al atoms came later.
What’s the qubit? The two states needed for computation (“0” and “1”) are two energy levels of the ion. Generally, these are two hyperfine levels of the ground state. These states are generally separated by a small amount of energy, and thus have extremely long lifetimes, long enough that you don’t need to worry about one spontaneously decaying to the other. The exact choice of states is different for different experiments.
How do you manipulate the qubits? In order to do quantum computation, you need to be able to manipulate the states of individual ions. This is done by hitting them with lasers tuned to drive transitions between states. Generally, this is done via “Raman transitions,” which use two lasers with very slightly different frequencies to make a coherent two-photon transition between states. The basic idea is that one photon excites the ion to a high energy state (much higher than either the “0” or “1” states), while the second causes stimulated emission back down into the other of the two computation states. This isn’t quite right– the atoms is never actually in the upper state– but it gets the idea.
You can use this Raman technique to do any excitation you want. You can take an ion in “0” and move it to “1,” you can take an ion in “1” and move it to “0,” or you can take an ion in either “0” or “1” and move it into an arbitrary superposition of “0” and “1” at the same time. It’s all a matter of timing– by leaving the lasers on for different amounts of time, you make different states.
How do you entangle the qubits? The ion trap system offers a novel method for entangling separated bits. In addition to the quantized internal states of the ions, the traps allow collective modes. If you have two or more ions in a trap, they can be sloshing back and forth in the trap in a couple of different ways. This motion is also quantized.
So, if you want to entangle two bits– say, the first one, and the fourth– you can do it using the collective motion as a “data bus.” You do an operation that 