Rydberg Atoms

(a) Energy of ground-to-Rydberg state transition as a function of distance from an already-excited Rydberg atom. The detuning caused by the Rydberg atom means that, within a radius rb, the excitation of the ground-state atom is forbidden. This is called “Rydberg blockade”. (b) Using Rydberg blockade can lead to an increase in spatial order of an atomic system. (c) An increase in spatial ordering, here quantified by rb/a, where “a” is the average interparticle seperation, leads to a reduction in electron/ion bunch temperature and a corresponding increase in bunch focusability.

Picture an atom: a positive nucleus with a cloud of electrons.  Add some energy to excite the atom, and you picture the electron cloud at larger radius.  Now picture that electron cloud when you excite not just one step, but several hundred.  You now have a Rydberg atom, an atom where the electron is only barely bound, with a radius of many microns.  It’s a very floppy atom, extremely sensitive to, well, just about everything.  Including any nearby Rydberg atoms.

The 2012 Nobel Prize in Physics was awarded to Serge Haroche for his pioneering studies using Rydberg atoms and microwave photons. They are the basis of quantum computing qubits, precision sensors of weak electric and magnetic fields, and promising as the basis of quantum simulators for studying complex many-body problems.

We are interested in Ryberg blockade, where one atom in a Rydberg state inhibits nearby atoms from also being exciting, creating a “super-atom”.  If we try to excite a bunch of randomly separated atoms to the same Rydberg state, closely-spaced atoms won’t both be excited, so we create a more evenly separated sample of atoms.  Then we ionise the Rydberg atoms, and because they are more evenly spaced, there is less variation in the ion-ion separation and lower spread in the ion-ion electrostatic potential energy. The ions are then colder than for a random disordered bunch.

Colder ions means better focusing for our cold ion microscope (see Cold Atom Electron Ion Source), and a neat quantum trick (called STIRAP) can be used to increase ionisation efficiency and therefore ion beam brightness, bycoherently exciting to a Rydberg state.  We’re also working on using Rydberg blockade to create a quasi-deterministic single-ion source.

For more information contact Rob Scholten.

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