Berkeley Quantum
Information & Computation
Center

Berkeley Quantum Information and Computation Seminar

Fortnightly on Tuesdays, 2:00 pm 410 Hearst Mining Building (map)

If you're interested in joining the seminar mailing list, presenting a talk, or just more information, contact Alireza Shabani: shabani (-at-) berkeley (dot) edu

2011-2012

9/7/2011 Special time and place: 11:00-12:00, in 325 LeConte Hall. (Joint QIC/AMO seminar)
Diedrich Leibfried (NIST)
Towards scalable quantum information processing and quantum simulation with trapped ions
I will give a brief general introduction to quantum information processing and then discuss experiments towards Quantum Information Processing (QIP) and Quantum Simulation (QS) with trapped ions. Most requirements for QIP and QS have been demonstrated in this system, with two big challenges remaining: Improving operation fidelity and scaling up to larger numbers of qubits. The architecture pursued a the Ion Stage Group at NIST is bsed on quantum information stored in long lived internal (hyperfine) states of the ions. We investigate the use of laser beams and microwave fields to induce both single-qubit rotations and multi-qubit gates mediated by the Coulomb interaction between ions. Moving ions through a multi-zone trap architecture allows for keeping the number of ions per zone small, while sympathetic cooling with a second ion species can remove energy and entropy from the system. After an introduction to these elements, I will discuss the current status of experiments and some future perspectives for QIP and QS as well as for other applications based on trapped ions. This work has been supported by IARPA, DARPA, NSA, ONR, and the NIST Quantum Information Program.
11/8/2011
Armand C. R. Niederberger (Stanford University)
Quantum circuits: from concept to future applications
Current experimental progress in quantum optics and nanophotonics is establishing a solid base for fascinating future applications. We may soon be able to create integrated circuits of nanophotonic components for ultra-low power and ultra-high speed optical switching. My theory seminar presents the methods with which we are currently studying photonic circuit models and discusses examples of circuits for classical photonic logic. First, I give a brief overview of the rationale behind the use of nanophotonics in general and the advantages of using optical interconnects over electronic interconnects in particular. Second, I present our high-level quantum hardware description language which links graphical circuit design tools with recent mathematical developments to describe open quantum optical networks, thus enabling scientists and engineers to simulate quantum circuits without having to deal with the details of quantum optics. Third, I show how we perform design optimization on nanophotonic circuits using adjoint calculus. This method is based on the use of Lagrange multipliers and drastically reduces the number of computations in parameter optimization and stability analysis.
11/22/2011
Paolo Zanardi (University of Southern California)
Does a closed quantum system equilibrate?
I will discuss the issue of whether and how we can make sense of the notion of equilibration("convergence" to equilibrium) for a large but finite quantum system with only internal degrees of freedom. (i.e., closed). I will illustrate our recent results on equilibration of the Loschmidt echo in nearly-critical quantum many-body systems evolving unitarily.
2/14/2012
Lin Tian (School of Natural Sciences, University of California, Merced)
Quantum wavelength conversion and transmission in optomechanical systems
Optomechanical systems with strong light-matter interaction can be explored as an interface between photon modes of distinct wavelengths, e.g. an optical mode and a microwave mode. In this talk, we study transient and adiabatic schemes for cavity state conversion and for photon transmission in the optomechanical system. Our results can be applied to various applications in optical quantum information processing, such as photon pulse generation and state manipulation, quantum repeaters, and conversion of information between different photon modes.
3/13/2012
Constantin Brif (Sandia National Laboratories)
Protecting quantum gates from control noise
External controls are necessary to enact quantum logic operations, and the inevitable control noise will result in gate errors in a realistic quantum circuit. We investigate the robustness of quantum gates to random noise in an optimal control field, by utilizing properties of the quantum control landscape that relates the physical objective (in the present case, the quantum gate fidelity) to the applied controls. An approximate result obtained for the statistical expectation value of the gate fidelity in the weak noise regime is shown to be in excellent agreement with direct Monte Carlo sampling over noise process realizations for fidelity values relevant for practical quantum information processing. Using this approximate result, we demonstrate that maximizing the robustness to additive/multiplicative white noise is equivalent to minimizing the total control time/fluence. Also, a genetic optimization algorithm is used to identify controls with improved robustness to colored noise.
