Information & Computation

Center

Tuesdays, 2:00 pm, unless a special time or location is announced.

Location: 290 HMMB

Contact Wilfredo Balza if you are interested in joining the seminar mailing list: wilfredo (-at-) berkeley (dot) edu

- 08/30/2013, *Special time*: 3:30 PM.

- John Gough (Aberystwyth University, Wales UK)
- Interconnection and Control of Quantum Systems
- We discuss the "SLH" formalism for connected Markovian input-output components, and consider the case of non-Markovian models.

- 09/24/2013, 2:00 PM

- Alireza Marandi (Stanford University)
- Network of Optical Parametric Oscillators for Solving NP-Complete Problems
- Quantum computers promise to provide computational powers beyond conventional computers. The most prominent forms, including circuit-based and adiabatic quantum computers are based on closed quantum systems; and computation with open dissipative quantum systems is largely unexplored. In this talk, we show how a network of degenerate optical parametric oscillators (OPOs) with configurable couplings represents a programmable artificial Ising spin system. We experimentally show that a 4-OPO network can solve an instance of NP-complete problem corresponding to a frustrated antiferromagnet. The experimental and simulation results suggest that such a scalable Ising machine can be used to efficiently solve NP-complete problems. Our approach of time division multiplexing of femtosecond OPOs using widely available optical technologies at room temperature can open opportunities to achieve computational powers not yet available.

- 11/12/2013, 2:00 PM

- Leigh Norris (University of New Mexico)
- Enhanced Spin Squeezing Through Quantum Control
- Spin squeezed states are entangled quantum many-body states with applications in metrology and quantum information processing. While there has been significant progress in producing spin squeezed states and understanding their properties, most spin squeezing research to date has focused on ensembles of two-level systems or "qubits". We explore squeezed state production in an ensemble of spin f>1/2 alkali atoms or "qudits". The Faraday effect, which couples the collective spin of the atomic ensemble and the polarization modes of an optical field, can be used to mediate entangling interactions between the atoms that generate spin squeezing. Although these entangling interactions are inherently nonlocal, we find that local control of the atomic qudits substantially enhances the entangling power of the atom-light interface. Further control of the atomic qudits converts entanglement into metrologically useful spin squeezing. The amount of spin squeezing we can achieve ultimately depends upon a balance between increased entanglement generation and increased susceptibility to decoherence.

- 11/26/2013, 2:00 PM

- Alexander Eisfeld (Max-Planck-Institute, Germany)
- Optical and Transport Properties of Molecular Aggregates
- The transition dipole-dipole interaction between closely spaced chromophores leads to delocalized excitonic states which often results in drastic changes in the optical properties, exemplified for example by the narrow red-shifted J-band of certain organic dye molecules. This interaction is also responsible for the transfer of electronic excitation between the chromophores. The optical and transfer properties depend not only on the arrangement of the chromophores within the aggregate, but also crucially on the interaction of the electronic excitation with nuclear degrees of freedom and the charge distribution of the environment. In this contribution I will discuss an quantum open-system approach to model these coupling to nuclear and environmental degrees of freedom. To solve the open system dynamics we use the non-Markovian Quantum State Diffusion approach [1,2]. I will present a new, numerically exact solution of the resulting stochastic Schrödinger equations. Finally, I will discuss how we calculate non-adiabatic excitation transport and optical spectra. Taking the Fenna-Matthews-Olson (FMO) light harvesting complex as example, I will show how the parameters needed in the open system model can be extracted using QM/MM simulations [3]. [1] G. Ritschel, J. Roden, W.T. Strunz, A. Aspuru-Guzik, and A. Eisfeld, J. Chem. Phys. Lett. 2 (2011) 2912 [2] G. Ritschel, J. Roden, W.T. Strunz, and A. Eisfeld, New J. Phys.}13 (2011) 113034 [3] S. Valleau, A. Eisfeld and A. Aspuru-Guzik, J. Chem. Phys.} 137 (2012) 224103

- 12/10/2013, 2:00 PM

- Sevag Gharibian (EECS, UC-Berkeley)
- Hardness of Approximation for Quantum Problems
- The polynomial hierarchy plays a central role in classical complexity theory. Here, we define a quantum generalization of the polynomial hierarchy, and initiate its study. We show that not only are there natural complete problems for the second level of this quantum hierarchy, but that these problems are in fact hard to approximate. Using these techniques, we also obtain hardness of approximation for the class QCMA. Our approach is based on the use of dispersers, and is inspired by the classical results of Umans regarding hardness of approximation for the second level of the classical polynomial hierarchy [Umans, FOCS 1999]. The problems for which we prove hardness of approximation for include, among others, a quantum version of the Succinct Set Cover problem, and a variant of the local Hamiltonian problem with hybrid classical-quantum ground states. This talk is based on joint work with Julia Kempe.

