Southwest Quantum Information and Technology

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

Full Program | Thursday Tutorials | Friday | Saturday | Sunday

SESSION 13: Breakout I - Quantum and Atom Optics
Session Chair:
12:00-12:30	Kavan Modi, University of Texas
	The role of state preparation in quantum process tomography
	Abstract. The immense computational power of a quantum computer comes with a cost - the fragility of entangled quantum states from coherence loss. Although decoherence is present in all physical systems, the effect of these logic errors can be eliminated by using error correcting codes provided gate errors fall below a fault tolerance threshold. This threshold depends on system architecture and specific forms of decoherence, but is likely to be in the 10^-4 range. The measurement of gate fidelity is thus a critical step for implementing fault tolerant quantum computing. Most experiments determine coherence through T1 and T2 measurements, which gives only a simple description of error process in qubits. A more full and precise measurement is based on density matrix measurements of qubit states, which leads to a description of coherence in terms of state and process tomography. We study the effects of preparation of input states in quantum process tomography experiments. We study two preparation procedures, stochastic preparations and preparations by measurements. We show that for stochastic preparation procedure, linear process maps adequately describe the process. But when linear process maps are obtained from systems initially prepared using von Neumann measurements, they cannot describe the process adequately. We introduce a quadratic process map that can describe the processes initialized by preparation by measurements. I will discuss the consequences of the quadratic map and its properties.
12:30-13:00	Cody Leary, Oregon Center for Optics, University of Oregon
	Single-Photon Spin-Orbit Coupling for Cluster State Quantum Computation
	Abstract. When a quasi-paraxial photon propagates through a cylindrically symmetric inhomogeneous transparent medium such that the inhomogeneity is slowly varying over the spatial extent of the photon’s transverse electric field, its spin angular momentum s and its orbital angular momentum l are coupled. That is, photons in eigenmodes with the formerly degenerate propagation constant k but different values of s and l undergo splitting in k according to k + k(A + B s l) in the presence of the inhomogeneity. The constants A and B are both small compared to unity and are determined by the properties of the medium. This is photon spin-orbit coupling (SOC). In the case of a multimode step-index optical fiber, this k splitting gives rise to a rotational effect in the transverse spatial field distributions of the higher order fiber modes, in which left (right) circularly polarized modes resembling free-space Hermite-Gauss (H-G) modes rotate clockwise (counterclockwise) as they propagate through the fiber. Due to these rotations, single-photon SOC can be used to exploit the transverse spatial photonic degrees of freedom in order to create cluster states for use in fiber-based linear optical quantum computation. We propose fiber-based spin-orbit fusion gate elements towards the creation of cluster states entangled in H-G mode.
13:00-13:30	Rolando Somma, Perimeter Institute
	Quantum Simulated Annealing
	Abstract. During the last years it has been shown that if a large quantum computer existed today, certain problems could be solved with them much more efficiently than their classical counterparts. Some of these problems include the quantum simulations of physical systems. In this talk I will show how quantum computers can be used to simulate and compute properties of classical systems in equilibrium. In particular, I will present a quantum algorithm that simulates annealing processes, where the (quantum) annealing rate greatly outperforms other classical methods like Markov chain Monte-Carlo based algorithms.
13:30-14:00	Fernando Cucchietti, Los Alamos National Laboratory
	Polarons in Bose-Einstein condensates
	Abstract. I will describe the behavior of impurities in a Bose-Einstein condensate using analogies with the problem of electrons in ionic crystals -- i.e. the "quantum simulation" of condensed-matter polarons using ultra-cold atoms. In the strong coupling regime, the impurities take on a self-localized state that is smaller than the healing length of the condensate. For intermediate to weak coupling, a different variational approach allows us to calculate analytic expressions for the effective mass of the BEC-polarons and its dispersion relation. I will discuss applications of this quantum simulation as well as its experimental viability.