You can almost hear the inner breathing from newsrooms around the world. The science journalists hid in the bathroom, crying softly. What is the reason for this despair? Someone released a paper with the word “topology” – something no one knows how to explain, forcing people to resort to metaphors about forcing cakes to become coffee cups, even though there’s no coffee or cake on display.
And while topology is central to the new findings, it’s also accidental to explain it (in my opinion, anyway). So what are those results?
One of the biggest problems with quantum computers is that they accumulate errors, and the speed with which this happens limits the complexity of the problems they can solve. This new paper shows how to reduce errors, not by engineering but by understanding (and using) the correct quantum states and coupling them to generate a system that is naturally more immune to certain types of noise. So grab a cup of coffee and a donut, and let’s dive into the bustling world of qubits.
Teen qubits don’t maintain noise
The researchers worked with a quantum computer based on 10 trapped ions. Each ion is one qubit (the quantum equivalent of one bit), with the values of one and zero determined by the quantum state of the ion. The quantum state of each ion can be changed by applying magnetic fields and bright lasers to it.
Unlike a digital system, where some part can be flipped from one to zero with certainty, a quantum computer operates in an analog world. The equivalent of bit inversion in the world of quantum computing means reversing the probability of a bit being measured as one or zero. For example, if the probability of a single qubit being 75 percent, a simple reflection would change that to 25 percent.
Also unlike the digital system, this process is somewhat prone to errors. To perform a bit flip on a qubit, a certain amount of energy must be applied to the qubit. This can be done by means of a laser that lights up for a specific duration with a certain intensity. But the laser isn’t perfect, so no operation goes exactly as planned. An inverted qubit is just, like Westley especially mirrored. After many imperfect bit fluctuations, the state of a qubit will be completely random and unusable.
One of the more insidious forms of error is called a correlated error, in which changes in the state of one of the qubits affect the qubits with which it is associated with the first. But you need this conjugation to do arithmetic operations, which is kind of a quandary.
As we discussed, qubits have a state. But in the quantum world, this state changes over time. This change should follow a predictable pattern so that the calculations can be timed correctly. The more predictably the state changes, the more coherent the system will be. In coherent faults, neighboring qubits exert a pull on each other so that they keep changing in a non-random manner (and thus are still coherent), but this pull causes the change to occur at a different rate. This means that the calculations will be timed incorrectly. You might think this is a systematic error, but it’s a different one for each account.
This new paper realizes a scheme that uses the Fibonacci sequence along with coherent coupling between qubits to slow the buildup of coherent errors.