Oct 04 2021

Incremental Advance for Quantum Computing

Quantum computing is an exciting technology with tremendous potential. However, at present that is exactly what it remains – a potential, without any current application. It’s actually a great example of the challenges of trying to predict the future. If quantum computing succeeds, the implications could be enormous. But at present, there is no guarantee that quantum computing will become a reality, and if so how long it will take. So if we try to imagine the world 50 or 100 years in the future, quantum computing is a huge variable we can’t really predict at this point.

The technology is moving forward, but significant hurdles remain. I suspect that for the next 2-3 decades the “coming quantum computer revolution” will be similar to the “coming hydrogen economy,” in that it never came. But the technology continues to progress, and it might come yet.

What is quantum computing? Here is the quick version – a quantum computer exploits the weird properties of quantum mechanics to perform computing operations. Instead of classical “bits” where a unit of information is either a “1” or “0”, a quantum bit (or qubit) is in a state of quantum superposition, and can have any value between 0 and 1 inclusive. This means that each qubit contains a vastly greater amount of information than a classical bit, especially as they scale up in number. A theoretical quantum computer with one million qubits could perform operations in minutes that would take a universe full of classical supercomputers billions of years to perform (in other words, operations that are essentially impossible for classical computers). It’s no wonder that IBM, Google, China, and others are investing heavily in this technology.

But there are significant technological hurdles that remain. Quantum computer operations leverage quantum entanglement (where the physical properties of two particles are linked) among the qubits in order to get to the desired answer, but that answer is only probabilistic. In order to know that the quantum computer is working at all, researchers check the answers with a classical computer. Current quantum computers are running at about a 1% error rate. That sounds low, but for a computer it’s huge, essentially rendering the computer useless for any large calculations (the ones that quantum computers would be useful for).

David Allcock, a physicist and coauthor on a new paper on quantum computer, said that we need to get the error rate at least down to 1 in 10,000 operations (two orders of magnitude better than current quantum computers), adding:

“We want to get to that point. Then you can use quantum computers for something useful. Right now they’re just toys.”

This is where the new study comes in. Allcock, David Wineland, and 10 other coauthors published a paper in Nature in which they present an alternate method of error reduction in quantum computers. Essentially what we have are technology companies trying to build better quantum computers based on our current understanding of physics, while physicists are trying to deepen that understanding and provide new methods for quantum computing. The current paper is an example of the latter.

Right now quantum computers use lasers to entangle qubits, which is part of how they undergo the desired operation. This new method uses trapped ions and does not require lasers to create the entanglement, which the physicists believe is a better and simpler method of entanglement which will produce fewer errors. They right (sorry if this gets technical):

Universal control of trapped ion qubits has been separately demonstrated using tightly focused laser beams or by moving ions with respect to laser beams, but at lower fidelities. Laser-free entangling methods may offer improved scalability by harnessing microwave technology developed for wireless communications, but so far their performance has lagged the best reported laser-based approaches. Here we demonstrate high-fidelity laser-free universal control of two trapped-ion qubits by creating both symmetric and antisymmetric maximally entangled states with fidelities of 1+00.00171−0.0017+0 and 0.9977+0.00100.00130.9977−0.0013+0.0010, respectively (68 per cent confidence level), corrected for initialization error.

Instead of using lasers they are using microwave technology to entangle a pair of qubits. According to their study this new method has a couple of key advantages. First the method can be simultaneously used on multiple pairs of trapped ions, and therefore would scale more easily to high qubit numbers (current quantum computers are approaching 1,000 qubits). Second they claim the system is “robust against multiple sources of decoherence.” Decoherence is what happens when entangled particles are no longer entangled, usually because one or the other interacts with the environment in some way.

For a quick primer on what entanglement is, this is a quantum state in which two particles have one or more physical properties linked. If we consider, for example, the property of spin – a particle may be spin up or spin down. Superposition means that a particle will be in a quantum state that is both spin up and down a the same time. If we have two particles in a superposition of spin up and spin down, and they are entangled, that means when one particle interacts with its environment so that is has to become either spin up or down, the other particle (wherever it is in the universe) will instantly become the opposite spin.

If the particles are not completely isolated from the environment, however, their entanglement can be “shared” with the environment, meaning that they lose it over time, undergoing decoherence. A quantum computer, therefore, needs to keep the qubits as isolated as possible so that pairs of qubits can remain entangled in order to carry out the quantum calculation. Decoherence, therefore, is the death of quantum computing, and needs to be prevented. Using a new method that is “robust against decoherence,” is potentially a huge advantage, therefore.

Now it remains to be seen what happens when and if the technology companies try to use this new method to build a quantum computer. There are multiple simultaneous hurdles that need to be overcome, and success is by no means guaranteed. But the potential is huge.

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