otons. If you tune your laser correctly, ions in “0” will absorb photons from the laser, and then re-emit them, while ions in “1” will do nothing. If you look at the trapped ions with a sensitive CCD camera, you’ll see bright spots at the positions of ions in “0” and nothing at the position of ions in “1.” That lets you read out the values of all the ions in the trap.
Does it scale? Yes and no. It’s possible to put multiple atoms in a single trap, but not enough of them to do useful computations (“useful” here meaning things like “factoring products of 100-digit prime numbers”). It’s also possible to build arrays of lots of little traps, and shuttle ions back and forth between computation and storage regions– Chris Monroe’s group has done some really excellent work in this area.
It’s hard work, but it looks like it’s at least in principle possible to build a large quantum computer using ion traps.
What about decoherence? One of the problems plaguing quantum computation is “decoherence,” a term referring to random interactions with the environment that destroy fragile quantum superposition states, and wreck the operation of the computer. In the ion trap system, the main source of decoherence is heating of the ions in the trap– if you cool all the ions in a trap down to the not-moving state, and hold them for a while, after some time, they start moving. Since this motion is your “data bus,” this is a killer.
Finding the source of this heating has been a long and tedious process. As I understand it, the current thinking is that it’s due to random areas of extra charge on the electrodes making up the trap, which push the ions in different directions.
A lot of hard work has gone into nailing this problem down, and it looks like they finally have a handle on it. The last decoherence rates I remember seeing were still a little too high to do practical quantum computing, but they’ve made great strides, and the future looks promising.
Summary: As I said at the beginning of this, ion traps are the most established of the candidate technologies, and probably the farthest along by most measures. The Wineland group in particular has been working on this stuff for better than twenty years, and they’ve got a lot of the bugs worked out, or at least identified.
They’re not ready to make an ion-based quantum computer yet, and the complexity needed for a large system might mean that it never will be the basis for a practical computer. Ion traps are the best test and demonstration system we have at the moment, and they’re likely to remain so for the forseeable future.
If they were running for President, they would be: Joe Biden. They’ve been around forever, and are a central part of the quantum computing establishment. They may not be the most exciting candidate out there, but nobody doubts that they could get the job done.
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