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 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 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 ph