Southwest Quantum Information and Technology

Twelfth Annual Meeting, February 18-21, 2010
Santa Fe, New Mexico

Full Program | Thursday | Friday | Saturday | Sunday

SESSION 1: CQuIC Kickoff Keynotes
Session Chair:
4:00-4:30	Carlton Caves, University of New Mexico (invited)
	Quantum-circuit guide to optical and atomic interferometry
	Abstract. Atomic (qubit) and optical or microwave (modal) phase-estimation protocols are placed on the same footing in terms of quantum-circuit diagrams. Circuit equivalences are used to demonstrate the equivalence of protocols that achieve the Heisenberg limit by employing entangled superpositions of Fock states, such as N00N states. The key equivalences are those that disentangle a circuit so that phase information is written exclusively on a mode or modes or on a qubit. The Fock-state-superposition phase-estimation circuits are converted to use entangled coherent-state superpositions; the resulting protocols are more amenable to realization in the lab, particularly in a qubit/cavity setting at microwave frequencies.
4:30-5:15	Gerard Milburn, The University of Queensland (invited)
	Quantum control and computation in circuit quantum electrodynamics.
	Abstract. The new field of circuit quantum electrodynamics (circuit QED for short) has developed in less than a decade driven by technological improvements in the ability to fabricate small circuits from superconducting metals. Much of this development has been motivated by the possibility of implementing quantum computing in such systems, but they are of much wider interest. In this talk I will discuss the feasibility of a number of schemes for quantum feedback control enabled by the new technology. Lehnert has recently demonstrated quantum limited interferometry with high readout efficiency, equivalent to that of a photo-detector reading out an ideal interferometor with efficiency=0.27. This opens up the possibility of doing some important quantum feedback control experiments that are very difficult to do in an atomic or quantum optical setting but very much more feasible in circuit QED. In a quantum optical setting, quantum limited feedback requires that we use all the light leaving the cavity in the measurement process. This is difficult to do in an optical setting but in principle easier in a circuit QED setting. Unlike in an optical setting, all the measured fields are voltages and currents at GHz frequencies on a superconducting wire and thus there is no need to convert from an optical frequency down to a fast electronic signal. Finally the time scales are slower in a circuit to what they are in an all-optical setting and thus fast feedback is more feasible, even with some in-line signal processing. On the other hand, circuit QED presents a difficulty that is not found in optics: we need to make quantum limited homodyne measurements on the cavity output. Lehnert's scheme uses a Josephson parametric amplifier (JPA) which is a phase sensitive amplifier. JPAs have long been used in superconducting electronics, but a key difference in the new devices is the presence of a significant Kerr nonlinearity. I will discuss the quantum noise performance of such devices in circuit QED.
6:45-7:30	William Phillips, Joint Quantum Institute (invited)
	Simulated Electric and Magnetic Fields for Quantum Degenerate Neutral Atoms
	Abstract. William D. Phillips, Robert L. Compton, Karina Jiménez-García, Yu-Ju Lin, James V. Porto, and Ian B. Spielman Joint Quantum Institute, National Institute of Standards and Technology, and University of Maryland, Gaithersburg, Maryland, 20899, USA We create an effective vector potential for ultra-cold neutral 87Rb atoms by applying laser beams that Raman-couple different magnetic sublevels having different linear momenta. The resulting distorted energy-momentum dispersion relationship is analogous to the Hamiltonian for a charged particle in a magnetic vector potential. A time-varying effective vector potential creates a synthetic electric field, and a spatially varying vector potential creates a synthetic magnetic field. Measuring the momentum imparted to the atoms allows a direct measurement of the impulse imparted by the synthetic electric field, and observation of vortices in the atom cloud reveals the action of the synthetic magnetic field. Such synthetic fields should address some of the difficulties in other approaches to using neutral atoms as quantum simulators of the integer and fractional quantum Hall effects. This work was supported by DARPA/ARO, the NSF, and the ONR.
7:30-8:15	Andrew Landahl, Sandia National Laboratories (invited)
	How to build a fault-tolerant logical qubit with quantum dots
	Abstract. After a brief overview of Sandia's quantum information science effort, I will focus on our current "Grand Challenge" QIST program, a large part of which is aimed at designing a fault-tolerant logical qubit with quantum dots. Because this technology may not be familiar to everyone, I will spend some time reviewing it with especial focus on why silicon might be a good material for quantum-dot qubits. Many theoretical analyses of fault-tolerant quantum error correction omit engineering-level constraints such as the space needed to route control wires to the qubits. We have found that these kinds of considerations have a HUGE impact on the accuracy threshold, and in fact can even cause the accuracy threshold to disappear altogether. I will discuss how theoretical ideas such as quantum local check codes and dynamical decoupling can ameliorate some of these constraints in the quantum dot setting. We have developed several logical qubit architectures based on these ideas, and using high-performance computing we have generated optimal schedules for processing them. Our Monte-Carlo simulations point to the accuracy threshold disappearing entirely if dynamical decoupling is not used in conjunction with fault-tolerant quantum error correction, and when it is, the threshold lies between roughly 10^{-5} to 10^{-3} depending on which local check code is used. Based on arXiv:0904.0003. This work was supported through the Laboratory Directed Research and Development program at Sandia National Laboratories. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.
8:15-9:00	Richard Hughes, Los Alamos National Laboratory (invited)
	Quantum Key Distribution: longer ranges and stronger security with superconducting detectors and decoy states
	Abstract. The past few years have seen dramatic advances in the range, rate and security of quantum key distribution (QKD) over optical fiber. These advances have arisen from the development of decoy-state protocols and new superconducting single-photon detector technologies. The former permit rigorous security without adversely impacting the signal-to-noise, and the later offer lower error rates with higher clock rates than conventional detectors. I will describe the results of QKD experiments using superconducting single photon detectors, and the prospects for incorporating decoy-state QKD into transparent optical fiber networks.