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Paths Not Taken in Quantum Computing: Why did the Winning Technologies Win and the Losers Lose?

By IQT News posted 02 Dec 2019

(ArsTechnica) An interesting examination of the quantum-computing landscape over the past decade asking “Why did the winners win and the losers lose? Science writer Chris Lee,examines the technological pathways not proven feasible for quantum computing.  Lee asks then if we will end up with one technology to rule them all? He believes that–for the most part, yes, a single technology will dominate. He believes photonic quantum computers will win out, even though superconducting qubits rule the roost at the moment.
Summarized next is his review of technologies trial-tested in the effort to construct quantum computers:
Early in the decade, nitrogen-vacancy centers, silicon vacancies (which took a lot longer), and solid-state materials were among the front-runners in quantum computing. These materials all operate on similar principles: a small percentage of a contaminant material is introduced to a crystal. Nitrogen is put in diamond, phosphorous in silicon, and ytterbium in yttrium-aluminum-garnet crystals.
There are fundamental disadvantages to these earlier quantum computing technologies. A good example of many of these can be seen in nitrogen-vacancy centers in diamond. Each qubit consists of a single electron left hanging by nitrogen’s inability to bond with a fourth carbon atom. This electron is addressed (set and read) optically. Hence, the first problem is to search through a crystal for the few isolated vacancies that can be individually addressed. Every chip of diamond will produce a different computer, with a different arrangement of qubits that have different properties. The wire routing to ensure that the local magnetic fields are truly local to the target qubit seems insanely difficult.
The case for ions in crystal, like ytterbium in yttrium-aluminum-garnet, is a bit different. Here, the quantum state is not generally stored in a single ytterbium ion. Instead, the state is spread over a population of ions, which makes it incredibly robust—these are some of the longest-lived quantum states. However, it also makes defining the location of the qubit a bit difficult.
Now compare these with ion-trap quantum computers and superconducting qubit computers. In the case of ion traps, the quantum state is stored in and read from an individual trapped ion.

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