- 02/18/2014, 2:00 PM *Special location*: Bixby North (on north side of Latimer Hall)

- Seung Woo Shin (EECS, UC Berkeley)
- How “ Quantum ” is the D-wave Machine?
- Recently there has been intense interest in claims about the performance of the D-Wave machine. Scientifically the most interesting aspect was the claim in Boixo et al., based on extensive experiments, that the D-Wave machine exhibits large-scale quantum behavior. Their conclusion was based on the strong correlation of the input-output behavior of the D-Wave machine with a quantum model called simulated quantum annealing, in contrast to its poor correlation with two classical models: simulated annealing and classical spin dynamics. In this paper, we outline a simple new classical model, and show that on the same data it yields correlations with the D-Wave input-output behavior that are at least as good as those of simulated quantum annealing. Based on these results, we conclude that classical models for the D-Wave machine are not ruled out. Further analysis of the new model provides additional algorithmic insights into the nature of the problems being solved by the D-Wave machine.

- 03/11/2014, 2:00 PM

- Nicolas Spethmann (Dept. of Physics, UC Berkeley)
- Optically Measuring Force near the Standard Quantum Limit
- The Heisenberg uncertainty principle sets a lower bound on the sensitivity of continuous optical measurements of force. This bound, the standard quantum limit, can only be reached when a mechanical oscillator subjected to the force is unperturbed by its environment, and when measurement imprecision from photon shot-noise is balanced against disturbance from measurement back-action. We apply an external force to the center-of-mass motion of an ultracold atom cloud in a high-finesse optical cavity. The optomechanically transduced response clearly demonstrates the trade-off between measurement imprecision and back-action noise. We achieve a sensitivity that is consistent with theoretical predictions for the quantum limit given the atoms' slight residual thermal disturbance and the photodetection quantum efficiency, and is a factor of 4 above the absolute standard quantum limit.

- 03/18/2014, 2:00 PM

- Claire Thomas (Dept. of Physics, UC Berkeley)
- Flatband physics in an optical kagome lattice
- Ultracold atoms in optical lattices offer a way to experimentally realize a Bose-Hubbard Hamiltonian with novel controls and no defects. Optical lattices are formed by interfering laser beams over the macroscopic wave function of a Bose-Einstein condensate. In such a system, the atoms experience a periodic potential from the optical interference in analog to the potential that an electron experiences due to ions in a crystalline solid. In our lab we load bosons into a kagome lattice structure, which has a dispersionless and highly degenerate second excited energy band that may support a stable supersolid phase at low lattice filling. Accessing this phase requires significant reduction of the filling factor in our lattice and inversion of the bandstructure so that atoms occupy the flat band in the ground state. I will discuss a proposal to experimentally realize such a system by first creating a Mott insulator with low filling, then inverting the tunneling parameter and finally lowering the lattice depths, allowing a transition from the Mott insulator to the flat band. I will also discuss the signatures of such a phase in our experiment.

- 03/25/2014, 2:00 PM

- Steve Weber (Dept. of Physics, UC Berkeley)
- Mapping the optimal route between two quantum states
- The length of time that a quantum system can exist in a coherent superposition is determined by how strongly it interacts with its environment. Unmonitored environmental fluctuations can be viewed as a source of noise, causing random evolution of the quantum system from an initially pure state into a statistical mixture. However, by accurately measuring the environment in real time, the quantum system can be maintained in a pure state and its time evolution described by a "quantum trajectory'' determined by the measurement outcome. We use weak measurements to continuously monitor a microwave cavity embedding a superconducting qubit and track the individual quantum trajectories of the system. We analyze ensembles of trajectories to determine statistical properties such as the most likely path and most likely time connecting pre- and post-selected quantum states. We compare our results with theoretical predictions derived from an action principle for continuous quantum measurement. Furthermore, by introducing a qubit drive, we investigate the interplay between unitary state evolution and non-unitary measurement dynamics.

- 04/29/2014, 2:00 PM

- Daniel Lidar (Dept. of Chemistry and CQIST, Univ. of Southern California)
- Probing the D-Wave processor: quantumness, benchmarking, and error correction
- Claims by D-Wave Inc. that they have constructed a large-scale quantum optimizer using superconducting flux qubits have attracted much attention in the popular media and scrutiny from the academic community. In this talk I will report on our progress in attempting to resolve the D-Wave processor quantumness question, its performance in benchmarking tests against classical algorithms, and our work on improving the processor's performance using error suppression and correction.

Atoms in a 2-dimensional addressable optical lattice. Unit occupancy of each site is achieved through a loading and compactification procedure.