Tunable flux qubit. (A) Double SQUID with two control coils. (B) Potential of the double SQUID in the symmetric case, relative energy levels. (C) Potential in the asymmetric case. Chiarello, cond-mat 0602464.

Tunable flux qubit manipulated by fast pulses MQC Group Chiarello evaluates the physical parameters for operation of a tunable flux qubit, calculating dissipation and decoherence factors, and discussing the potential for employment of integrated rapid single flux quantum (RSFQ) logic for qubit control.

High fidelity state tomography of capacitively shunted phase qubits UCSB Steffen et al. introduce a novel design concept for superconducting qubits – separating the capacitive element from the Josephson junction for improved qubit performance. Environmental coupling to the qubit is reduced by an order of magnitude; measurement fidelity improves to 90%. "This improved design enables the first demonstration of quantum state tomography with superconducting qubits using single shot measurements."

High-contrast dispersive readout of a superconducting flux qubit Delft Lupascu et al. demonstrate high-contrast state detection of a superconducting flux qubit by probing the microwave transmission of a nonlinear resonator based on a SQUID. "Measured contrast of Rabi oscillations is as high as 87%; of the missing 13%, only 3% is unaccounted for. Experiments involving two consecutive detection pulses are consistent with preparation of the qubit state by the first measurement."

Feedback control for communication with non-orthogonal states LSU Kurt Jacobs examines continuous implementation of optimal measurement for distinguishing between two non-orthogonal states. "Feedback control can be used during measurement to increase the rate at which the information regarding the initial preparation is obtained. Enhancement in the rate of information gain is achieved at the expense of reducing the total information which the measurement can extract in the long-time limit."

Frontiers in Quantum Nanoscience Queensland/PiTP "Within a few years the lives of most people will be touched by the quantum revolution – a change as profound as cars, flight, antibiotics or the Internet. Most people have heard of nanotechnology as the building of new materials at the molecular or atomic scale. That's the stone-axe age compared to what's coming." Nanoscience and nanotechnology receive much attention in the media today. However almost all current work concentrates on very small scale classical devices. This conference looks ahead to the far more revolutionary developments expected once nanoscience 'goes quantum', and begins to use the full potential of quantum mechanical superposition, phase coherence, and entanglement. Conference resources include public surveys on classical and quantum nanoscience.

Frequency dependence of multiphoton interference fringes in a superconducting qubit. Qubit switching probability plotted as a function of frequency and flux detuning in the limit of (A) strong driving and (B) weak driving signals. Symmetric patterns in peaks and valleys due to quantum interference are clearly observable. Oliver et al. Science 310.

Superconducting circuits and quantum information RIKEN You and Nori discuss recent advances in quantum information processing with superconducting circuits in the charge, flux and phase regimes. "The device can test Bell inequalities, produce Schrödinger cat states, and simulate the Einstein-Podolsky-Rosen experiment. Quantum engineering of macroscopic entangled states will surely play a central role in several future technologies."

Mach-Zehnder interferometry in a strongly driven superconducting qubit Lincoln Lab In Science 310 and cond-mat 0512691, Oliver et al. demonstrate Mach-Zehnder interferometry in a flux qubit. "The development of artificial atoms with lithographically defined superconducting circuits presents a new paradigm of quantum solid state physics, allowing the realization and exploration of new macroscopic quantum phenomena, and holding promise for applications in quantum computing [...] The generalization of optical Mach-Zehnder interferometry, performed in qubit phase space, provides an alternative means to manipulate and characterize the qubit in the strongly driven regime."

Dephasing of a superconducting qubit induced by photon noise Delft In PRL 95, 257002, Bertet et al. evaluate photon noise-induced dephasing in a superconducting flux qubit coupled to a harmonic oscillator. "Retaining quantum coherence is a central requirement in quantum information processing. Solid-state qubits, including superconducting ones, couple to environmental degrees of freedom that potentially lead to dephasing [...] By careful tuning of flux and current bias, long coherence times can be achieved with flux qubits."

Heisenberg limited measurements with superconducting circuits JPL Guillaume and Dowling describe an assembly of superconducting qubits in a single-mode cavity. Performing collective manipulations of the assembly to generate maximally entangled states, "this method can thus enable Heisenberg limited sensor technology with electric charge or magnetic field superconducting devices."

Quantum coherent oscillations in a charge qubit Y. Nakamura, Yu.A. Pashkin, and J.S. Tsai. Nature, 398:786, 1999.

Tunable coupling scheme for flux qubits CREST-JST Niskanen, Nakamura and Tsai introduce a design for tunably coupling two flux qubits via a third high-frequency qubit, allowing the qubits to remain optimally-biased and shielded from harmful low-frequency flux noise. "The presented scheme is an experimentally realistic way of carrying out two-qubit gates, and should be easily extended to multiqubit systems."

Quantum phase slip junctions Kavli Institute Delft Quantum phase slip is the exact dual to Cooper pair tunneling in the Josephson junction. In cond-mat 0511535, Mooij and Nazarov propose coherent quantum phase slip junctions. If experimentally verified, these junctions could yield applications as resonators or in fundamental current standards.

Scalable controlled gate operations KU In a recent submission to Physical Review A, Han and Yang present a novel approach to realize scalable, controlled-U gate operations with superconducting qubits coupled to a microwave cavity or in atomic qubits within cavity QED. "The method operates essentially by creating a single photon through one of the control SQUIDs, and then performing an arbitrary unitary transformation on the target SQUID with the assistance of the cavity photon."

Decoherence and quantum measurement of Josephson qubits Stony Brook Doctoral dissertation, Kristian Rabenstein