Home Computers Immunizing quantum computers against errors
Computers - February 28, 2019

Immunizing quantum computers against errors

When building a quantum computer, one needs to reckon with errors — in both senses of the phrase. Quantum bits or “qubits,” that may take on the logical values zero and 1 at the same time and for that reason carry out calculations faster, are extraordinarily vulnerable to perturbations. A viable remedy for this is quantum mistakes correction, which means that each qubit is represented “redundantly” in several copies, such that errors can be detected and sooner or later corrected without stressful the delicate quantum kingdom of the qubit itself. Technically this is very demanding. However, several years ago an opportunity idea came up in which information is not stored in numerous redundant qubits, however rather in the many oscillatory states of a single quantum harmonic oscillator. The studies group of Jonathan Home, the professor on the Institute for Quantum Electronics at ETH Zurich, has now realized this type of qubit encoded in an oscillator. Their outcomes have been published within the clinical journal Nature.

Periodic oscillatory states

In Home’s laboratory, Ph.D. Student Christa Flühmann and her colleague’s paintings with electrically charged calcium atoms which can be trapped via electric fields. Using accurately chosen laser beams, these ions are cooled all the way down to very low temperatures at which their oscillations inside the electric fields (internal which the ions slosh from side to side like marbles in a bowl) are defined by means of quantum mechanics as so-known as wave functions. “At that factor things get interesting,” says Flühmann, who is the first writer of the Nature paper. “We can now control the oscillatory states of the ions in any such way that their position and momentum uncertainties are disbursed amongst many periodically organized states.”

Here, “uncertainty” refers to Werner Heisenberg’s famous formula, which states that in quantum physics the product of the size uncertainties of the location and velocity (greater exactly: the momentum) of a particle can in no way cross below a nicely-described minimal. For instance, if one desires to control the particle a good way to recognize its role thoroughly — physicists call this “squeezing” — one robotically makes its momentum less sure.

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Reduced uncertainty

Squeezing a quantum country on this way is, on its own, only of a restricted price if the aim is to make specific measurements. However, there may be a clever manner out: if on the pinnacle of the squeezing, one prepares an oscillatory nation in which the particle’s wave feature is sent over many periodically spaced positions, the measurement uncertainty of every position and of the respective momentum may be smaller than Heisenberg might allow. Such a spatial distribution of the wave feature — the particle can be in numerous locations without delay, and most effective a measurement decides where one absolutely reveals it — is paying homage to Erwin Schrödinger’s well-known cat, that’s simultaneously useless and alive.

This strongly reduced measurement uncertainty also manner that the tiniest change in the wave characteristic, for instance by means of a few outside disturbance, can be decided very precisely and — as a minimum in precept — corrected. “Our consciousness of those periodic or comb-like oscillatory states of the ion is an critical step in the direction of such an error detection,” Flühmann explains. “Moreover, we are able to put together arbitrary states of the ion and perform all feasible logical operations on it. All that is important for constructing a quantum pc. In the next step, we want to combine that with errors detection and blunders correction.”

Applications in quantum sensors

A few experimental limitations should be triumph over on the way, Flühmann admits. The calcium ion first needs to be coupled to every other ion by means of electric forces, so that the oscillatory state may be studied out without destroying it. Still, even in its gift shape, the technique of the ETH researchers is of a wonderful hobby for applications, Flühmann explains: “Owing to their extreme sensitivity to disturbances, those oscillatory states are a terrific tool for measuring tiny electric fields or other bodily quantities very exactly.”

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