Berkeley Quantum
Information & Computation
Center

Berkeley Quantum Information and Computation Seminar

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

Location: 775 Tan Hall

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

2014-2015

11/18/2014, 2:00 pm 775 Tan Hall
Ty Volkoff (UC Berkeley)
Schrodinger cat states in separable Hilbert space: optimal branch distinguishability and minimal algebras for metrological usefulness
Two measures of macroscopicity for quantum superpositions in countably infinite dimensional Hilbert space will be introduced: one depending on the optimal distinguishability of the components of the superposition under measurements of subsets of particles and another based on the ratio of the quantum Fisher information of the superposition to that of its components. I will argue that these measures allow large quantum superpositions (e.g., Schrodinger cat states) to be considered as quantum resources (distinct from entangled states) for sub-Heisenberg limit Hamiltonian parameter estimation. Examples will be taken from multimode quantum optics, BECs, and (2+1)-dimensional quantum dimers. I will show that the optimal metrological usefulness of a Schrodinger cat state depends on the Lie algebra containing the observables of the system and I will provide a method for constructing the optimal Lie algebra by compressing unbounded observables to a subspace containing the cat state.
10/28/2014, 2:00 pm 775 Tan Hall
Yasaman Bahri (UC Berkeley)
Protected quantum coherence at infinite temperature in many-body systems
I discuss two recent conceptual advances in condensed matter physics -- that of (i) symmetry-protected topological (SPT) phases and (ii) many-body localization (MBL) -- and the novel consequences that arise when they are combined. While SPT phases are often characterized by special boundary edge states, these states disappear at higher energies due to mobile thermal excitations. I will describe how combining SPT phases and MBL gives rise to protected edge states even at arbitrarily high energies without any fine tuning. This combination is currently the only known "robust" way in which topological physics can be extended to higher energies. In particular, I describe our work demonstrating that quantum information stored in a boundary qubit in such a system appears to be lost but can in fact be retrieved. These new advances pave the way for future studies of topologically protected quantum coherent dynamics, with relevance for quantum computation, in a wide array of systems.
10/15/2014, *Special time*: 12:00 pm *Special location*: 306 Soda
John Martinis (UC Santa Barbara)
Superconducting qubits poised for fault-tolerant quantum computation
Superconducting quantum computing is now at an important crossroad, where "proof of concept" experiments involving small numbers of qubits can be transitioned to more challenging and systematic approaches that could actually lead to building a quantum computer. Our optimism is based on two recent developments: a new hardware architecture for error detection based on "surface codes", and recent improvements in the coherence of superconducting qubits. I will explain how the surface code is a major advance for quantum computing, as it allows one to use qubits with realistic fidelities, and has a connection architecture that is compatible with integrated circuit technology. We have also recently demonstrated a universal set of logic gates in a superconducting Xmon qubit that achieves single-qubit gate fidelity of 99.92% and a two-qubit gate fidelity up to 99.4%. This places Josephson quantum computing at the fault-tolerant threshold for surface code error correction. Our quantum processor is a first step towards the surface code, using five qubits arranged in a linear array with nearest-neighbor coupling. Using this device we have further demonstrated generation of the five-qubit Greenberger-Horne-Zeilinger (GHZ) state using the complete circuit and full set of gates, giving a state fidelity of 82% and a Bell state (2 qubit) fidelity of 99.5%. These results demonstrate that Josephson quantum computing is a high-fidelity technology, with a clear path to scaling up to large-scale, fault-tolerant quantum circuits.
10/01/2014, *Special time*: 11:00 am *Special location*: 325 Old LeConte
Mark Saffman (University of Wisconsin)
Towards scalable quantum computing with cold atoms and Rydberg blockade
We are exploring several different approaches towards scalable quantum computing based on neutral atom qubits with long range Rydberg blockade interactions. I will start with an introduction to the atomic physicsof Rydberg blockade. The blockade interaction can be used in several different ways to entangle atomic qubits, and to create hybrid quantum interfaces between atoms and photons or between atoms and other matter based qubits. I will present entanglement results from experiments with single Cs atoms performed in a 2D array of optical traps, as well as progress towards entanglement in atomic ensembles and in a hybrid experiment designed to couple atoms to superconductors.
07/29/2014, 2:00 pm *Special location*: 775 Tan Hall
Enrique Solano (University of the Basque Country, Bilbao, Spain)
Quantum theatre of impossible physics
I will review the main goals of quantum simulations as our quantum theatre in current quantum technologies, ranging from aesthetics and the exchange of knowledge between different fields to the possibility of beating classical computers. I will explain then how it is possible to simulate unphysical operations, as complex conjugation and time reversal. Furthermore, I will show how this leads to the concept of embedding quantum simulators, allowing the simplified measurement of physical quantities that otherwise would require full tomography. Finally, I will give some examples on how to apply these concepts in current quantum technologies as trapped ions and superconducting circuits.
07/22/2014, 2:30 pm *Special location*: 412 O'Brien Hall
Mikel Sanz (University of the Basque Country, Bilbao, Spain)
Quantum biomimetics and bio-inspired quantum devices
For a long time, human beings mimicked nature to create or optimize devices and machines, as well as industrial processes and strategies. In particular, biomimetics is the branch of science which designs materials and machines inspired in the structure and function of biological systems. Analogously, novel quantum protocols may be envisioned by mimicking macroscopic biological behaviors at the microscopic level, in what we call quantum biomimetics. We will provide a survey of our last results in the topic, including bio-inspired protocols for cloning particle quantum information, ideas of quantum artificial life, quantum neurons and quantum neuron networks, quantum memristoric devices, among others. Finally, we will briefly discuss possible experimental implementations with current technologies.
07/22/2014, 3:00 pm *Special location*: 412 O'Brien Hall
Unai Alvarez-Rodriguez (University of the Basque Country, Bilbao, Spain)
Biomimetic cloning of quantum observables
We propose a method for cloning partial quantum information beyond the classical limit, which is inspired in the self-replication property of biological systems. This is a particular case of our Quantum Biomimetics project, aiming at creating quantum information models based on fundamental biological properties. Following the same line, I will explain the model of an artificial environment that mimics most ingredients of the natural selection process using quantum information tools, as well as an artificial quantum neural network that learns to find the shortest path between different points.
Past seminars

2013-2014

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.