Boffins show off speedy quantum CNOT gate - in silicon

This gives us building blocks for coupla-qubits quantum computers

German and American boffins have claimed a speed record for a quantum CNOT gate: 200 nanosecond operation, which would equate to 5 MHz clock speeds.

Just as important, the researchers created the quantum CNOT* gate in silicon, in the form of electron spins controlled by microwave pulses.

As this announcement from the University of Konstanz in Germany explained, stability is an important challenge in quantum computing, because quantum states are highly sensitive to external noise.

The researchers' gates provide a building block for two qubit systems, using individual electrons in silicon to store information, “and they can precisely control and read out the interaction of two quantum bits”, the announcement said, meaning “the experiment includes all necessary basic operations of the quantum computer.”

To get just one electron from the billions available, the boffins used a combination of electromagnetic attraction and repulsion, before making the electrons captives, “held in a floating state”, the university said.

To control the angular momentum of individual electrons, Konstanz physicists Guido Burkard and Maximilian Russ applied a nano-electrode to each electron.

“Using a magnetic field gradient, the physicists can create a position-dependent magnetic field the electrons can be individually accessed with, therefore enabling the researchers to control the angular momentum of the electrons.”

That provided single qubits, whose states were linked are controlled by a second nano-electrode between the two electrons, to control the coupling between two spins.

The group, which included researchers from Princeton University, NIST, and the University of Maryland, says the single qubit fidelity of their experiment was over 99 per cent, and for two interacting qubits reached 80 per cent, “significantly more stable and more accurate than in previous attempts”.

As they write in the abstract of their paper, “We use the CNOT gate to generate a Bell state with 75 per cent fidelity, limited by quantum state readout”.

The experiment was published in Science December 7 (pre-press at arXiv). ®

*Bootnote: The CNOT gate is a fundamental building block, because it can be used to simulate any other quantum circuit. Wikipedia explains that it's used to create and destroy entanglement.

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