Measurement and control of quantum cavities with electronic atoms

This chapter describes a method to control and measure quantum cavities, achieved using a superconducting phase qubit, a type of electronic atom. The phase qubit has been under intensive development for use as a quantum processing element in a quantum computer. We have recently used it to demonstrate the quantum control and measurement of excitations in resonant cavities, including controlling photons in superconducting microwave resonators (Hofheinz et al., 2008; Wang et al., 2008; Hofheinz et al., 2009; Wang et al., 2009a; Mariantoni et al., 2011; Wang et al., 2011), as well as phonons in microwave-frequency mechanical resonators (OConnell et al., 2010). The discussion here focusses on the quantum control of electromagnetic resonators; we refer the interested reader to other recently-published references for further information on quantum control of mechanical resonators (Marquardt et al., 2011; OConnell et al., 2010). The phase qubit is a particular implementation of the Josephson junction. I begin by describing the physics of the Josephson junction, and how the junction is embedded in an electrical circuit to provide the critical nonlinearity needed for quantum circuit operation. We have used this electrical circuit as a quantum two-level system, the prototypical qubit, and as a three-level system, where it can be called a “qutrit”. I then describe how we have used the phase qubit in conjunction with highly linear microwave resonators to perform a number of interesting experiments, including the on-demand creation and storage of photon Fock states, and the synthesis of arbitrary superpositions of Fock states. I also briefly summarize more recent results, including entangling photons stored in two physically separate microwave resonators, as well as playing a “shell game” with the photons stored in three microwave resonators.