Speaker: John Martinis (University of California Santa Barbara)

Title: Synthesizing arbitrary photon states in a superconducting resonator: The quantum digital to analog converter

Two-level systems, or qubits, can be prepared in arbitrary quantum states with exquisite control, just using classical electrical signals. Achieving the same degree of control over harmonic resonators has remained elusive, due to their infinite number of equally spaced energy levels. Here we exploit the good control over a superconducting phase qubit by using it to pump photons into a high-Q coplanar wave guide resonator and, subsequently, to read out the resonator state. This scheme has previously allowed us to prepare and detect photon number states (Fock states) in the resonator and to measure their decay. Using a generalization of this scheme by Law and Eberly, we can now create arbitrary quantum states of the photon field with up to approximately 10 photons. We analyze the prepared states by directly mapping out the corresponding Wigner function, which is the phase-space equivalent to the density matrix and provides a complete description of the quantum state.

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Speaker: David G. Cory (MIT)

Title: The Design and Function of Quantum Information Processors

Quantum information theory provides a new framework for the development of sensors and actuators that rely on quantum dynamics to obtain efficiencies beyond their classical counterparts. Today we can build laboratory examples of small quantum devices from spin systems, optics, superconducting systems and even neutron beams. I will introduce some of the concepts underlying these devices. In particular showing simple schemes for improving device performance via quantum engineering. I will also explore some near term examples of practical quantum sensors.

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Speaker: Stephen Bartlett (The University of Sydney)

Title: Quantum computers: A new state of matter?

Coauthors: Andrew Doherty, Sean Barrett, Terry Rudolph, David Jennings

A recent breakthrough in quantum computing has been the realization that quantum computation can proceed solely through single-qubit measurements on an appropriate quantum state. One exciting prospect is that the ground or low-temperature thermal state of an interacting quantum many-body system can serve as such a resource state for quantum computation. The system would simply need to be cooled sufficiently and then subjected to local measurements.

It would be unfortunate, however, if the usefulness of a ground or low-temperature thermal state for quantum computation was critically dependent on the details of the system's Hamiltonian; if so, engineering such systems would be difficult or even impossible. A much more powerful result would be the existence of a robust ordered phase which is characterized by the ability to perform measurement-based quantum computation.

Using some simple toy models, we investigate the existence of such a computational phase of matter. In one model, we identify such a phase by using the fidelity of quantum gates as order parameters. In another, we are able to show the existence of a transition in quantum computational power – from a region in parameter space where every state is useful for measurement-based quantum computation, to a region where all local measurements can be efficiently simulated on a classical computer. Remarkably, this transition occurs despite there being no phase transitions in the model at all. Together, these results reveal that the characterization of computational phases of matter has a rich, complex structure – one which is still poorly understood.

Paper reference: arXiv:0802.4314 (accepted to PRL); arXiv:0807.4797

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Speaker: Daniel Burgarth (Imperial College London)

Title: Scalable quantum computation via local control of only two qubits

We apply quantum control techniques to control a large spin chain by only acting on two qubits at one of its ends, thereby implementing universal quantum computation by a combination of quantum gates on the latter and swap operations across the chain. It is shown that the control sequences can be computed and implemented efficiently. We discuss the application of these ideas to physical systems such as superconducting qubits in which full control of long chains is challenging.

Paper reference: arXiv:0905.3373