other seminars:

:: Fri 10/24/14, 1:30 in 6C442 Chetan Nayak (Microsoft Research, Station Q and UC Santa Barbara) :: Fri 11/7/14, 1:30 in 6C442 Alan AspuruGuzik (Harvard University) Density functional theory (DFT) and it's timedependent counterpart (TDDFT) have been cornerstones in the field of numerical simulations, e.g. in computational materials science, ad computational physics and chemistry. The DFT theorems demonstrate that the exact energy of the ground state of a quantum system is a functional of the density of the system. The timedependent counterpart (TDDFT) demonstrates that a timeevolving density is enough, in theory, to obtain all observables of a quantum system. In this talk, I will discuss our group's work on the connections of timedependent density functional theory (TDDFT) and quantum information. In particular, we have proved the TDDFT theorems for the case of open quantum systems. I will discuss the implications of this to problems such as decoherence and the emergence of density functionals for dissipation and relaxation. We also recently showed that TDDFT theorems also exist for the case of distinguishable spin 1/2 systems (e.g. qubits) that are able to perform universal quantum computation. I will discuss the theorems and some potential applications to quantum simulation and also to the possibility of approximating the results of quantum information processing tasks. Recently, we have worked on the complexity of TDDFT as a procedure and show that, within certain physical and computational considerations, it belongs to the bounded quantum polynomial complexity class. This is joint work with Joel YuenZhou, currently a Silbey postdoctoral fellow at MIT, David Tempel, currently a postdoctoral researcher in my group, as well as James Whitfield, former PhD. student currently in Vienna, Sergio Boixo, now at Google and ManHong Yung now Assistant Professor at Tsinghua University. :: Fri 11/14/14, 1:30 in 6C442 Lorenza Viola (Dartmouth College) Hamiltonian engineering via unitary openloop quantum control provides a versatile and experimentally validated framework for precisely manipulating a broad class of nonMarkovian dynamical evolutions of interest, with applications ranging from dynamical decoupling and dynamically corrected quantum gates to noise spectroscopy and quantum simulation. In this context, transferfunction techniques directly motivated by control engineering have proved invaluable for obtaining a transparent picture of the controlled dynamics in the frequency domain and for quantitatively analyzing control performance. In this talk, I will show how to construct a general filterfunction approach, which overcomes the limitations of the existing formalism. The key insight is to identify a set of "fundamental filter functions", whose knowledge suffices to construct arbitrary filter functions in principle and to determine the minimum "filtering order" that a given control protocol can guarantee. Implications for dynamical control in multiqubit systems and/or in the presence of nonGaussian noise will be discussed. :: Fri 11/21/14, 1:30 in 6C442 John Preskill (California Institute of Technology) TBA :: Fri 12/5/14, 1:30 in 6C442 Jess Riedel (Perimeter Institute) TBA Brian Swingle (Stanford University) I will propose a mechanism whereby a dynamical geometry obeying Einstein's equations emerges holographically from entanglement in certain quantum manybody systems. As part of this broader story I will discuss in particular two crucial results: one establishing a geometric representation of entanglement in the vacuum state of a wide class of (lattice regulated) quantum field theories and one showing how the equivalence principle of gravity is encoded in the universality of entanglement. I will also briefly indicate how the first result opens the door to solving previously intractable strongly interacting models of relevance for experiments in the solid state and elsewhere. Thus I will argue that the fundamental physics of entanglement provides a window into nonperturbative quantum field theory and quantum gravity. :: Fri 2/27/15, 1:30 in 6C442 Ramis Movassagh (MIT) Entanglement is a quantum correlation which does not appear classically, and it serves as a resource for quantum technologies such as quantum computing. The area law says that the amount of entanglement between a subsystem and the rest of the system is proportional to the area of the boundary of the subsystem and not its volume. A system that obeys an area law can be simulated more efficiently than an arbitrary quantum system, and an area law provides useful information about the lowenergy physics of the