Southwest Quantum Information and Technology

Eleventh Annual Meeting, February 19-22, 2009
Seattle, Washington

Full Program | Thursday | Friday | Saturday | Sunday

SESSION 7: Quantum Measurement
Session Chair:
1:00-1:30	Anil Shaji, The University of New Mexico
	Heisenberg limited phase estimation with mode-entangled coherent states
	Abstract. We investigate phase estimation in a Mach-Zehnder type interferometer using the ``0BB0" state which is a mode-entangled state formed by superposing a state with the vacuum in the first arm of the interferometer and a coherent state in the second arm and another state with the coherent state in the first arm and the vacuum in the second. The quantum Cramer-Rao bound on the measurement uncertainty in the estimate of an unknown phase shift between the two arms of the interferometer scales inversely with the mean photon number in the 0BB0 state (Heisenberg limited scaling). We discuss how 0BB0 states can be created and also the measurements that must be performed on the output state of the interferometer in order to find the phase shift. We compare the performance of the 0BB0 states in phase estimation with that of ``N00N" states. In the presence of photon loss, using 0BB0 states instead of N00N states, lead to lower measurement uncertainties.
1:30-2:00	John Sidles, University of Washington
	Renewing and Uniting Two Challenges of John von Neumann and Richard Feynman: Atomic-Resolution Biomicroscopy and Simulating Quantum Physics with Computers
	Abstract. In two renowned lectures, Richard Feynman (in 1959) challenged mathematicians, scientists, and engineers to "see the individual atoms" in biological molecules and (in 1982) to "make a simulation of nature [that is] quantum mechanical." An earlier statement of these same challenges can be found in a 1946 letter from John von Neumann to Norbert Weiner. The status of these two challenges is reviewed. Atomic-resolution biomicroscopy is treated as a problem in quantum communication whose fundamental quantum limits can be calculated by combining Feynman's formalism for quantum measurement with Shannon's formalism for information channel capacity. Modern advances in quantum information theory and simulation science suggest avenues for further analysis. The assessment concludes that both of von Neumann's and Feynman's challenges are rapidly approaching scientific and technological feasibility.
2:00-2:30	Kevin Young, University of California - Berkeley
	Optimal experiment design for parameter estimation as applied to dipole- and exchange-coupled qubits
	Abstract. We consider the problem of quantum parameter estimation with the constraint that all measurements and initial states are separable. Two qubits are presumed coupled through the dipole and exchange interactions. The resulting Hamiltonian generates a unitary evolution which, when combined with arbitrary single-qubit operations, contributes to a universal set of quantum gates. However, while the functional form of the Hamiltonian is known, a particular experimental realization depends on several free parameters - in this case, the position vector relating the two qubits and the magnitude of the exchange interaction. We use the Cramer-Rao bound on the variance of any point estimator to construct an optimal series of experiments to estimate these free parameters. Our method of transforming the constrained optimal estimation problem into a convex optimization is powerful and widely applicable to other systems.
2:30-3:00	Robert Cook, University of New Mexico
	Continuous measurement of a quantum phase transition in a collective atomic system weakly coupled to a single optical mode
	Abstract. We consider an atomic ensemble that is dispersively coupled to a high finesse optical cavity. The application of cavity assisted Raman transitions generate two body interactions that are symmetrically distributed across the entire ensemble. If the cavity mode rapidly decays to an external field, adiabatic elimination of the cavity produces effective atomic dynamics that are equivalent to a dissipative Lipkin-Meshkov-Glick model, which exhibits a zero temperature quantum phase transition. In the framework of quantum stochastic calculus, we derive the propagator that describes the effective coupling between the collective atomic spin and the external field. We then derive a filter that describes the atomic state conditioned on a continuous measurement of the external field. Finally, we simulate this measurement as the system is tuned though its critical parameter range.