Some time back, I wrote about what you need to make a quantum computer. Given that it’s election season, I thought I’d revisit the topic by looking in detail at the candidate technologies for quantum computing.
The first up is Ion Trap Quantum Computing, probably the most well-established of any of the candidates. The field really starts with Dave Wineland’s group at NIST, though there is outstanding stuff being done by Chris Monroe at Maryland, and a host of others.
So, how do they stack up? Here are the facts about ion traps as a quantum computing system:
What’s the system? Ion traps are, as the name suggests, devices for trapping atoms that have lost one or more electrons. They use the fact that these ions interact very strongly with electric fields to confine them to small volumes using high-voltage electrodes.
Once they’re stuck in the trap, the ions can be cooled to very low temperatures using laser cooling. Laser cooling was actually first demonstrated with ions, by the Wineland group– cooling of neutral 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 oth