puts the collective motion into a state that depends on the state of the first bit– not moving if the state is “0,” moving if the state is “1.” Then you do an operation on the fourth bit that depends on the state of the motion– flip the bit if the ions are moving, or leave it alone if they’re not. Then you stop the collective motion.
You now have a system in which the state of the fourth ion depends on the state of the first ion. If the fourth ion started as a “0,” it’s a “1” if the first bit was a “1” and a “0” if the first bit was a “0.” And if the first bit was in a superposition of “0” and “1,” the fourth bit is now in a superposition of “0” and “1,” entangled with the state of the first bit.
This lets you do all the operations you need to do to make a quantum computer.
How do you read the result out? You can detect the state of an individual bit by illuminating it with a laser tuned to a “cycling transition,” which drives the ion back and forth between two energy states, absorbing and emitting lots of photons. 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 ra