Schrödinger’s Cat 2.0: New Thought Experiment Combines Quantum Entanglement And Superposition

If you thought Erwin Schrödinger’s famous (or infamous) cat-killing thought experiment could not get any more bizarre, think again. A team of researchers has now come up with a new twist to the experiment — one that proves that not only is the fickle feline both alive and dead until someone observes it, it is also in two places at once.

In the original version of the thought experiment, which Schrödinger came up with in 1935 to discredit what he felt were apparent contradictions in the interpretation of quantum mechanics, a cat is trapped in a sealed box along with radioactive material and a vial of poison that will shatter if an atom of the radioactive substance decays. If the laws of quantum physics are extrapolated, they tell us that until someone opens the box, the cat would be in a state of “superposition,” wherein it would be simultaneously alive and dead.

Although this statement seems absurd, it describes a very real behavior that subatomic particles exhibit. In fact, superposition is one of two basic properties of subatomic particles that researchers hope to utilize in building a quantum computer, with the other being quantum “entanglement,” which Albert Einstein once derisively called “spooky action at a distance.”

The new version of the experiment, described in a study published in the journal Science, marries these two central concepts of quantum physics.

The experiment, which scientists have been envisioning for the past two decades, puts the cat in two boxes. In this particular case, the “cat” is made of a standing wave of two separate wavelengths of microwave light confined in two cavities, such that hundreds of particles simultaneously exhibit two distinctly different “states.”

So, if you want to figure out whether the cat is dead or alive, you would need to open both the boxes.

“This cat is big and smart. It doesn't stay in one box because the quantum state is shared between the two cavities and cannot be described separately,” lead author Chen Wang, a postdoctoral associate at Yale University, said in a statement. “One can also take an alternative view, where we have two small and simple Schrödinger's cats, one in each box, that are entangled.”

Not only does the new twist add another level of quirkiness to the already counterintuitive experiment, it also gives researchers an opportunity to investigate significant real world implications.

Scientists have long been trying to create quantum computers that exploit the inherently unstable nature of quantum particles, or “qubits,” and can therefore perform operations many orders of magnitude faster than conventional computers. However, in their efforts toward doing so, they have hit a formidable hurdle — under the rules governing quantum particles, the very act of observing a system to check for mistakes is likely to create an error, as it would alter the spin of the entangled particles. This makes error checking — a basic operation of any computer — a seemingly insurmountable problem.

“It turns out 'cat' states are a very effective approach to storing quantum information redundantly, for implementation of quantum error correction,” Robert Schoelkopf, director of the Yale Quantum Institute, said in the statement. “Generating a cat in two boxes is the first step towards logical operation between two quantum bits in an error-correctible manner.”