions in Magnesium ultrastable laser systems with very narrow linewidth in the Hz regime are required. This is achieved by stabilizing diode lasers to ultrastable optical resonators. These resonators are designed to be very insensitive to environmental perturbations such as vibrational noise or temperature fluctuations. For this purpose, special materials such as Ultra-Low Expansion Glass (ULE) are used and resonator designs are developed with the help of finite element simulations. The experimental setup must provide a very stable environment for the resonators, therefore these are placed into vacuum chambers on active vibration isolation platforms and are encapsulated in a multistage thermal isolation.
Since the performance of state-of-the-art optical clocks is limited by the interrogation lasers, the search for methods to improve the performance of the ultrastable laser systems is a research field of high importance.
In our research group we investigate new materials and approaches to improve the overall attainable performance of these ultrastable resonators.
State-of-the-art optical resonators are stable to one part in 1015 or better in 1 s and are limited by thermal fluctuations in the resonator material. 10-15 stability in frequency is, in analogy, comparable to the stability of the length of one meter to the radius of an atomic nucleus.
Phase Stable Transfer Via Optical Telecommunication Fiber Link
The dissemination of accurate clock signals is important for the practical utility of a clock. The complex setup of an optical clock makes it difficult to transport it from one location to another. Methods for comparing optical clocks over long distances at the demanding level of accuracy and stability thus becomes an important area of research.
The direct transfer of ultrastable optical frequencies via optical fiber links is, among other techniques like GPS transfer, a very promising method. For a successful transfer, the fiber needs to be actively phase-stabilized to cancel out the frequency fluctuations imprinted on the fiber by acoustic, seismic and thermal noise.
In cooperation with the Physikalisch-Technische Bundesanstalt (PTB) we established a 70 km long fiber link in a telecommunication network, connecting PTB with our institute (see Fig. 7). We are currently characterizing our Magnesium optical frequency standard via this fiber link, thus allowing a remote comparison to an optical frequency standard, a hydrogen maser and a Cs fountain clock at PTB.
We have demonstrate the potential of long-distance telecommunication fiber links to compare ultrastable optical frequencies at the 10-15 level in a few seconds.