More To Quantum Computing Than Qubits

Why researchers are now proposing hybrid systems rather than relying on one type of qubit.

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A couple of weeks ago, I posted an article about qubits based on the nitrogen-vacancy (N-V) center in diamond. I’m working on one about qubits based on superconducting loops with Josephson junctions. But it’s important to remember that the qubit technology alone tells only part of the story of a quantum computer. Quantum computers, like conventional computers, need ways to store data and ways to transmit information among elements of the system. As it turns out, not all proposed qubit technologies are equally capable at all of these tasks.

For example, as my upcoming article will discuss in more detail, superconducting qubits are relatively easy to fabricate because of their macroscopic dimensions, and are readily manipulated by magnetic fields. Diamond N-V qubits, in contrast, are made by ion implantation; researchers are still learning how to produce them consistently. However, they offer long coherence times and can share data through optical transitions.

Thus, rather than depending on one type of qubit exclusively, a number of researchers have proposed hybrid systems, using the strengths of one qubit type to offset the weaknesses of another. Researchers at NTT Research Laboratories have taken the first steps toward using diamond N-V qubits as quantum repeaters to transfer information between groups of superconducting qubits. In other work, the same group has used diamond N-V qubits as memory elements, transferring information from superconducting qubits to diamond N-V qubits and back.

Practical systems for quantum computing are in their infancy. It is far too early to say which technologies will be best suited to which elements of a complete system. But results like these serve as an important reminder that, just as the transistor was only the first of a long list of advances leading to the computers we use today, qubits are only the beginning, not the final answer, to the challenges of quantum computation.