(SpectrumIEEE) Intel’s director of quantum hardware, Jim Clarke, says that quantum computing will really have arrived when it can do something unique that can change our lives, calling that point “quantum practicality.
This article is an interview by IEEE Spectrum with Clarke. The questions begin with asking Clarke? “Why do you think silicon has the best chance of leading to a useful quantum computer?”
Clarke explains that silicon spin qubits look exactly like a transistor. … The infrastructure is there from a tool fabrication perspective. We know how to make these transistors. So if you can take a technology like quantum computing and map it to such a ubiquitous technology, then the prospect for developing a quantum computer is much clearer.
He concedes that today silicon spin-qubits are not the most advanced quantum computing technology out there. There has been a lot of progress in the last year with superconducting and ion trap qubits.
Clarke discusses the silicon spin qubit: You turn on a transistor, and you have a flow of electrons from one side to another. In a silicon spin qubit, you essentially trap a single electron in your transistor, and then you put the whole thing in a magnetic field [using a superconducting electromagnet in a refrigerator]. This orients the electron to either spin up or spin down. We are essentially using its spin state as the zero and one of the qubit.
That would be an individual qubit. Then with very good control, we can get two separated electrons in close proximity and control the amount of interaction between them. And that serves as our two-qubit interaction.
So we’re basically taking a transistor, operating at the single electron level, getting it in very close proximity to what would amount to another transistor, and then we’re controlling the electrons.
Clarke also addresses a problem: “The challenge is that silicon, even a single crystal, may not be as clean as we need it. Some of the defects—these defects can be extra bonds, they can be charge defects, they can be dislocations in the silicon—these can all impact that single electron that we’re studying. This is really a materials issue that we’re trying to solve.”
The interview is extensive and worth a read. This summary encompasses the silicon spin qubit section, and the entire interview includes:

Why quantum computers will be made of silicon
How silicon spin qubits work
What needs to happen before quantum error correction works
“Hot” silicon spin qubits
What problems keeps him up at night

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