Quantum Machines: Measurement Control of Engineered Quantum Systems

This book gathers the lecture notes of courses given at the 2011 summer school in theoretical physics in Les Houches, France, Session XCVI. What is a quantum machine? Can we say that lasers and transistors are quantum machines? After all, physicists advertise these devices as the two main spin-offs of the understanding of quantum mechanical phenomena. However, while quantum mechanics must be used to predict the wavelength of a laser and the operation voltage of a transistor, it does not intervene at the level of the signals processed by these systems. Signals involve macroscopic collective variables like voltages and currents in a circuit or the amplitude of the oscillating electric field in an electromagnetic cavity resonator. In a true quantum machine, the signal collective variables, which both inform the outside on the state of the machine and receive controlling instructions, must themselves be treated as quantum operators, just as the position of the electron in a hydrogen atom. Quantum superconducting circuits, quantum dots, and quantum nanomechanical resonators satisfy the definition of quantum machines. These mesoscopic systems exhibit a few collective dynamical variables, whose fluctuations are well in the quantum regime and whose measurement is essentially limited in precision by the Heisenberg uncertainty principle. Other engineered quantum systems based on natural, rather than artificial degrees of freedom can also qualify as quantum machines: trapped ions, single Rydberg atoms in superconducting cavities, and lattices of ultracold atoms. This book provides the basic knowledge needed to understand and investigate the physics of these novel systems.PART I: LECTURES ; 1. Real-time feedback control of quantum optical input-output systems ; 2. Quantum noise and quantum measurement ; 3. Circuit QED: Superconducting qubits coupled to microwave photons ; 4. Quantum logic gates in superconducting qubits ; 5. Exploring quantum matter with ultracold atoms ; 6. Readout of superconducting qubits ; 7. Quantum error correction ; 8. Quantum optomechanics ; 9. Micromechanics and superconducting circuits ; 10. Two electron spin qubits in GaAs: Control and dephasing due to nuclear spins ; 11. Exploring the quantum world with photons trapped in cavities and Rydberg atoms ; 12. SQUID amplifiers ; 13. Quantum information science: Experimental implementation with trapped ions ; PART II: SEMINARS ; 14. An introduction to laser cooling optomechanical systems ; 15. Tomography schemes for characterizing itinerant microwave photon fields ; 16. Using a "friction-less" pendulum for quantum measurement ; 17. Quantum Bayesian approach to circuit QED measurement ; 18. Superconducting quantum circuits: Artificial atoms coupled to 1D modes ; 19. A superconducting artificial atom with two internal degrees of freedom