New Quantum 'Cat State' Can Be in Two Places at Once

Image: Michael S. Helfenbein/Yale University
Image: Michael S. Helfenbein/Yale University

Chances are you’re familiar with the Schroedinger’s cat paradox, whereby a hypothetical cat inside a box is both dead and alive until we open the box to see for ourselves. Now physicists at Yale University have figured out how to make a quantum cat that both lives and dies in two boxes at once. They described their work last week in Science.


Technically, this isn’t an actual cat, but a so-called “cat state” in which two (or more) particles are in two different states at the same time. For decades, Schroedinger’s cat was just a morbid thought experiment, but in 2005 physicists at the National Institute of Standards and Technology successfully created an actual “cat state” in the laboratory. They used six atoms all in simultaneous “spin up”and “spin down” states—think of it as spinning clockwise and counter-clockwise at the same time. Since then, other physicists have created their own large cat states with photons.

The Yale physicists have added a new twist: not only are the photons in a superposition of states, they are also entangled, meaning that changing the state of one will change the state of the other, even if they are separated. It’s one of the strangest aspects of quantum mechanics. Albert Einstein dubbed it “spooky action at a distance.”

To create the state, they built a small chamber with two separate cavities made of aluminum. The microwave photons bounced around inside the cavities, and the team managed to connect them with a sapphire superconducting artificial atom. The result: two alive-and-dead cats made of microwave light in two different cat boxes at the same time.

“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 said in statement. “One can also take an alternative view, where we have two small and simple Schroedinger’s cats, one in each box, that are entangled.”

This research has implications for quantum computing. Unlike a classical computer, with bits representing 0's and 1's, a quantum computer stores information in “qubits.” A qubit can be in two states at once, both 0 and 1, just like Schroedinger’s cat is simultaneously alive and dead until a measurement or observation is made. But it’s a very delicate state. The quantum information must be shielded from all external noise in the surrounding environment. The slightest bit of interference—a single photon bumping into the atom you’ve used to encode and store your information, for instance—will cause the entire system to “decohere,” such that the superposition is lost.


Cat states are of interest in part because they could be very useful for storing quantum information. And being able to create cat states in two different boxes is “the first step towards logical operation between two quantum bits” that also enables error correction, according to co-author Robert Schoelkopf.


Video: Yvonne Gao/Yale University



Fun fact: “Schrödinger’s Cat” is a thought experiment proposed by physicist Erwin Schrödinger in 1935 as a tongue-in-cheek critique of the Einstein, Podolsky , Rosen paper on quantum entanglement. It is a reductio ad absurdum argument against macroscopic quantum entanglement. Here is what Schrödinger actually said in a journal article:

One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter, there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer that shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.

It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naively accepting as valid a “blurred model” for representing reality. In itself, it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.