Monday 28 December 2020

Generate enough incidental electromagnetic noise to cause decoherence

 The second challenge lies in controlling the qubit to perform logical functions, often achieved through a finely tuned pulse of electromagnetic radiation. This manipulation process itself can generate enough incidental electromagnetic noise to cause decoherence. To scale up quantum computers, engineers will have to strike a balance between protecting qubits from potential disturbance and still allowing them to be manipulated for calculations. This balance could theoretically be attained by a range of physical systems, though two technologies currently show the most promise: superconductors and trapped ions.

A superconducting quantum computer uses the flow of paired electrons — called “Cooper pairs” — through a resistance-free circuit as the qubit. “A superconductor is quite special, because below a certain temperature, its resistance goes away,” says William Oliver, who is an associate professor in MIT’s Department of Electrical Engineering and Computer Science, a Lincoln Laboratory Fellow, and the director of the MIT Center for Quantum Engineering.

The computers Oliver engineers use qubits composed of superconducting aluminum circuits chilled close to absolute zero. The system acts as an what is the difference between computer science and computer engineering oscillator with two energy states, corresponding to 0 and 1, as current flows through the circuit one way or the other. These superconducting qubits are relatively large, about one tenth of a millimeter along each edge — that’s hundreds of thousands of times larger than a classical transistor. A superconducting qubit’s bulk makes it easy to manipulate for calculations.


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