system. It was widely believed that the area law could not be violated by more than a logarithmic factor (e.g. based on critical systems and ideas from conformal field theory) in the system’s size. We introduce a class of exactly solvable onedimensional models which we can prove have exponentially more entanglement than previously expected, and violate the area law by a square root factor. We also prove that the gap closes as n^{c}, where c ge 2, which rules out conformal field theories as the continuum limit of these models. In addition to using recent advances in quantum information theory, we have drawn upon various branches of mathematics and computer science in our work and hope that the tools we have developed may be useful for other problems as well. (Joint work with Peter Shor) :: Fri 3/20/15, 1:30 in 6C442 Richard Cleve (IQC, University of Waterloo) We present exact unitary 2designs on n qubits that can be implemented with Õ(n) elementary gates from the Clifford group. This is essentially a quadratic improvement over all previous constructions that are exact or approximate (for sufficiently strong approximations). This is joint work with Debbie Leung, Li Liu, and Chunhao Wang. :: Fri 4/17/15, 1:30 in 6C442 Lior Eldar (MIT) Quantum entanglement is considered to be a very delicate phenomenon that is hard to maintain in the presence of noise, or nonzero temperatures. In recent years however, and motivated by a quest for a quantum analog of the PCP theorem, researches have tried to establish whether or not we can preserve quantum entanglement at constant temperature that is independent of system size. This would imply that any quantum state with energy at most, say 0.05 of the total available energy of the Hamiltonian, would be highlyentangled. However to this date, no such systems were found. Moreover, it became evident that even embedding local Hamiltonians on robust, albeit "nonphysical" topologies, namely expanders, does not guarantee entanglement robustness. In this talk, I'll provide indication that such robustness may be possible after all, by showing a local Hamiltonian with the following property of inapproximability: any quantum state that violates a fraction at most 0.05 of all local terms cannot be even approximately simulated by classical circuits whose depth is sublogarithmic in the number of qubits. In a sense, this implies that even providing a "witness" to the fact that the local Hamiltonian can be "almost" satisfied, requires longrange entanglement. William Wooters (Williams College) Two orthogonal bases for a Hilbert space are called mutually unbiased if each vector in one basis is an equalmagnitude superposition of all the vectors in the other basis. The maximum number of mutually unbiased bases in a space of dimension d is d+1; so a set of d+1 such bases is called complete. Complete sets of mutually unbiased bases play significant roles in quantum cryptography and quantum tomography. For certain values of d, it is possible to find a single unitary transformation that, by repeated application, generates a complete set of mutually unbiased bases starting with the standard basis. This talk reviews our current understanding of such cycling unitaries and related transformations, and shows how their effects can be pictured in a discrete phase space. Beni Yoshida (Caltech) We propose a family of exactly solvable toy models for the AdS/CFT correspondence based on a novel construction of quantum errorcorrecting codes with a tensor network structure. Our building block is a special type of tensor with maximal entanglement along any bipartition, which gives rise to an exact isometry from bulk operators to boundary operators. The entire tensor network is a quantum errorcorrecting code, where the bulk and boundary degrees of freedom may be identified as logical and physical degrees of freedom respectively. These models capture key features of entanglement in the AdS/CFT correspondence; in particular, the RyuTakayanagi formula and the negativity of tripartite information are obeyed exactly in many cases. That bulk logical operators can be represented on multiple boundary regions mimics the Rindlerwedge reconstruction of boundary operators from bulk operators, realizing explicitly the quantum errorcorrecting features of AdS/CFT. This is a joint work with Daniel Harlow, Fernando Pastawski and John Preskill :: Fri 5/15/15, 1:30 in 6C442 Sergey Bravyi (IBM) TBA :: Mon 5/18/15, 1:30 in 6C442 Lidia del Rio (University of Bristol) TBA :: Tue 6/23/15, 1:30 in 6C442 Rotem Arnon Friedman (ETH Zurich) TBA :: Fri 6/26/15, 1:30 in 6C442 Marco Tomamichel (The University of Sydney) TBA 