For more than two decades, scientists have been “watching” electrons in atoms make the jump between energy levels in real time. “Atoms have energy levels, and when electrons ‘jump’ from one level to another, you can detect this optically. You can encode information in real atoms to make a quantum bit, or qubit,” Irfan Siddiqi tells PhysOrg.com.
While the formation of a qubit from real atoms has the advantage of long coherence times, Siddiqi points out that by the same token single atoms are difficult to couple to each other and have fixed parameters. Qubits made from ‘artificial atoms’ have the advantage of tunability and can, in principle, be produced en masse. The use of superconducting electrical circuits, engineered to have discrete energy levels, is a way to develop artificial atoms.
“These artificial atoms make use of circuitry so that we can control the parameters. We are able to realize analogues of experiments performed with real atoms, and are even able to access different parameter regimes but at the expense of short-lived quantum coherence,” Siddiqi says. “A problem with these superconducting circuits, though, is that it has not been possible to continuously monitor their state with high fidelity,” he continues. This lack of measurement sensitivity has limited the possibility of real time quantum feedback. Until now.