T. B. Mitchell

Laser-cooled ion plasmas in Penning traps

A laser-cooled ion plasma in a Penning trap provides a rigorous realization of a strongly coupled one-component plasma. After a brief review of the crystal structures that have been observed in Penning traps, we summarize two recent experiments. First we describe careful measurements of the stability of the plasma rotation which is controlled by a rotating electric field. We then discuss the excitation of plasma wakes produced by radiation pressure from a laser.

Sympathetically laser-cooled positrons

We present results on trapping and cooling of positrons in a Penning trap. Positrons from a 2 mCi 22 Na source travel along the axis of a 6 Tmagnet and through the trap after which they strike a Cu reflection moderator crystal. Up to a few thousand positrons are trapped and lose energy through Coulomb collisions (sympathetic cooling) with lasercooled 9 Be þ . By imaging the 9 Be þ laser-induced fluorescence, we observe centrifugal separation of the 9 Be þ ions and positrons, with the positrons coalescing into a column along the trap axis. This indicates the positrons have the same rotation frequency and comparable density (� 4 � 10 9 cm � 3 ) as the 9 Be þ ions, and places an upper limit of approximately 5 K on the positron temperature of motion parallel to the magnetic field. We estimate the number of trapped positrons from the volume of this column and from the annihilation radiation when the positrons are ejected from the trap. The measured positron lifetime is >8 days in our room-temperature vacuum of 10 � 8 Pa. Published by Elsevier Science B.V.

Experimental dynamics of stressed, strongly correlated magnetized plasmas

We study the control of the rotation of a laser-cooled ion crystal in a Penning trap by a rotating electric field perturbation. We show that application of a small torque produces sudden angular jumps or “slips” of the crystal orientation spaced by intervals when the crystal is phase-locked or “stuck” relative to the rotating perturbation. The distribution of angular slips is described by a power law, where the power-law exponent depends on the applied torque. We believe this system is driven by a constant force and that small perturbations or thermal effects trigger the slips.

Laser-Induced Wakes in Ion Crystals

Wakes in a Coulomb crystal are produced by “pushing” with radiation pressure on a rotating spheroidal cloud of laser-cooled 9Be+ ions. The wakes are stationary in the lab frame and are caused by the interference of “drum-head” type oscillations. Velocity images of these wakes are obtained directly through the dependence of the ion fluorescence on Doppler shifts, and new analytical calculations accurately reproduce these experimental wake images. The technique demonstrates a way to excite and study modes, that were not accessible with previous techniques.

Quantum information processing in a Penning ion trap

We discuss the potential of the Penning ion trap for quantum information processing. An ion plasma in a Penning trap rotates about the trap (or magnetic field) axis. We have demonstrated phase-locked control of this rotation by a rotating electric field perturbation (a “rotating wall”). This precise control enables the formation of planar arrays of ions that could potentially be used for quantum computation with the Cirac-Zoller scheme, mimicking a “hard disk” geometry. Current experiments with miniature rf traps observe significant heating and decoherence which is believed to be due to fluctuating patch fields on the trap electrodes. This heating should be several orders of magnitude less in the larger Penning trap.