Abstract
Electrical circuits might be regarded as rather mundane pieces of classical engineering. But their electromagnetic fields are, like light, a quantum object whose energy comes in discrete units — photons. In cavity quantum electrodynamics (QED), atoms or quantum dots are made to strongly interact with single photons. Recent work showed that a regime of 'strong coupling' can be obtained, where a single photon is absorbed and re-emitted many times. Schuster et al. have built a special type of cavity QED system that is embedded within an electronic circuit; in it a superconducting quantum bit (qubit) interacts with photons from a microwave transmission line. A novel regime can be produced in this system, namely the strong dispersive limit, where a single photon has a large effect on the qubit without being absorbed. This opens the possibility of nondestructive counting of photons that are present in the cavity. This effect could be used as a basis for qubit–photon conditional logic, a requirement for quantum computing.
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