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Ion Laser Cooling & Trapping

Ion Laser Cooling and Trapping with Diode Lasers

  • Ion traps
  • Ionization of neutral atoms
  • Laser cooling of trapped ions
  • Coherent manipulation of ions

Since their first demonstration in the 1950s, ion traps have become powerful tools for spectroscopy, metrology, fundamental quantum physics experiments and quantum computation. The inventors of the Paul trap and the Penning trap were rewarded with the Nobel Prize in physics in 1989 (Wolfgang Paul and Hans Dehmelt, shared with Norman F. Ramsey) “for contributions of importance for the development of atomic precision spectroscopy”. Ions can be stored for very long times up to many months. Laser cooling of trapped ions is possible in mainly two ways, either Doppler cooling or sideband cooling. The latter is used to cool ions to the vibrational ground state of the trap enabling high resolution spectroscopy. Addressing such laser cooled trapped ions with lasers, one was able to demonstrate quantum gates which are elementary operations needed to realize quantum computers. Quantum logic based on the entanglement between the movement of ions in a trap and their internal electronic state was used to realize the most accurate atomic clock in the world.  Many experiments explore the quantum world with trapped ions and help to understand many different aspects of quantum physics and of the atom light interaction.

Ion traps are powerful instruments to confine charged particles. The Paul trap, named after its inventor Wolfgang Paul, is a 3d trap that uses oscillating electric fields to generate a time averaged energy minimum in space for ions. It is closely related to a quadrupole mass filter, a 2-d ion trap which is widely used in chemical analysis. A Penning trap, on the contrary, uses a combination of electric and magnetic fields to keep ions at a well defined point in space. Modifications of the original trap designs were developed in order to match the requirements of novel types of experiments. Linear traps, for example, are developed for the demonstration of quantum computing. Micro traps or linked traps are promising for the generation of quantum registers. Storage rings, used in accelerators are also special ion trap version. To load an ion trap, one either suddenly switches on the electric fields while ions are within the trapping region or the fields are on all the time and one suddenly ionizes neutral atoms when they are traversing the trap.

Ionization of neutral atoms, making ions out of atoms, is a prerequisite for ion trapping. One way is shoot fast electrons onto atoms and take advantage of inelastic collisions. The fast electrons can kick out electrons from the atomic shell creating an atom with at least one electron less than protons, so a positively charged ion. Instead of electrons, one can also use lasers photons with an energy higher that is above the atomic ionization energy to ionize neutral atoms by inelastic collisions. Since ionization energies are in the several eV range, one typically uses lasers in the deep UV or high power pulsed lasers in the UV. In the latter case, two photons are needed for the ionization. While quite straight forward to implement using electron guns or available lasers, these methods allow one to ionize different isotopes or even different elements. This might be advantageous if many species have to be studied but is a disadvantage if a very special isotope has to be trapped. In the latter case, one uses resonantly enhanced two-photon ionization. In a first step, a photon from a laser with a wavelength that is matched to an electronic transition excites the atom. This can be done element or isotope specific using a narrow linewidth laser. Then a second photon, usually from a different laser (e.g. that laser that will be used for ion cooling) having enough photon energy, ionizes the already excited atom. Such a selective ionization process is also used in resonance ionization mass spectroscopy.

Laser cooling of ions can be performed in different ways. Most common methods are Doppler cooling and Sideband cooling. The first one is analog to Doppler cooling of atoms and uses the Doppler effect-induced imbalance in photon scattering from different laser beams in order to give photon recoil kicks in such a way that the ion looses kinetic energy. Sideband cooling makes use of the high trapping frequencies of ion traps on the order of many MHz. If the linewidth of an electronic transition within the ion is smaller than the trapping frequency, one can selectively address the ion with a narrow linewidth laser in such a way that each internal excitation comes with a reduction in kinetic energy. For this, the laser frequency is adjusted an amount of the trap frequency below the transition frequency of the ion. This way, exciting the ion takes away one quanta of vibration energy (oscillation of ion in the trap). If the confinement of the ion in the trap is very strong (Lamb-Dicke regime) the spontaneous emission does not alter the kinetic energy of the ion which is essentially a Mößbauer effect (Nobel Prize in Physics 1961). So each step of excitation/spontaneous emission can be seen as anti-Stokes Raman scattering and leads to a cooling by one quanta of ion oscillation within the trap.  The excitation stops if the atom is in the lowest vibrational state, the ground state of the ion trap.

Coherent manipulation of ions is needed to perform algorithms for quantum computing and also for special types of precision measurements. Typically, two states of the ion are coupled with one laser or with two phase-locked lasers. This way, as a function of time, the ion state oscillates phase coherently between the two states. If the light is switched on only for a time given by a quarter of the oscillation period (Pi/2 pulse) the ion is in an equal superposition of the two states. So the light acts like a 50/50 beam splitter for states of the ions.  If the light is on for one half of the oscillation period the ion is transferred coherently from one state to the other. It is worthwhile to mention that any superposition between the states can be generated and that the ion states can be entangled internal and external (vibrational) states. The criteria for achieving this coherent operations are quite constraining (laser linewidth < atomic linewidth, oscillation period << than lifetime of the states). In order to fulfill especially the second criterion, instead of one laser acting on ground and excited electronic state of the ion quite frequently two phase-locked lasers act in a Raman-type transition on two ground (or at least metastable) states. 

TOPTICA’s added value
Ion trapping itself relies on static or rf electric and static magnetic fields. So no lasers are needed here. However, selective ionization and cooling require tunable lasers at the right wavelengths, which are usually in the blue or UV spectral range.  Those wavelengths are most conveniently realized by frequency converting high power cw diode lasers. Depending on the ion, frequency-doubled or even frequency quadrupled diode laser systems are needed, usually in conjunction with other lasers for optical pumping or coherent manipulation. TOPTICA offers customized solutions for one requested wavelength. Usually, also complete packages are available for selected ions. If lasers with extremely small linewidth (e.g. Hz) are required, TOPTICA is the right partner for your attempt to actively narrow the linewidth of a special version of an ECDL diode laser. Our locking modules for fast feedback are unique, much faster than any other solution on the market. We also supply a versatile function generator (VFG) which allows one to phase-coherently address lasers (e.g. using AOMs) for coherent manipulation. Discuss with TOPTICA experts that have strong interest and experience in ion trapping details of your experiment.