3/20/2012
Na Young Kim (Stanford University)
Exciton-Polariton Quantum Emulators
Microcavity exciton-polaritons are hybrid light-matter quasi-particles arising from the mixed states between cavity photons and quantum well excitons. The inherent lightmatter duality provides experimental advantages: the stimulated scattering among interacting particles and the small effective mass (~ 10e-8 times the hydrogen atom) form coherent condensate states at high temperatures (e.g. 4 K in GaAs and room temperature in GaN materials). In addition, the dynamics of exciton-polaritons are accessed by capturing the leaked photons out of the cavity due to the short lifetime. Utilizing coherence and open-dissipative nature of exciton-polariton condensates, we engineer a two-dimensional (2D) polariton-lattice system for investigating exotic quantum phase order. Via micro-photoluminescence measurements in both real and momentum spaces, we have observed d-orbital condensate states, vortex-antivortex phase order, massless Dirac dispersions in 2D square, honeycomb, and triangular lattices respectively. These results demonstrate that the polariton-lattice systems will be promising solid-state quantum emulators in the quest for better understanding strongly correlated materials and in the development of novel optoelectronic devices.
4/4/2012 **Special Date**
Daniel Lidar (University of Southern California)
Benchmarking and Protecting Adiabatic Quantum Computation
USC and Lockheed Martin recently jointly founded a new quantum computing center housing the 128 qubit Rainier chip built by D-Wave. I will report on our efforts to benchmark the chip, by comparing its performance in solving random instances of spin glasses against classical solvers. I will also describe our very recent theoretical and experimental work on designing decoherence-protected adiabatic quantum computation.
4/5/2012 **Special Date and time: 10:30 AM**
Howard Wiseman (Griffith University, Australia)
How many bits does it take to track an open quantum system?
In general if one obtain information about an open quantum system by measuring its environment, that measurement will alter the future evolution of the system. However in the Markovian case this back-action is negligible and one can "track" the system i.e. assign it a (stochastically evolving) pure state at all times without disturbing its (deterministic) average evolution. In general this stochastic evolution creates a trajectory passing through infinitely many different pure states, even for a finite dimensional quantum system. Hence an infinite classical memory would be required to track such evolution. Here we show that, for any ergodic master equation, there should exist a monitoring scheme (which in general must be adaptive) on the environment that can confine the system state to jumping between finitely many states, so that only a finitely large classical memory is required [1]. We prove explicitly that one bit is always sufficient to track a qubit [1]. We also investigate the stability of these monitoring schemes [2]. [1] R. I. Karasik and H. M. Wiseman, Phys. Rev. Lett. 106, 020406 (2011). [2] R. I. Karasik and H. M. Wiseman, Phys. Rev. A 84, 052120 (2011).
4/10/2012
Adrian Feiguin (University of Wyoming)
Using symmetries to understand molecular devices and magnetic ad-atoms on substrates
Realizing a quantum transistor built of molecules or quantum dots has been one of the most ambitious challenges in nanotechnology. Even though remarkable progress has been made, being able to gate and control nanometer scale objects, as well to interconnect them to achieve scalability remains extremely difficult. Most experiments concern a single quantum dot or molecule, and they are made at ultra low temperature to avoid decoherence and tunnelling. We propose to use canonical transformations to design quantum devices that are protected by symmetry, and therefore, may be operational at high temperatures. We illustrate the idea with an example of a quantum transistor controlled by a gate electrode in a three terminal setup. We consider the effects of interactions, and we find that the same principles can be applied to design a device that could operate as an electrically controlled spin qubit. I will show that similar but more sophisticated principles can be used to improve our understanding of the effects of magnetic ad-atoms on substrates, such as graphene.
5/03/2012
Mark Johnson (D-Wave Systems Inc.)
Quantum Annealing with Superconducting Flux Qubits
D-Wave Systems has implemented a processor based on Quantum Annealing, an algorithm for finding the ground state of a system of interacting spins. The technology is built on a superconducting chip composed of flux qubits that enable a quantum annealing algorithm, and digital components that apply programmable on-chip flux biases. In this presentation, I will review Quantum Annealing, and then give a brief overview of the processor architecture. I will then discuss a method for observing the system dynamics during the annealing process for a sample eight spin problem instance, and describe how the temperature dependence of these dynamics provides a signature of Quantum Annealing.
Past seminars