Short answer: Resolved-sideband cooling drives a trapped ion toward its motional ground state. Three experiments report it: a single calcium-40 ion in a room-temperature macroscopic rf trap reached 99 +/- 1% ground-state population (about 6 microkelvin), a calcium-40 ion in a Penning trap reached 98(1)% in one dimension, and a separate experiment cooled an ion using radio-frequency radiation instead of lasers, reaching 88(7)%. These are single-ion laboratory results.
Room-temperature macroscopic trap
An arXiv paper reports resolved sideband laser cooling of a single calcium-40 ion in a macroscopic linear radio-frequency trap with a radial diagonal electrode spacing of 7 mm and an rf drive frequency of just 3.7 MHz. For an oscillation frequency of 585 kHz along the rf-field-free axis, the authors report a ground state population of 99 +/- 1%, corresponding to a temperature of only 6 microkelvin. For oscillation frequencies in the range 285 to 585 kHz, they measure heating rates below one motional quantum per second at room temperature. They say the lowest measured heating power is about an order of magnitude lower than reported previously in room-temperature and cryogenically cooled traps.
Penning trap
A 2016 Physical Review Letters paper reports laser cooling of a single calcium-40 ion in a Penning trap to the motional ground state in one dimension. Cooling is performed in the strong binding limit on the 729 nm electric quadrupole transition, broadened by a quench laser. The authors find a final ground-state occupation of 98(1)% and a heating rate of about 0.3(2) per second for trap frequencies from 150 to 400 kHz, which they say is consistent with the large ion-electrode distance.
Cooling with radio-frequency radiation
A 2015 Physical Review Letters paper demonstrates ground-state cooling of a trapped ion using radio-frequency radiation. The authors present it as a tool for quantum operations in which rf or microwave radiation, instead of lasers, drives motional quantum state engineering. They measure a mean phonon number of 0.13(4) after sideband cooling, corresponding to a ground-state occupation probability of 88(7)%. They then demonstrate motional state engineering by driving Rabi oscillations between the zero and one phonon states. They also use the ground-state cooling to measure the motional heating rate and report a reduction by almost two orders of magnitude from their earlier result, which they attribute to carefully eliminating sources of electrical noise.
How occupation is measured
In the rf experiment, the authors measure population after sideband cooling during a frequency scan over the red sideband and another over the blue sideband. The imbalance between the two scans gives the mean phonon number.
Limits
The three papers use different ions, trap types and cooling methods, so the occupation figures are not a ranking. Each is a single-ion result. They do not show cooling of long ion chains.
FAQ
What is the goal of sideband cooling?
To bring a trapped ion's motion to its ground state so that quantum operations on the motion start from a known state.
Can it work at room temperature?
An arXiv paper reports 99 +/- 1% ground-state population in a room-temperature macroscopic rf trap.
Does it need lasers?
A 2015 paper reports ground-state cooling with radio-frequency radiation, reaching 88(7)%.
How low was the heating rate?
Below one motional quantum per second in the room-temperature trap, and about 0.3 per second in the Penning trap, per the two papers.
Is this done for many ions at once?
The papers report single-ion experiments.
All field notes