Researchers at ETH Zurich have used trapped calcium ions to demonstrate a new method for making quantum computers immune to errors. To do so, they created a periodic oscillatory state of an ion that circumvents the usual limits to measurement accuracy.
When building a quantum computer, one needs to reckon with errors – in both senses of the word. Quantum bits or “qubits”, which can take on the logical values 0 and 1 at the same time and thus carry out calculations faster, are extremely susceptible to perturbations. A possible remedy for this is quantum error correction, which means that each qubit is represented “redundantly” in several copies, such that errors can be detected and eventually corrected without disturbing the fragile quantum state of the qubit itself. Technically this is very demanding. However, several years ago an alternative suggestion came up in which information isn’t stored in several redundant qubits, but rather in the many oscillatory states of a single quantum harmonic oscillator. The research group of Jonathan Home, professor at the Institute for Quantum Electronics at ETH Zurich, has now realised such a qubit encoded in an oscillator. Their results have been published in the scientific journal Nature.
Periodic oscillatory states
In Home’s laboratory, PhD student Christa Flühmann and her colleagues work with electrically charged calcium atoms that are trapped by electric fields. Using appropriately chosen laser beams, these ions are cooled down to very low temperatures at which their oscillations in the electric fields (inside which the ions slosh back and forth like marbles in a bowl) are described by quantum mechanics as so-called wave functions. “At that point things get exciting”, says Flühmann, who is first author of the Nature paper. “We can now manipulate the oscillatory states of the ions in such a way that their position and momentum uncertainties are distributed among many periodically arranged states.”
Here, “uncertainty” refers to Werner Heisenberg’s famous formula, which states that in quantum physics the product of the measurement uncertainties of the position and velocity (more precisely: the momentum) of a particle can never go below a well-defined minimum. For instance, if one wants to manipulate the particle in order to know its position very well – physicists call this “squeezing” – one automatically makes its momentum less certain.
Squeezing a quantum state in this way is, on its own, only of limited value if the aim is to make precise measurements. However, there is a clever way out: if, on top of the squeezing, one prepares an oscillatory state in which the particle’s wave function is distributed over many periodically spaced positions, the measurement uncertainty of each position and of the respective momentum can be smaller than Heisenberg would allow. Such a spatial distribution of the wave function – the particle can be in several places at once, and only a measurement decides where one actually finds it – is reminiscent of Erwin Schrödinger’s famous cat, which is simultaneously dead and alive.
This strongly reduced measurement uncertainty also means that the tiniest change in the wave function, for instance by some external disturbance, can be determined very precisely and – at least in principle – corrected. “Our realisation of those periodic or comb-like oscillatory states of the ion are an important step towards such an error detection”, Flühmann explains. “Moreover, we can prepare arbitrary states of the ion and perform all possible logical operations on it. All this is necessary for building a quantum computer. In a next step we want to combine that with error detection and error correction.”
Applications in quantum sensors
A few experimental obstacles have to be overcome on the way, Flühmann admits. The calcium ion first needs to be coupled to another ion by electric forces, so that the oscillatory state can be read out without destroying it. Still, even in its present form the method of the ETH researchers is of great interest for applications, Flühmann explains: “Owing to their extreme sensitivity to disturbances, those oscillatory states are a great tool for measuring tiny electric fields or other physical quantities very precisely.”
Learn more: Immunising quantum computers against errors
The Latest on: Quantum computers
via Google News
The Latest on: Quantum computers
- The quantum supremacy enigma: Can Google’s claim withstand scrutiny? on November 18, 2019 at 2:18 pm
If quantum computing is ever to transcend the realm of experiment and become a legitimate industry, it will need to sustain a legitimate business model. Businesses invest in machines and technologies ...
- Quantum computers learn to mark their own work on November 18, 2019 at 6:05 am
A new test to check if a quantum computer is giving correct answers to questions beyond the scope of traditional computing could help the first quantum computer that can outperform a classical ...
- Cambridge Quantum Computing Announces New Release of Updated Software Stack t|ket ™ on November 17, 2019 at 3:31 pm
t|ket provides essential tools that allow users to maximise the performance of quantum computers in the NISQ era CAMBRIDGE, England, Nov. 18, 2019 /PRNewswire/ -- Cambridge Quantum Computing ("CQC") ...
- Here’s How Quantum Computer Supremacy Will Impact Self-Driving Cars on November 16, 2019 at 10:01 am
That’s not to say that the use of their 54-qubit Sycamore processor wasn’t notable, and in fact, does provide another handy stride toward achieving viable quantum computing, but whether it was the ...
- D-Wave sticks with its approach to quantum computing on November 15, 2019 at 1:04 pm
While D-Wave’s 2000Q only played a small part in this process, it’s nevertheless a sign that quantum computing is slowly getting ready for production use and that D-Wave’s approach, somewhat ...
- Quantum Leap for Computing Is a Small Step for Computers on November 15, 2019 at 7:00 am
Ariel Procaccia is an associate professor in the computer science department at Carnegie Mellon University. His areas of expertise include artificial intelligence, theoretical computer science and ...
- Dell Technologies on democratising 5G and the future of quantum computing on November 14, 2019 at 2:27 pm
"And this next cycle, trust me, we have no other goal, than broad adoption of these technologies." Quantum computing, it could go either way When asked during a media session what Dell Technologies ...
- Can Google's New Quantum Computer Hack Bitcoin? on November 14, 2019 at 9:15 am
Nearly everyone agreed that was an extremely remote possibility. But now, some folks fear that day may be closer than expected. The reason: Google claims to have built a quantum computer. It’s a ...
- Quantum Computing Holds Promise for Banks, Executives Say on November 8, 2019 at 1:36 pm
BROOKLYN, N.Y.—When quantum computing hits the market, the financial-services industry could be the first to benefit, a Goldman Sachs Group Inc. executive said at a quantum-computing panel event. “In ...
- Hewlett Packard Enterprise explains why it gave up on quantum computing right as it was splitting off from HP Inc on November 8, 2019 at 6:06 am
Hewlett Packard Enterprise explains why it stopped its quantum computing efforts five years ago while IBM, Google, Microsoft, and Intel continue.
via Bing News