So suppose you are thinking of building an ion trap quantum computer. Since putting all your ions in one trapping region is just silly when you get to large numbers of ions, you are led to think about using multiple ion traping regions. Then you are led to two possible design choices: whether to move the ions between these regions or whether you can couple the ions to flying qubits like photons which can then couple back to ions. If you’re going to do the former, you are quickly led to the notion that it is better to have the qubits packed pretty densely, both for reasons of not wanting a gigantic quantum computer, but also for reducing the amount of moving you’ve got to do. So making small traps is of great import and a lot of groups have been doing just that, with trap sizes a few tens of microns in size.
But, uhoh, what happens if you make your traps smaller? Well one effect is that the heating rate of your ions goes up. This heating is believed to come from fluctuating patch potentials on the trap surface. This noise scales roughly like one over the size of your trap to the fourth power, and so you can imagine that going to small traps might make for some rough heating. And indeed for the small traps you’d like to use for ion trap quantum computers, this heating becomes pretty unbeareble. That is at room temperature it becomes unbareable. But there is evidence that this heating effect is thermally activated which is a fancy way of saying that if you cool your trap down this heating starts to go away. Indeed, Chris Monroes’ group has cooled a needle trap (think two ball point pens pointing at each other) to 150 Kelvin and seen a surpression in this heating rate. Indeed, one of my favorite lines at FTQC II was when Chris Monroe said of this heating that “we’ll get rid of it before we understand it.”
And indeed, today on the archive there is even more good news for getting rid of this noise. In 0706.3763 the ion trapping group of Isaac Chaung lab at MIT describes experiments with a surface electrode trap which has been cooled down to near 4 Kelvin:
Title: Suppression of Heating Rates in Cryogenic Surface-Electrode Ion Traps
Authors: Jaroslaw Labaziewicz, Yufei Ge, Paul Antohi, David Leibrandt, Kenneth R. Brown, Isaac L. Chuang
Dense arrays of trapped ions provide a promising avenue towards large scale quantum information processing. However, the required miniaturization of ion traps is currently limited by sharply increasing decoherence at sub-100 um ion-electrode distances. We characterize heating rates in cryogenically cooled surface-electrode traps, with characteristic sizes ranging from 75 um to 150 um. At 6 K, the measured rates are suppressed by 7 orders of magnitude below room temperature values, and are two orders of magnitude lower than previously published data. The observed heating depends strongly on thermal processing of the trap, which suggests further improvements are possible.
Yep they get 7 orders of magnitude slower heating in their trap by cooling from room temperature to 6 Kelvin (which, I guess, is near 4 Kelvin ) And two orders of magnitude lower than for previously ion traps. Interestingly the effect depends on how the trap is made. Shiny traps are bad, apparently
So is the future of ion trap quantum computing to work at 4 Kelvin? Or might understanding the properties of the annealing of the trap which lead to heating suppresion give clues as to how to design room temperature traps without this heating present?