Institute for Quantum Information

Seminar Abstracts

Below are the abstracts for some of the seminars listed on the "Seminars & Workshops" page.

July 17: IQI Seminar
      Arka Majumdar, Stanford University
      3:00 pm, 107 Annenberg

Title: Strong photon-photon and photon-phonon interaction in quantum dot CQED


Quantum dots (QDs) coupled to optical cavities constitute a scalable, robust, on-chip, semiconductor platform for probing fundamental cavity quantum electrodynamics, as well as for building classical and quantum information processing devices. Very strong interaction between light and matter can be achieved in this system as a result of the field localization inside sub-cubic wavelength volumes, leading to vacuum Rabi frequencies in the range of tens of GHz.
We have employed a platform consisting of a single InAs QD in a GaAs photonic crystal cavity to study quantum optics and cavity QED. We probed the optical nonlinearity that occurs in the strong coupling regime, and employed it to perform classical optical switching at single photon level as well as to generate non-classical light.  Additionally, we have been able to achieve an experimentally novel regime of cavity QED by strongly coupling a single QD to a photonic molecule consisting of two proximity-coupled cavities. Finally, we showed how the interaction of the quantum dot with acoustic phonons gives rise to novel phenomena unique to a solid state cavity QED system. We used such strong photon-phonon interaction to efficiently read out the QD states by the off-resonant cavity emission.

June 12: IQI Seminar
      Olivier Landon-Cardinal, University of Sherbrooke
      3:00 pm, 107 Annenberg

Title: Variational tomography

Characterizing a quantum state is essential to benchmark quantum devices and, more broadly, to compare theoretical predictions to experimental realizations. However, standard techniques fundamentally require a number of experiments and a post-processing effort that scales exponentially with the number of particles. Recently, we've developed schemes that circumvent the exponential cost of tomography.

Taking a more targeted approach, we have developed schemes that enable (i) estimating the fidelity of an experiment to a theoretical ideal description [1], (ii) learning which description within a variational class of states best matches the experimental data [2-3].

In this talk, I will focus on task (ii), which we call variational tomography. I will present methods for identifying a state inside interesting variational classes such as matrix products states (MPS) [2], and multi-scale entanglement renormaisation ansatz (MERZ) [3]. For MERA, I will describe how to learn a state from a small number of efficiently-implementable measurements and fast prost-processing, without requiring unitary control

[1] da Silva, L.-C. and Poulin, PRL 107, 210404 (2011). [2] Cramer, Plenio, Flammia, Somma, Gross, Bartlett, L.-C., Poulin, Liu, Nature Cummun. 1, 149 (2010). [3] L.-C., Poulin, arXiv: 1204.0792.

May 29: IQI Seminar
      Peter Maurer, Harvard University
      3:00 pm, 107 Annenberg

Title: Quantum control of diamond defects for quantum information and bio-imaging

In recent years, much attention has focused on the quantum control of nanoscale systems under ambient condition; these efforts span a variety of goals ranging from quantum metrology to quantum communication and computation. In this talk, I will focus on recent efforts toward the realization of these goals by taking advantage of the spin properties of individual Nitrogen Vacancy (NV) color centers in diamond. I will begin by introducing a novel technique, which allows for the detection and manipulation of individual spins with resolution beyond the diffraction limit [1]; this constitutes an important building block for an NV-based quantum processor [2] and opens the door to nanoscale bio-imaging/sensing. I will then discuss the extraordinary readout and coherence properties of NV-based quantum registers, demonstrating single shot readout and storage times well beyond a second at room temperature [3]. The ability to store quantum information on a macroscopic time scale in small, portable devices is an important step toward the practical realization of "quantum money" type encryption primitives.

[1] Maurer et. al - Far-field optical imaging and manipulation of individual spins with nanoscale resolution - Nature Physics 6 - 912 to 918 - Sep. 2010
[2] Yao et. al. - Scalable architecture for a room temperature solid-state quantum information processor - Nature Communications 3 - 800 - Mar. 2012
[3] Maurer et. al. - Room temperature quantum bit memory exceeding one second - Science - in press

April 6: IQIM Seminar
      Glen Evenbly
      4:30 pm, 114 E Bridge

Title: The Scale-Invariant MERA and quantum criticality

The multi-scale entanglement renormalization ansatz (MERA) is a class of tensor network state for quantum lattice systems, which is based upon a coarse-graining transformation known as entanglement renormalization. MERA has been demonstrated to accurately represent the ground states of a variety of strongly correlated systems lattice systems in one and two spatial dimensions and, in particular, have been seen to offer a very natural description of quantum critical systems.

In this talk I will discuss the theoretical foundations of the MERA and also describe the application of MERA for the study of scale-invariant critical systems, including a prescription for extracting the conformal data (including the scaling dimensions, OPE coefficients, central charge etc.) of a scale-invariant theory.

March 14: Group Meeting
      Iman Marvian, University of Waterloo
      5:30 pm, 204 Annenberg

Title: Symmetry, Asymmetry and quantum information

The asymmetry properties of a state relative to some symmetry group specify how and to what extent the given symmetry is broken by the state. Characterizing these is found to be surprisingly useful for answering the following question: when a system’s dynamics has a particular symmetry, how does this constrain which final states of the system can be reached from a given initial state?  This question can be considered for both open-system and closed-system dynamics. It turns out that even for closed-system dynamics, one can find constraints on the possible state evolutions which are stronger than the conservation laws implied by Noether's theorem. Another motivation for the study of asymmetry comes from the field of quantum metrology. It turns out that the degree of success one can achieve in many metrological tasks depends only on the asymmetry properties of the state used for metrology. So a systematic study of these properties can help to develop optimal protocols and strategies for dealing with practical constraints such as noise.

