Quantum researchers are making an error correction cat

Yale physicists have developed an error-correcting cat – a new device that combines Schrödinger’s cat concept of superposition (a physical system that exists in two states simultaneously) with the ability to repair some of the most troublesome errors in a quantum computation.

It’s Yale’s latest breakthrough in trying to master and manipulate the physics needed for a useful quantum computer: Correcting the flow of errors that occur under fragile pieces of quantum information, called qubits, by performing a task.

A new study reporting on the discovery appears in the journal Nature. The senior author is Michel Devoret, Yale’s FW Beinecke professor of applied physics and physics. The co-authors of the study are Alexander Grimm, a former postdoctoral associate in Devoret’s lab who is now a scientist at the Paul Scherrer Institute in Switzerland, and Nicholas Frattini, a student in Devoret’s lab.

Quantum computers have the potential to transform a variety of industries, from pharmaceutical products to financial services, by enabling computations that are orders of magnitude faster than today’s supercomputers.

Yale – led by Devoret, Robert Schoelkopf, and Steven Girvin – builds on another two decades of groundbreaking quantum research. Yale’s approach to building a quantum computer is called ‘circuit QED’ and uses particles of microwave light (photons) in a superconducting microwave resonator.

In a traditional computer, information is coded as 0 or 1. The only errors that occur during calculations are “bit-flips”, when a bit of information is accidentally changed from 0 to 1 or vice versa. The way to correct it is by building in redundancy: using three “physical” information to guarantee one “effective” – ​​or accurate – bit.

In contrast, quantum information bits – qubits – are subjected to both bit-flips and “phase-flips”, in which a qubit randomly flips between quantum superpositions (when two opposite states exist simultaneously).

So far, quantum researchers have attempted to correct errors by adding greater redundancy, and an abundance of physical qubits required for each effective qubit.

Enter the cat white – named for Schrödinger’s cat, the famous paradox used to illustrate the concept of superposition.

The idea is that a cat will be placed in a sealed box with a radioactive source and a poison that will be triggered when an atom of the radioactive substance decays. The superposition theory of quantum physics suggests that until someone opens the coffin, the cat is both alive and dead, a superposition of states. Opening the box to observe the cat causes it to change its quantum status at random, forcing it to be alive or dead.

“Our work streams from a new idea. Why not use a sensible way to encode information in one physical system so that one type of error is instantly suppressed?” Frege Devoret.

Unlike the multiple physical qubits needed to maintain one effective qubit, a single cat qubit can prevent phase flips by itself alone. The cat qubit encodes an effective qubit in superpositions of two states within a single electronic circuit – in this case a superconducting microwave resonator whose oscillations correspond to the two states of the cat qubit.

“We achieve all this by applying microwave frequency signals to a device that is not significantly more complex than a traditional superconducting qubit,” Grimm said.

The researchers said they are able to change their cat qubit from one of the superposition states to any other superposition state, on command. In addition, researchers have developed a new way of reading – or identifying – the information encoded in qubit.

“This makes the system we have developed a versatile new element that will hopefully find use in many aspects of quantum computing with superconducting circuits,” Devoret said.