Southwest Quantum Information and Technology

Fourteenth Annual Meeting, February 16-19, 2012
Albuquerque, New Mexico

Full Program | Thursday | Friday | Saturday | Sunday | All Sessions

SESSION 11c: Breakout C - Decoherence, Entanglement and Error Correction - Alvarado "C"
Session Chair:
4:15pm-4:45pm	Zhang Jiang, University of New Mexico
	Conditions imposing physical ancillary states in Stinespring dilations
	Abstract. While unitary transformations are used to describe state evolutions in closed quantum systems, the formalism of quantum operations is the more general approach for open systems. A valid quantum operation has to be completely positive, i.e., the output state for any physical input state, even those entangled with a third party, should also be physical. Often a quantum operation can be described by a Kraus representation. An alternative representation is by a measurement model or ancilla model, which is also called a Stinespring dilation by mathematicians. In an ancilla model, a quantum operation is realized by tracing out the ancilla after a joint unitary is applied on the primary system and the ancilla. Here we answer the following question: given an ancilla model with a particular joint unitary, what are the conditions on the joint unitary so that the ancilla state must be physical, i.e., a density operator, in order that the measurement model gives rise to a valid quantum operation.
4:45pm-5:15pm	Peter Rose, Carleton College
	Decoherence Leads to Non-monotonicity in the "Quantumness" of Fock States
	Abstract. We consider the evolution of Fock states \|n> of a harmonic oscillator coupled to a Markovian bath. The master equation in the number basis is an infinite number of coupled, first order differential equations which can be solved analytically at any temperature. Using the negative volume of the Wigner function as a metric of "quantumness", we show that in the absence of environmental coupling, quantumness increases with n, but the presence of any environmental interaction causes high-n states to lose their quantum features more rapidly leading to a time-dependent quantumness peak across the eigenstates. Our results are consistent with recent experiments.
5:15pm-5:45pm	Chris Cesare, Center for Quantum Information and Control, Department of Physics and Astronomy, University of New Mexico
	Relationships Between Defect Encodings for Topological Codes
	Abstract. Several schemes have been proposed in the literature for performing quantum computations using defects in topological codes. In the color codes, there are three schemes for storing qubits using defects: using single defects tethered to boundaries, using two defects tethered to one another, or using three defects tethered to each other. The three defect approach stands apart as the only known way to retain the transversality property of certain color code gates. As such, one might ask whether there is any relationship between this transversality-preserving encoding and the others, and if there exists a way to convert between them. We demonstrate this relationship by presenting such a method of conversion.
5:45pm-6:15pm	Gerardo Paz-Silva, University of Southern California
	Open-loop methods for protection of encoded information
	Abstract. We study the interplay of two well-known open-loop decoherence suppression methods, the Quantum Zeno effect and Dynamical decoupling, with quantum error correction codes. For the first part, the quantum Zeno effect case, we analyze the decoherence suppression induced only by the weak (non-selective) syndrome measurements in every error correction round, i.e. multiple rounds of error correction without the recovery step. We show that there is indeed a suppression effect despite the absence of recovery operations. For the second part, dynamical decoupling, we discuss the use of elements of the code as pulses and show that they provide an advantage over brute force multiqubit dynamical decoupling: they not only generate shorter sequences but impose no additional locality constraints on the noise besides the ones demanded by fault-tolerant models.