Berkeley Quantum Information and Computation Seminar
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 .
 G. Ritschel, J. Roden, W.T. Strunz, A. Aspuru-Guzik, and A. Eisfeld, J. Chem. Phys. Lett. 2 (2011) 2912
 G. Ritschel, J. Roden, W.T. Strunz, and A. Eisfeld, New J. Phys.}13 (2011) 113034
 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)
- 03/25/2014, 2:00 PM
- Steve Weber (Dept. of Physics, UC Berkeley)