Southwest Quantum Information and Technology

Eleventh Annual Meeting, February 19-22, 2009
Seattle, Washington

Full Program | Thursday | Friday | Saturday | Sunday

SESSION 1: Condensed Matter QI
Session Chair:
6:00-6:45	Jack Harris, Yale University (invited)
	Optomechanical systems
	Abstract. Very sensitive mechanical detectors are rapidly approaching a regime in which either the mechanical device itself or its readout should demonstrate quantum behavior. The main technical barrier to reaching this regime has been the difficulty of integrating ultrasensitive micromechanical devices with high-finesse optical cavities. Recently we have developed a robust means for addressing this issue, and have integrated a 50 nm-thick membrane (with a quality factor > 1,000,000) into an optical cavity with a finesse ~ 200,000. Although the membrane is nearly transparent, it couples to the optical cavity dispersively. This coupling is strong enough to laser-cool the membrane from room temperature to 7 mK. In addition, the dispersive nature of the optomechanical coupling allows us to realize a sensitive "displacement squared" readout of the membrane. Such a readout is a crucial requirement for measuring quantum jumps in a mechanical oscillator. We will describe these results, as well as our progress towards observing quantum effects in this system.
6:45-7:15	Malcolm Carroll, Sandia National Laboratories
	Development of a Silicon Physical Qubit and Single Logical Qubit Design
	Abstract. An overview will be given of both experimental and theoretical development of a single error corrected logical qubit using silicon based hardware. The physical qubit research centers on demonstrating a basic qubit fabricated in an accumulation mode silicon metal oxide semiconductor (MOS) structure. The experimental component of the logical qubit focuses on the classical-quantum circuit interface and its impact on error correction. The logical qubit effort includes both hardware development, such as cryogenic complementary metal oxide semiconductor (CMOS), and a theoretical component, which examines a quantum error correction circuit architecture. The theoretical analysis accounts for more realistic constraints suggested by the physical qubit research while providing insight and feed-back about choices of lay-out, transport and error code choice. We note that some insight drawn from constraints of working in a cryostat may be more generally useful to other quantum computing architectures using cryogenics. In summary, the goal of this combined engineering effort is to more completely understand the design of a single solid-state logical qubit and work towards development of the required silicon qubit hardware elements (e.g., single qubit and read-out) with which to build it.
7:45-8:15	Thaddeus Ladd, Stanford University
	Recent Progress in Quantum Computing with Optically Controlled Semiconductors
	Abstract. I will present two recent experimental results from the Yamamoto group at Stanford. The first is the rapid initialization and subsequent coherent manipulation of a single electron spin qubit in a self-assembled InAs quantum dot using ultra-fast laser pulses. This result demonstrates a complete single qubit gate set at the highest possible clock speed for the system. The second is the generation of indistinguishable single photons from two separate semiconductor sources based on isolated donor-bound excitons in ZnSe/ZnMgSe quantum wells. This result demonstrates a tool of great importance for linear optics quantum computing; it also shows promise for mass-production of homogeneous, optically connected semiconductor qubits. I will also briefly indicate some theoretical work on implementing all-optical quantum logic and designing a complete quantum computer architecture around these elements.
8:15-8:45	Haitao Quan, Los Alamos National Laboratory
	Quantum Fidelity and Thermal Phase Transitions
	Abstract. We study the quantum fidelity approach to characterize thermal phase transitions. Specifically, we focus on the mixed-state fidelity induced by a perturbation in temperature. We consider the behavior of fidelity in two types of second-order thermal phase transitions (based on the type of non-analiticity of free energy), and we find that usual fidelity criteria for identifying critical points is more applicable to the case of $\lambda$ transitions (divergent second derivatives of free energy). Our study also reveals that for fixed perturbations, the sensitivity of fidelity at high temperatures (where thermal fluctuations wash out information about the transition) is reduced. From the connection to thermodynamic quantities we propose slight variations to the usual fidelity approach that allow us to overcome these limitations. In all cases we find that fidelity remains a good pre-criterion for testing thermal phase transitions, and we use it to analyze the non-zero temperature phase diagram of the Lipkin-Meshkov-Glick model.