There are 15 to 18 technologies being investigated for quantum computing. Each have advantages and limitations. Blatt’s group is working on a qubit based on an optical transition of trapped ions of calcium-40.
Trapped ion qubits “have exquisite coherence properties; they can be prepared and measured with nearly 100% efficiency; and they are readily entangled with each other through the Coulomb interaction or remote photonic interconnects,” writes Chris Monroe of the Joint Quantum Institute in Science. His group is using ytterbium ions; other groups are studying other trapped ions. Both the Innsbruck and JQI groups have scaled experiments to 15 or 16 qubits, about halfway to the 30 qubits that Monroe says is needed to simulate behavior of a quantum-mechanical system too complex for digital computers to handle.
Other types of qubits may be better for other types of operations, says Klaus Ensslin of the Swiss Federal Institute of Technology (ETH; Zurich, Switzerland). Swiss researchers are studying many types of qubits for potential applications. One concern is the short lifetime of quantum states when they couple to the outside world. “To operate a quantum computer, you must isolate the quantum system from its environment, but you also must read it out,” says Ensslin. The spin of the single electron in a quantum dot is attractive because it couples weakly to its environment. Quantum-dot pin qubits are hard to manipulate, but he says their big attraction is the possible ease of scaling in well-understood semiconductor nanostructures. Others are studying approaches where topological protection is quantum-engineered to enhance coherence and reduce noise.
Other types of quantum computing technology include:
• Neutral atoms and molecules with long-lived internal states, cooled, trapped, and entangled to create qubits.
• Superconducting Josephson junction circuits.
• Optical measurement of the quantum states of photons.
• Nuclear magnetic resonance effects