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# 6.2 A few examples of quantum technologies

This section is to provide you with a starting point if you would like to learn more about quantum computer technologies. The difficulties of quantum computing means that the number of systems that have been proposed as quantum computers is almost as large as the number of tasks in which quantum computers can (in principle) outperform classical computers. In this section there is a brief introduction to three, the transmon qubit, NMR and cold atom technology. It is not possible to expect this section to keep up-to-date as advances are being made all the time and often announcements appear in the news. You can watch out for these and follow up your interests if you would like to. Some links are provided at the end to get you started.

The transmon qubit

The transmon qubit is based on a miniaturised superconducting circuit, built from a capacitor and a non-linear inductor called a Josephson junction. This qubit is at the heart of the IBM quantum computers, which makes it one of the most advanced platforms.

Within superconducting circuits, quantum logic gates may be implemented by applying an AC voltage to the qubits. The qubits in the superconductor involve the charge carriers which respond to the applied potential.

The NMR qubit

NMR was used as an early test of quantum computing because the technology is well-developed as a result of medical research and its use for medical purposes (MRI scanners). In NMR quantum computing, each qubit is realised as a collection of particles called an ensemble. The ensemble is all the molecules in a sample. The qubits are molecular sites in the target molecule. Therefore, the sample contains many copies of the qubits. To increase the number of qubits, the molecule must become more complex, but this is difficult because as the size of the molecule increases, environmental effects mean that each molecule is likely to be different. This leads to the conclusion that NMR quantum computing is limited to about 20 qubits and it is unlikely that this number of qubits can be increased.

Using cold-atom technology

Cold-atom technology (which includes cold ions) is able to take advantage of techniques developed to build atomic clocks. The appeal of cold-atom systems is the degree of control that can be achieved. For example, from the gas phase, individual atoms can be trapped at specific sites in a vacuum chamber, and then addressed by individual laser beams for the purposes of implementing gates or measurements. The weakness of the cold-atom approach is that, so far, its complexity scales poorly with the number of qubits.

Links

Here are some links to websites that explain current activities in quantum computing from different companies.

IBM https://www.ibm.com/quantum/technology

Amazon https://aws.amazon.com/what-is/quantum-computing

Quantum Insider https://thequantuminsider.com/2023/06/06/types-of-quantum-computers

Xanadu AI and From a state of light to state of the art

Google Quantum AI and What our quantum computing milestone means or

Microsoft https://quantum.microsoft.com/en-us/explore/concepts/topological-qubits

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OpenLearn - Introduction to quantum computing
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