Guest Lecturer Schedule

Monday, April 29, 2002

Speaker: Carl Williams -- NIST

Title: Quantum Computing with Neutral Atoms:

Gates, Loading, and Traps

Abstract: This talk will begin with a general introduction to quantum computing with neutral atoms and will then briefly describe the experimental approach at NIST. Based on the general physical requirements that are necessary to build a scalable quantum processor or computer, I will describe those requirements that are easy and those which are difficult for neutral atoms. During this description I will also discuss basic physical limits that undermine some proposed approaches to quantum computation. I will a brief description of some of the various types of traps that are being investigated for neutral atoms. After this extended introduction to neutral atom quantum computing, I will describe in more detail theoretical work we have done on neutral atom gates, including a two-qubit motional gate and how quantum interference can be used to improve many two-qubit gate implementation schemes. I will also describe one approach to neutral atom qubit initialization based on the Mott-Insulator transition. I will conclude with my outlook on the benefits of neutral atom quantum computing and some general conclusions.

Wednesday, May 1, 2002

Speakers: Gavin Brennen & David Song – NIST

Title: Quantum Bus and Efficient Non-Local Operations

Abstract: Many protocols for quantum information processing (QIP) use a control sequence or circuit of interactions between qubits and control fields wherein arbitrary qubits can be made to interact with one another. Often the errors that accumulate in these schemes fall into two distinct classes: static errors which occur when the qubits are fixed and non-interacting with other qubits, and dynamic errors accumulated during single and two qubit operations or during the transmission of quantum information either by physically moving the qubits or swapping information from one location to another. The chosen architecture in which to perform QIP given a physical system will depend on which errors will dominate in that system for a given task.

We investigate gate protocols to efficiently perform nonlocal gates between separated static logical qubits using a bus of dynamic qubits that can be used as a refreshable entanglement resource. In particular, entanglement swapping methods are used to create the necessary network among dynamic qubits. A scheme to perform a nondeterministic nonlocal Toffoli gate using a multibody entangled resource is given. This protocol is shown to be valuable as a technique to simulate nonlocal decoherence. The noise is produced by a nonfactorisable superoperator acting as global phase noise over the qubits. Further it is shown that this noise can be coherently triggered by one or more qubits.

Monday, May 6, 2002

Speaker: Bryan Jacobs – JHU/APL

Title: An Optical Approach to Quantum Information Processing

Abstract: Knill, Laflamme, and Milburn recently showed that non- deterministic quantum logic operations involving photonic qubits can be performed using only linear optical elements, additional ancilla photons, and post-selection based on the output of single-photon detectors [Nature 409, 46 (2001)]. These operations give the desired result with certainty when a specific output from the detectors is obtained, but that will only occur for some fraction of the events.

I will talk about recent experimental demonstrations of two basic logic devices of this kind, a quantum parity check and a destructive controlled-NOT gate. These two devices can be combined with a pair of entangled photons to implement a conventional controlled-NOT gate that succeeds with a probability of 1/4. The use of fast "feed-forward" and controlled single-qubit operations to increase the success probability of the gates will also be discussed.

Wednesday, May 8, 2002

Speaker: Dennis Lucareli – JHU/APL

Title: Holonomic Quantum Computation with

Squeezed Coherent States

Abstract: When a quantum system undergoes adiabatic evolution subject to a periodic Hamiltonian it acquires a phase after one complete cycle. Berry's surprising discovery was that in addition to the well known dynamical phase associated to the evolution, there is a phase of purely geometric origin. Recently, non-Abelian geometric phases have been proposed as a way of constructing logic gates in a quantum computer. In this talk, I will introduce Holonomic Quantum Computation from the perspective of geometric control theory. Universality, entanglement, and decoherence will be discussed for a model in which adiabatic squeezing and displacing devices are employed to control the quantum information.

Monday, May 13, 2002

Speaker: Sam Osofsky – Metron, Inc.

Title: Fault-tolerant quantum computation

Abstract: Quantum algorithms are predicted to perform certain calculations much faster than classical computers, but in their simplest form the predictions assume that states can be prepared, maintained, manipulated with unitary operations, and have measurements made on them, all without error. However, it is likely that each of these steps will be beset by noise in at least the first generation of quantum computers. Once realistic noise processes are included in the performance predictions for quantum algorithms, will they will still beat classical algorithms? The answer appears to be yes: it has been shown that quantum computations can be made "fault-tolerant" -- in effect, able to successfully perform a large calculation with a reasonable probability of success -- given reasonable assumptions about the noise processes, as long as the probability of error for quantum gate is below a certain threshold. Furthermore, fault tolerance can be achieved without sacrificing the ability of quantum computers to outperform classical machines. This class will survey some of the key concepts in fault-tolerant quantum computation.