Most of our superconducting cavities are used in particle accelerators, but once in a while, one takes an unconventional turn. Like the cavity used by Serge Rosenblum’s group at the Weizmann Institute to store a Schrödinger cat quantum state with sizes up to 1024 photons[[there is no sense in which the cat we prepared can be thought of as 1024 qubits - unfortunately]] for unprecedented times. Which is why we like to say that, technically, we built a house for Schrödinger’s cat.
Schrödinger’s cat is a famous thought experiment from quantum mechanics (1935). It was designed to illustrate some of the weird, counterintuitive aspects of quantum theory, in particular the principle of superposition and the role of observation in quantum systems. It does this by connecting quantum physics to our macroscopic human world.
Long story short: A cat is placed in a closed box along with a radioactive substance and a vial of poison. Radioactive decay triggers the release of the poison and kills the cat. If there is no decay, the cat remains alive. According to quantum mechanics, until the box is opened and observed, the cat exists in a superposition of states, both dead and alive, represented by what is called a quantum wave function. It is only upon observation that this wave function collapses, and the cat is found to be either alive or dead.
In a recent preprint, Serge’s group demonstrates storage of a quantum state with up to 1024 photons for more than 30ms [[a single photon qubit (i.e. a superposition of zero or one photons) can be stored for more than 30 ms, but a cat state with a size of 1024 photons can be stored for 50 microseconds ]], which is more than an order of magnitude higher than in previous experiments. The key to such a long lifetime is removing any and all impurities in the superconducting niobium the cavitiy is made of, even minute oxidized surface layers which would absorb the electro-magnetic energy. Using what is probably the world’s smallest electropolishing apparatus we removed the surface oxide layer in a buffered chemical polishing (BCP) process.
While 50 us would be a terribly short lifetime for a cat in our macroscopic world, it is ample time for conducting all kinds of experiments on such a well-defined state in the quantum world. The coherence times of the experiment by Serge and his team could open many new options to quantum information processing, for instance to quantum error correction, quantum memories or quantum networks.
And Schrödinger’s cat is no longer a thought experiment. It is an observable reality.