Reading Out Multiple Ions Simultaneously
I'm catching up a little on my reading, so expect a fair number of paper reviews over the next week or so. Most of these will be quantum computing papers with a focus on how large systems are going to develop.
Starting from the top of the stack, Mark Acton and company from Chris Monroe's lab at Michigan just posted the paper Near-Perfect Simultaneous Measurement of a Qubit Register on the arXiv. This is one of a string of excellent papers to come out of Monroe's lab in the last half year or so.
Acton et al. are measuring ions in an ion trap using an intensified CCD. They use the hyperfine states of 111Cd+ ions in a linear trap as the qubits. A photocathode and fluorescent screen are used to intensify the photons coming from the ions themselves, then focused onto a small CCD. In their example, a 4x4 group of pixels is grouped into a single pixel, and a 7x7 block of those is used to detect each ion. Readout time is 15msec for the whole CCD right now; they claim reducing that to 2.5usec/ion is doable with currently available CCDs. One limit in their current setup is the waist of the detection beam is about 10 microns, compared to an ion spacing of 4um. They achieved readout accuracy of 98%, and a significant part of the inaccuracy is believed to be due to neighboring ions influencing each other. Done on a chain of three ions, the paper includes images of binary counting from 000 to 111.
This paper is important because concurrent readout of ions is critical for making quantum error correction work, as well as generally running algorithms faster. Rather than reading the ions in-place in a single trap, of course, the ions could be moved apart first, but that movement can create errors, and of course if they are separated then you have to have multiple laser beams for the detection. Beyond the beam waist problem, it's not clear to me what limits the number of ions this can scale to, but in general we are interested in only a few per trap at a time, so it should work.
When I visited Andrew Steane at Oxford in January, he had an old printout on his door showing pictures of binary states up to some larger number; 63 or maybe even 127? At first glance, this appears to be similar to that work, though I don't know much about the Steane experiments. I suspect those were more about the preparation of the numeric states, whereas this is about how to build a detector that can work reliably for multiple ions. At any rate, the accuracy of Acton's detection is remarkably good, and bodes well for continuing work on ion traps.
[Update, 12/2: the pontiff has a posting on a couple of other recent, good ion trap papers.]