Error-correcting 'cat qubit' advances practical quantum computing

August 14, 2020 //By Rich Pell
Error-correcting 'cat qubit' advances practical quantum computing
Researchers at Yale University say they have developed a new device that combines the Schrödinger's cat concept of superposition - a physical system existing in two states at once - with the ability to fix some of the trickiest errors in a quantum computation.

The discovery reflects the researchers' latest efforts to master and manipulate the physics necessary for useful quantum computing - correcting the stream of errors that crop up among fragile bits of quantum information, called qubits, while performing a task. The researchers' approach to building a quantum computer - called " circuit quantum electrodynamics (QED)" - employs particles of microwave light (photons) in a superconducting microwave resonator.

In a traditional computer, information is encoded as either a "0" or "1." The only errors that crop up during calculations are "bit-flips," when a bit of information accidentally flips from 0 to 1 or vice versa. The way to correct it is by building in redundancy: using three "physical" bits of information to ensure one "effective" - or accurate - bit.

In contrast, quantum information bits (qubits) are subject to both bit-flips and "phase-flips," in which a qubit randomly flips between quantum superpositions - i.e., when two opposite states exist simultaneously. Until now, quantum researchers have tried to fix errors by adding greater redundancy, requiring an abundance of physical qubits for each effective qubit.

In their approach, the Yale researchers instead used what they call a "cat qubit" - named for Schrödinger’s cat , the famous paradoxical thought experiment used to illustrate the concept of quantum superposition . In the hypothetical scenario, a cat is placed in a sealed box with a radioactive source and a poison that will be triggered if an atom of the radioactive substance decays.

The superposition theory of quantum physics suggests that until someone opens the box, the cat is both alive and dead - a superposition of states. Opening the box to observe the cat causes it to abruptly change its quantum state randomly, forcing it to be either alive or dead.

"Our work flows from a new idea," says Michel Devoret, Yale's F.W. Beinecke Professor of Applied Physics and Physics and senior author of a paper

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