Southwest Quantum Information and Technology

Tenth Annual Meeting, February 14-17, 2008
University of New Mexico, Santa Fe, New Mexico

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SESSION 12: Quantum Processes and Decoherence
Session Chair: Andrew Landahl
10:45-11:15	David Poulin, California Institute of Technology
	Preserved information in quantum processes
	Abstract. I will derive a general structure theorem characterizing the information that can be preserved by a quantum process (CPTP map). This characterization builds on a very simple yet powerful operational definition of the notion of being preserved: a set of quantum states is preserved by a process if the states are as distinguishable before and after the process. This definition encompasses noiseless subsystems, decoherence-free subspaces, pointer bases, and error-correcting codes. More generally, I will demonstrate that all such information-preserving structure (IPS) is isomorphic to a matrix algebra. This provides a simple and efficient algorithm for finding all noiseless and unitarily noiseless IPS.
11:15-11:45	Anil Shaji, University of New Mexico
	Resources and decoherence in qubit metrology
	Abstract. In quantum parameter estimation, accuracies that beat the standard quantum limit can be obtained by using the quantum properties of the probes and by modulating the nature of the interaction between the probe and the measured system. When qubits are used to construct a quantum probe, it is known that initializing n qubits in an entangled state, rather than in a separable state, can improve the measurement uncertainty by a factor of $1/\\sqrt{n}$. It is also known that if the interaction between the probe and the measured system involves $k$-qubit couplings then the best possible scaling of the measurement uncertainty is $1/n^k$ for a probe initialized in an entangled state and $1/n^{k-1/2}$ for a probe initialized in a product state. We investigate how the measurement uncertainty is affected when the individual qubits in a probe are subjected to decoherence in measurement schemes involving both linear and nonlinear couplings. In the face of such decoherence, we regard the rate $R$ at which qubits can be generated and the total duration $\\tau$ of a measurement as fixed resources, and we determine the optimal use of entanglement among the qubits and the resulting optimal measurement uncertainty as functions of $R$ and $\\tau$.