Multi-Qubit Quantum Coherent Operations College Park "The purpose of this meeting is to identify – on a technology-specific basis – the primary difficulties that are likely to be encountered as qubit systems beyond typical current experiments with one, two, or three qubits are pursued. Topics of interest include, but are not limited to: controllable qubit interactions, miniaturization and integration to increase qubit density, new physics arising from the assembly of many-qubit systems, and process and state validation for larger systems. The workshop aims to address the following questions:

Given a particular qubit technology that has already demonstrated full single qubit control, what are the challenges facing this system as it grows into the multiple qubit regime? In a system with multiple qubits, what will be the mechanism(s) through which these qubits interact? How can these interactions be well enough controlled to achieve reliable, scalable logic gates? Do they contribute to decoherent processes?

What new physics must be taken into account for the benefit of, or hindrance to, many qubit operations? Are there any fundamental results that will inform the community of possible roadblocks that may be encountered when growing systems to the multiple qubit regime?

How can cross-talk from the control/readout channel for one qubit, to a bystander qubit, be reduced to a satisfactory level? What are the specific technologies required to ensure that a robust multi-qubit information processing system can be realized?

As systems grow larger, quantum state and, and even more so, process tomography will become impractical. What other measures will be effective for validating the operations of these systems?"

National Workshop on Quantum Information Science OSTP|NTSC Via John Preskill, Dave Bacon, Ian Durham and Scott Aaronson — NSTC has announced a workshop in response to the recent Federal Vision for Quantum Information Science: "In January 2009, the United States National Science and Technology Council issued the report A Federal Vision for Quantum Information Science. The report proposes that

“The United States … create a scientific foundation for controlling, manipulating, and exploiting the behavior of quantum matter, and for identifying the physical, mathematical, and computational capabilities and limitations of quantum information processing systems in order to build a knowledge base for this 21st century technology.”

This Workshop on Quantum Information Science (QIS) has been organized in response to the NSTC report. It brings together leading theorists and experimenters drawn from physical science, computer science, mathematics, and engineering who will assess recent progress in QIS and identify major goals and challenges for future research."

Adaptive quantum networks – We introduce a robust, error-tolerant adaptive training algorithm for generalized learning in high-dimensional, superposed quantum networks, or adaptive quantum networks. The formalized procedure applies standard backpropagation training to a coherent ensemble of discrete topological configurations of individual neural networks, each of which is formally merged under linear superposition. Quantum parallelism facilitates simultaneous training and revision within this coherent state space, resulting in accelerated convergence to optima. The protocol provides quantitative, numerical indicators for optimization of both single-neuron activation functions and reconfiguration of global network topology.

Density plot of qubit purity ξ after t =20 kicks as a function of chaos parameter K=kT and nonlinear parameter g for T=2, δ=0.2 and ε=0.5.

Chaotic dynamics of a Bose-Einstein condensate coupled to a qubit Shepelyansky, Martin, Georgot arXiv 0812.5067: "We study the coupling between a qubit and a Bose-Einstein condensate moving in a kicked optical lattice. In the regime where BEC size is smaller than the lattice period, the chaotic dynamics of the BEC is effectively controlled by the qubit state. This gives an example of exponentially sensitive control over a macroscopic state by internal qubit states. The realization of such coupled systems is within reach of current experimental techniques."

Interference of interacting matter waves Innsbruck arXiv 0812.4836: "The phenomenon of matter wave interference lies at the heart of quantum physics. It has been observed in various contexts in the limit of non-interacting particles as a single particle effect. Here we observe and control matter wave interference whose evolution is driven by interparticle interactions. In a multi-path matter wave interferometer, the macroscopic many-body wave function of an interacting atomic Bose-Einstein condensate develops a regular interference pattern, allowing us to detect and directly visualize the effect of interaction-induced phase shifts. We demonstrate control over the phase evolution by inhibiting interaction-induced dephasing, and by refocusing a dephased macroscopic matter wave in a spin-echo type experiment. Our results show that interactions in a many-body system lead to a surprisingly coherent evolution – possibly enabling narrow-band and high-brightness matter wave interferometers based on atom lasers."