Experiment Success: Twenty Year-Old Protocol Doubles Quantum Bit – Qubit – Lifetime
Quantum error correction achieved using oscillator grid states
Selected notes ~
+ A practical implementation of a quantum error correction protocol first proposed back in 2001 has been achieved by physicists in the US and France. The protocol increases the coherence time of quantum memory and although the work is still preliminary, it could potentially allow for the much more economical use of quantum bits (qubits) in quantum computers.
Preskill is impressed. “Until now they’ve only demonstrated a fairly modest improvement over what they can do without using these special error-resistant states,” he says, “but it’s an important step.”
+ Back in 2001, theoretical physicists Daniel Gottesman, Alexei Kitaev and John Preskill proposed storing information non-locally in exotic quantum states of an oscillator that became known as GKP states. “Everybody knows that the uncertainty principle says you can’t measure with arbitrary precision both the position and the momentum of a harmonic oscillator,” explains Preskill, who is at Caltech. “But it turns out that you can prepare a state of a particle or an oscillator, then someone – while your back is turned – can come along and shift it a little in position and momentum, and you can measure both shifts to arbitrary accuracy if you promise that they are small. If you look at one of these states in position space it looks like a grid, and if you look at it in momentum space it looks the same way; so if you shift the comb by less than half the distance between the teeth, you can measure the distance by which it’s been shifted. So you can encode information in these states and the noise in the lab can be measured and corrected.”
+ Last year, researchers at ETH Zurich in Switzerland created GKP states in qubits made of trapped ions, which are a leading contender for practical quantum computers. Now, Devoret and colleagues at Yale and the Inria Paris Research Centre have demonstrated quantum error correction of an encoded qubit in the other leading quantum-computing technology – qubits made from superconducting circuits. Double the lifetime The researchers initialized qubits in specific states and measured their lifetimes with and without applying the quantum error-correction algorithm. They found that the error-correction algorithm doubled the lifetime of all the quantum states to around 250 μs. “In our experiment we’re just protecting memory,” says Devoret, “The next step is to show fault-tolerant computation: you want to protect an operation and error correct while you are computing.”
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