Schrödinger’s cat can’t even seem to catch a break. The unfortunate imaginary feline is famous for being alive and dead at the same exact time, as long as it remains hidden inside a box. Scientists have now gone a step further, splitting one living-dead cat between two boxes.
Animal lovers may relax — there aren’t any actual cats involved. Instead, physicists used microwaves to mimic the cat’s weird quantum behavior. The new advance, reported on May 26 in Science, brings scientists a step closer to building quantum computers out of such systems.
Schrödinger’s cat is the hapless participant in a hypothetical experiment dreamt up by a physicist named Erwin Schrödinger in 1935. He imagined that a cat in a closed box with a lethal poison that will be released if a sample of radioactive material decays. After any given amount of time should pass, quantum math can provide only the odds that the material has decayed and released the poison. So from the quantum perspective, the cat happens to be in a state of superposition — both dead and alive. It remains in limbo until the box is at some point opened, and out comes a purring kitty or a lifeless corpse (SN: 10/20/2010, p. 15).
In a real laboratory version of this experiment, microwaves inside a superconducting aluminum cavity take the place of the cat. Inside the specially designed cavity, the microwaves’ electric fields may be pointing in two opposing directions at the same time — just as Schrödinger’s cat can be simultaneously alive and dead. These states are commonly known as “cat states.” Now, physicists have created such cat states in two linked cavities, thereby splitting the cat into two “boxes” at one time.
Though the idea of one cat in two boxes is “kind of whimsical,” states Chen Wang of Yale University, a coauthor of the paper, it’s not that far off from the real-world situation. The state of the cat “is shared in two boxes because it’s a global quantum state.” In other words, the cat is not only in one box or the other, but it stretches out to occupy both.
Because the states of the two boxes are linked — or in quantum parlance, entangled — if the cat turns out to be alive in one box, it must also be alive in the other (SN: 10/20/2010, p. 22). Chen compares it to a kitty with two symptoms of life: just an open eye in the first box and a heartbeat in the second box. Measurements from the two boxes will always agree on the cat’s status. For microwaves, this means the electric field can always be in sync in both cavities. The scientists measured the cat states produced and found a fidelity of 81 percent — a measure of just how close the state was to the ideal cat state. This fidelity is comparable to that achieved in similarly complex systems, the researchers say.
The result is a good step toward quantum computing with such devices. The two cavities could serve the purpose of two quantum bits, also called qubits. One stumbling block for quantum computers is that errors inevitably slip in to calculations due to the interactions with the outside environment that muck up the qubits’ quantum properties. The cat states are more resistant to errors than other types of qubits, the researchers believe, so the system could eventually lead to more fault-tolerant quantum computers.
“I think they’ve made some really great advances,” states Gerhard Kirchmair of the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences in Innsbruck. “They’ve come up with a very nice architecture to realize quantum computation.”
The demonstration of entanglement in the two-cavity system is quite important, states Sergey Polyakov of the National Institute of Standards and Technology in Gaithersburg, Md. “The next step will be to demonstrate that this approach is actually scalable” by adding more cavities to the mix to build a bigger quantum computer.