Southwest Quantum Information and Technology

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

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

SESSION 13: Quantum Computation and Error Correction - Franciscan Room
Session Chair:
10:45am-11:15am	Sergio Boixo, Information Sciences Institute at the University of Southern California
	Progress on theoretical studies and practical applications of quantum annealing and D-Wave One
	Abstract. A D-Wave One quantum optimizer has been installed at the newly created USC-Lockheed Martin Quantum Computing Center. This chip implements quantum annealing at finite temperature as a computational resource, with 90 working qubits. Quantum annealing is a particularly simple branch of adiabatic quantum computation. We report work in progress on exploring practical applications of quantum annealing in general, and this chip in particular. We will also discuss entanglement tests with realistic numerical simulations of the physical devices implemented in the chip. Some of this work is done in collaboration with Aspuru-Guzik's group at Harvard, and D-Wave.
11:15am-11:45am	Nathan Wiebe, Institute for Quantum Computing
	Improved Error-Scaling for Adiabatic Quantum Evolutions
	Abstract. We present a new technique that improves the scaling of the error in the adiabatic approximation with respect to the evolution duration, thereby enabling the design of more efficient adiabatic quantum algorithms and adiabatic quantum gates. Our method is conceptually different from previously proposed techniques: it exploits a commonly overlooked phase interference effect that occurs predictably at specific evolution times, suppressing transitions away from the adiabatically transferred eigenstate. Our method can be used in concert with existing adiabatic optimization techniques, such as local adiabatic evolutions or boundary cancellation methods. We perform a full error analysis of our phase interference method along with existing boundary cancellation techniques and show a tradeoff between error-scaling and experimental precision. We illustrate these findings using two examples, showing improved error-scaling for an adiabatic search algorithm and a tunable two-qubit quantum logic gate.
12:15pm-12:45pm	Matteo Mariantoni, Department of Physics and California NanoSystems Institute, University of California, Santa Barbara
	The Photon Shell Game and the Quantum von Neumann Architecture with Superconducting Circuits
	Abstract. Superconducting quantum circuits have made significant advances over the past decade, allowing more complex and integrated circuits that perform with good fidelity. We have recently implemented a machine comprising seven quantum channels, with three superconducting resonators, two phase qubits, and two zeroing registers. I will explain the design and operation of this machine, first showing how a single microwave photon \|1> can be prepared in one resonator and coherently transferred between the three resonators. I will also show how more exotic states such as double photon states \|2> and superposition states \|0>+\|1> can be shuffled among the resonators as well [1]. I will then demonstrate how this machine can be used as the quantum-mechanical analog of the von Neumann computer architecture, which for a classical computer comprises a central processing unit and a memory holding both instructions and data. The quantum version comprises a quantum central processing unit (quCPU) that exchanges data with a quantum random-access memory (quRAM) integrated on one chip, with instructions stored on a classical computer. I will also present a proof-of-concept demonstration of a code that involves all seven quantum elements: (1), Preparing an entangled state in the quCPU, (2), writing it to the quRAM, (3), preparing a second state in the quCPU, (4), zeroing it, and, (5), reading out the first state stored in the quRAM [2]. Finally, I will demonstrate that the quantum von Neumann machine provides one unit cell of a two-dimensional qubit-resonator array that can be used for surface code quantum computing. This will allow the realization of a scalable, fault-tolerant quantum processor with the most forgiving error rates to date. [1] M. Mariantoni et al., Nature Physics 7, 287-293 (2011); [2] M. Mariantoni et al., Science 334, 61-65 (2011). Matteo Mariantoni acknowledges support from an Elings Postdoctoral Fellowship. This work was supported by IARPA under ARO award W911NF-08-1-0336 and W911NF-09-1-0375.
11:45am-12:15pm	Jonas Anderson, University of New Mexico
	Homological Stabilizer Codes
	Abstract. The discovery of quantum error correction and fault-tolerance were major theoretical breakthroughs on the road towards building a full-fledged quantum computer. Since then thresholds have increased and geometric constraints on the underlying architecture have been added. Homological stabilizer codes provide a method for constructing stabilizer codes constrained to a 2D plane. In this talk I will define and proceed to classify all 2D homological stabilizer codes. I will show that Kitaev's toric code and the topological color codes arise naturally in this classification. I will finally show, up to a set of equivalence relations, that these are the only 2D homological stabilizer codes.