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No sugar coating: Donut math yields a way to make qubits last longer

    Image of an I-shaped piece of electronics on a dark background.
    enlarge / An ion trap, the quantum hardware used for this work.

    You can almost hear the inhaled breath from newsrooms around the world. Specialist science journalists have been hiding in the bathroom to cry quietly. The cause of such despair? Someone has released a paper using the word “topology” – something that no one knows how to explain, forcing people to resort to metaphors about donuts being forced to become coffee cups despite no coffee or donuts being offered.

    And while topology is fundamental to the new results, it’s also tangent to explaining them (in my view anyway). So what are those results?

    One of the big problems with quantum computers is that they accumulate errors, and the speed at which that happens limits the complexity of the problems they can solve. This new paper shows how errors can be reduced, not through engineering but by understanding (and using) the correct quantum states and their coupling to generate a system that is naturally more immune to certain types of noise. So grab a coffee and a donut, and let’s dive into the noisy world of qubits.

    Teen qubits never keep the sound down

    The researchers worked with a quantum computer based on 10 trapped ions. Each ion is a single qubit (the quantum equivalent of a single bit), with the values โ€‹โ€‹of one and zero defined by the quantum state of the ion. The quantum state of any ion can be changed by applying magnetic fields and shining lasers on it.

    Unlike a digital system, where a bit can be switched from one to zero with certainty, a quantum computer operates in an analog world. The equivalent of a bit flip in the world of quantum computers means reversing the probability of a bit being measured as a one or a zero. For example, if the probability of a qubit being a one was 75 percent, flipping it a bit would change that to 25 percent.

    Also unlike a digital system, this process is somewhat prone to errors. To perform a bit flip operation on a qubit, a certain amount of energy must be applied to the qubit. This can be done by a laser shining at a certain power for a certain duration. But lasers aren’t perfect, so no surgery goes exactly as planned. The inverse qubit, like Wesley, is only largely other way around. After several imperfect bit flips, the qubit state will be completely random and useless.

    A more insidious form of error is called a “coherent error,” where changes in the state of one qubit affect the qubits the first is associated with. But you need that link to do calculations, which presents a bit of a dilemma.

    As we discussed, a qubit has a state. But in a quantum world, that state changes over time. This change must follow a predictable pattern so that math operations can be timed just right. The longer the state changes predictably, the more coherent the system is. In coherent errors, the adjacent qubits exert an attraction on each other so that they still change in a non-random way (and so they are still coherent), but that attraction causes the change to happen at a different rate. This means that computational operations are timed incorrectly. You might think of this as a systematic error, but it’s one that’s different for every calculation.

    This new paper realizes a scheme that uses the Fibonacci sequence in conjunction with the coherent coupling between qubits to slow the build-up of coherent errors.