March 13: IQI Seminar
      Benjamin Lev, Stanford University
      3:00 pm, 107 Annenberg

Title: Exploring quantum soft matter in an AMO setting

Laser-cooled and trapped gases of neutral atoms can serve as versatile testbeds for exploring the organizing principles of quantum matter.  Although recent experiments can access the strongly correlated physics of gases and insulators, quantum realizations of everyday soft matter---glasses and liquid crystals that lie intermediate between canonical examples of order (crystals) and disorder (gases)---have yet to be created using ultracold atoms.  My group aims to elucidate the interplay between superfluidity, crystallinity, and magnetism in quantum soft matter using novel techniques developed to: 1) realize quantum dipolar gases for exploring quantum liquid crystal physics; 2) manipulate ultracold atoms near cryogenic surfaces for high-resolution, high-sensitivity imaging of (topologically protected) transport in, e.g., unconventional superconductors; 3) realize supersmectic, superglass, and spin glass phases in a many-body, multimode cavity QED context.   As a step forward, we recently created the first quantum degenerate dipolar Fermi gas as well as a strongly dipolar Bose-Einstein condensate by laser cooling and trapping the highly complex and most magnetic element, dysprosium. 

February 22: Group Meeting
Volkher Scholz, Leibniz University Hannover
      5:30 pm, 206 Annenberg

Title: The smooth entropy formalism on operator algebras and applications to the security analysis of CV crypto schemes

We first discuss the extension of the smooth entropy formalism of Renner to arbitrary physical systems with no bound on the number of degrees of freedom, and then show how to employ a version of the entropic uncertainty relation to provide a security analysis for continuous variable quantum key distribution protocols. We discuss protocols based on the transmission of squeezed vacuum states measured via homodyne detection, and give a lower bound on the number of secret bits which can be extracted from a finite number of runs of the protocol. This bound is valid under general coherent attacks, and gives rise to keys which are composably secure.

Based on arXiv:1107.5460, 1112.2179. This is joint work with Mario Berta, Fabian Furrer as well as with Torsten Franz, Marco Tomamichel, Reinhard Werner.

February 21: IQI Seminar
Andre Chailloux, UC, Berkeley
      3:00 pm, 107 Annenberg

Title:The complexity of the Local Separable Hamiltonian

We study variants of the canonical Local-Hamiltonian problem where, in addition, the witness is promised to be separable. We define two variants of the Local-Hamiltonian problem. The input for the Separable-Local-Hamiltonian problem is the same as the Local-Hamiltonian problem, i.e. a local Hamiltonian and two energies a and b, but the question is somewhat different: the answer is YES if there is a separable quantum state with energy at most a, and the answer is NO if all separable quantum states have energy at least b.
The Separable-Sparse-Hamiltonian problem is defined similarly, but the Hamiltonian is not necessarily local, but rather sparse. We show that the Separable-Sparse-Hamiltonian problem is QMA(2)-Complete, while Separable-Local-Hamiltonian is in QMA. This should be compared to the Local-Hamiltonian problem, and the Sparse-Hamiltonian problem which are both QMA-Complete. To the best of our knowledge, Separable-SPARSE-Hamiltonian is the first non-trivial problem shown to be QMA(2)-Complete.

oint work with Or Sattath. 

February 14: IQI Seminar
      Fernando Pastawski, Max Planck Institute
      3:00 pm, 107 Annenberg

Unforgeable noise-tolerant quantum tokens

The realization of devices which harness the laws of quantum mechanics represents an exciting challenge at the interface of modern technology and fundamental science. A perfect example of the power of such quantum primitives is the concept of "quantum money". A dishonest holder of a quantum bank-note will invariably fail in any forging attempts; indeed, under assumptions of ideal measurements and decoherence-free memories such security is guaranteed by the no-cloning theorem. In any practical situation, however, noise, decoherence and operational imperfections abound. Thus, the development of secure "quantum money"-type primitives capable of tolerating realistic infidelities is of both practical and fundamental importance. We focus on approaches with minimal technological requirements consisting of the ability to prepare, store and measure single qubit. In this talk I will present and formally analyze the security and noise tolerance of such protocols and find tight fidelity thresholds.

February 7: IQI Seminar
David Perez-Garcia, Complutense University of Madrid
      3:00 pm, 107 Annenberg

Title: Understanding RVB states with PEPS

Using the Projected Entangled Pair State (PEPS) toolbox, we are able to map the Resonating Valence Bond State (RVB) in a Kagome lattice in the Quantum Dimer Model (QDM) to the toric code in a square lattice. This (i) gives a local Hamiltonian for the QDM-RVB with degeneracy 4 (which in addition can be chosen to be gapped or gapless) and (ii) gives a clear picture of the excitations and topological character of the RVB. Moreover, the same map gives (i) and (ii) for the SU(2)-RVB and allows to interpolate between the two (QDM-RVB and SU(2)-RVB). Numerical calculations using also the PEPS toolbox suggest the presence of a phase transition along this path. We will show finally how thePEPS picture of the SU(2)-RVB can help in designing ways of engineering it in a quantum computer.