Schrödinger's cat is the classic physics analogy. You know how it goes: There's a cat hidden inside a box. The box turns out to be some sort of cruel death-box and features a modification designed to dispense cat-killing poison. Whether or not the poison is dispensed is determined by whether or not a radioactive atom decays, an occurrence with the neat property of being random, or about as random as it gets.
The box is an analogy for a fundamental feature of quantum mechanics, which is quantum superposition. There's not really an in-between half-dead, half-alive state for a cat (or most living things), but that's kind of how it goes in the quantum world. Particles get to hang out in many possible states at once—if a particle could be a dead particle, it might just be a half-dead particle. At least until we look at the particle, thus interfering with it. Then, it has to "choose" a state and things are back to normal, at least the way we think of it.
As described in the current issue of Science, a team of physicists mostly hailing from Yale University has added an interesting and potentially quite useful twist to the normal Schrödinger's cat picture: another cat. Here, the cats are actually two microwave fields inhabiting two different cavities, which you might as well imagine as two boxes full of photons (photons being the carriers of the electromagnetic force, and so microwaves). These boxes are then entangled across a distance, with the result being essentially one Schrödinger's cat in two places.
While the cat-in-a-box is usually considered as an analogy, there is a specific physical thing known as a "cat state." It's basically what's described above—a bunch of photons trapped in a box—but all of the photons are sharing the same state, which is a superposition of two states. It's not as macroscopic as a cat, but it's a bit more so than the usual picture of a single superimposed particle. And unlike the half-dead/half-alive cat, this cat state is a real quantum state, a real example of quantum superposition. Two opposites together as one.
"If you look at both boxes together, you can look at it as one big cat state spread across the boxes—or you can look at it as two boxes, each with a cat state correlated such that their fates are entangled to each other," study co-author Yvonne Gao told me.
The entangled property in this case is maybe not so obvious as the things we usually hear about, like particles being in more than one place at once or spinning in opposite directions. Here, the property we want to know about, what's correlated across both boxes, is called parity.
"The question we're asking these two boxes is, together, do you guys have an even or odd number of photons?," Gao said. "We don't want to know the exact number of photons, or the parity of the individual boxes, but we want to know the parity number of photons across these two boxes."
This is much more than a cool trick. Among the more pressing challenges (or even the most pressing challenge) in quantum computation and quantum information, generally, is increasing the amount of information that can be represented. This means increasing the number of particles in the computing system, which is hard to do because quantum states are very, very fragile things.
The total number of photons dealt with in the two-cat experiment topped out at around 100, but as far as maintaining coherent quantum states goes, that's a couple of big honkin' boxes of photons.