Physicists from Yale University have developed an "error-correcting cat". This device combines the concept of superposition from the famous Schrödinger's cat experiment, and the ability to fix some of the persisting problems with quantum computation.
The Yale team presented their report regarding the stabilization and operation of a Kerr-cat qubit, or their error-correcting cat, in the latest Nature journal. Michael Devoret, FW Beinecke Professor of Applied Physics and Physics, serves as the senior author for the study.
He is joined by Alexander Grimm, a former postdoctoral associate in Devoret's lab and now a tenure-track scientist at the Paul Scherrer Institute in Switzerland, and Nicholas Frattini, a graduate student. Shruti Puri, Shantanu Mundhada, and Steven Touzard from Yale also worked on the study and Shyam Shankar of the University of Texas - Austin and Mazyar Mirrahimi of Inria Paris, France.
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The Cat Qubit
In quantum computing, the basic unit of information is the qubit, an analog to the bits in classical computers. The classical bit is identifiable in its two independent states: either 0 or 1. The most common error with the bit is the bit-flip, which occurs when a zero accidentally turns into one or vice versa. It is corrected through redundancy, using additional bits to check whether the target bit is accurate upon transfer or computation.
Qubits, however, can be in any combination of a one and zero at the same time. Aside from its version of a bit-flip error, qubits can also experience a phase-flip error. The slightest external signal, or noise, can throw off a qubit's value—either bit or phase-wise.
Before the Yale cat qubit, physicists have tried to rectify bit qubit errors through a similar redundancy approach as the classical bit. However, qubits require more qubits to ensure the accuracy of a single qubit.
"Our work flows from a new idea. Why not use a clever way to encode information in a single physical system so that one type of error is directly suppressed?" Devoret asked in a Yale news release.
Whereas existing redundancy methods require additional physical qubits to check a single qubit, the cat qubit can prevent phase-flips on its own. It encodes an effective qubit into superpositions of two states in a single electronic circuit. The researchers generated and stabilized these states by using the Kerr nonlinearity and a single-mode squeezing technique in a superconducting microwave resonator.
"We achieve all of this by applying microwave frequency signals to a device that is not significantly more complicated than a traditional superconducting qubit," Grimm explained. They reported that the cat qubit could completely shift its superposition state to any other state as needed, on command.
Schrödinger's Cat
The Schrödinger's cat experiment is a thought experiment first proposed by the Austrian-Irish physicist Erwin Schrödinger back in 1935. It challenges the Copenhagen interpretation of quantum mechanics, applied to a macroscopic setting.
In the thought experiment, a cat, a container of poison and a radioactive source are all inside a sealed box. Once the internal monitor detects radioactivity, the poison is released, thus killing the cat. According to the Copenhagen interpretation, the cat in the container can be thought of as both dead and alive. However, once observed, the cat will be either dead or alive.
It illustrates the nature of quantum superposition described by the Copenhagen interpretation - a quantum system exists as a combination of two or more states (superposition) and remains this way until it interacts with, or is observed, by an external observer. At that point, the quantum system collapses into one of the particular states.
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Coincidentally, Erwin Schrödinger was born on this day, August 12, 1887, in Vienna, Austria.
To better visualize how the famous Schrödinger's cat experiment, see below a video from TED-Ed:
