Physicists from the Department of Physics and the Department of Applied Physics at Yale University, the Paul Scherrer Institute and Inria Paris have developed an error-correcting cat — a new device that combines the Schrödinger’s cat concept of superposition with the ability to fix some of the trickiest errors in a quantum computation.
Error-correcting Schrödinger’s cats. Image credit: Michael S. Helfenbein.
Quantum computers have the potential to transform an array of industries, from pharmaceuticals to financial services, by enabling calculations that are orders of magnitude faster than today’s supercomputers.
The team’s approach to building a quantum computer is called circuit QED and employs photons in a superconducting microwave resonator.
In a traditional computer, information is encoded as either 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 — 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.
Enter the cat qubit — named for Schrödinger’s cat, the famous paradox used to illustrate the concept of superposition.
“Our work flows from a new idea,” said Yale University’s Professor Michel Devoret, senior author of the research.
“Why not use a clever way to encode information in a single physical system so that one type of error is directly suppressed?”
Unlike the multiple physical qubits needed to maintain one effective qubit, a single cat qubit can prevent phase flips all by itself.
The cat qubit encodes an effective qubit into 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 of this by applying microwave frequency signals to a device that is not significantly more complicated than a traditional superconducting qubit,” said first author Dr. Alexander Grimm, a scientist at the Paul Scherrer Institute.
The researchers are able to change their cat qubit from any one of its superposition states to any other superposition state, on command.
In addition, they developed a new way of reading out — or identifying — the information encoded into the qubit.
“This makes the system we have developed a versatile new element that will hopefully find its use in many aspects of quantum computation with superconducting circuits,” Professor Devoret said.
The team’s work was published in the journal Nature.
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A. Grimm et al. 2020. Stabilization and operation of a Kerr-cat qubit. Nature 584, 205-209; doi: 10.1038/s41586-020-2587-z
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