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Seminar Listing

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[photo of a speaker] (c) D.Nagaj

Typical Weekly Calendar

other seminars:


2014 - 2015 (academic year)


:: Fri 10/24/14, 1:30 in 6C-442

Chetan Nayak (Microsoft Research, Station Q and UC Santa Barbara)
Recent Progress Towards a Majorana Zero Mode Quantum Computer

:: Fri 11/7/14, 1:30 in 6C-442

Alan Aspuru-Guzik (Harvard University)
Time-dependent density functional theory and quantum information

Density functional theory (DFT) and it's time-dependent 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 time-dependent counterpart (TDDFT) demonstrates that a time-evolving 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 time-dependent 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 Yuen-Zhou, 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 Man-Hong Yung now Assistant Professor at Tsinghua University.

:: Fri 11/14/14, 1:30 in 6C-442

Lorenza Viola (Dartmouth College)
A general transfer-function approach to noise filtering in open-loop quantum control

Hamiltonian engineering via unitary open-loop quantum control provides a versatile and experimentally validated framework for precisely manipulating a broad class of non-Markovian dynamical evolutions of interest, with applications ranging from dynamical decoupling and dynamically corrected quantum gates to noise spectroscopy and quantum simulation. In this context, transfer-function 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 filter-function 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 multi-qubit systems and/or in the presence of non-Gaussian noise will be discussed.

:: Fri 11/21/14, 1:30 in 6C-442

John Preskill (California Institute of Technology)
Quantum information and black holes


:: Fri 12/5/14, 1:30 in 6C-442

Jess Riedel (Perimeter Institute)
Toward an objective principle for decomposing the wavefunction into classical branches


:: Fri 2/6/15, 1:30 in 6C-442

Brian Swingle (Stanford University)
Einstein's Equations From Entanglement

I will propose a mechanism whereby a dynamical geometry obeying Einstein's equations emerges holographically from entanglement in certain quantum many-body 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 non-perturbative quantum field theory and quantum gravity.

:: Fri 2/27/15, 1:30 in 6C-442

Ramis Movassagh (MIT)
A counterexample to the area law for quantum matter

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 low-energy 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 systemís size. We introduce a class of exactly solvable one-dimensional 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 6C-442

Richard Cleve (IQC, University of Waterloo)
Near-linear constructions of exact unitary 2-designs

We present exact unitary 2-designs on n qubits that can be implemented with ’(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 6C-442

Lior Eldar (MIT)
On Quantum Inapproximability: or Can Entanglement Survive Outside the Fridge?

Quantum entanglement is considered to be a very delicate phenomenon that is hard to maintain in the presence of noise, or non-zero 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 highly-entangled. However to this date, no such systems were found. Moreover, it became evident that even embedding local Hamiltonians on robust, albeit "non-physical" 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 sub-logarithmic 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 long-range entanglement.

:: Fri 5/1/15, 2:00 in 6C-442

William Wooters (Williams College)
Cycling through mutually unbiased bases

Two orthogonal bases for a Hilbert space are called mutually unbiased if each vector in one basis is an equal-magnitude 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.

:: Fri 5/8/15, 1:30 in 6C-442

Beni Yoshida (Caltech)
Holographic quantum error-correcting codes: Toy models for the AdS/CFT correspondence

We propose a family of exactly solvable toy models for the AdS/CFT correspondence based on a novel construction of quantum error-correcting 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 error-correcting 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 Ryu-Takayanagi 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 Rindler-wedge reconstruction of boundary operators from bulk operators, realizing explicitly the quantum error-correcting features of AdS/CFT. This is a joint work with Daniel Harlow, Fernando Pastawski and John Preskill

:: Fri 5/15/15, 1:30 in 6C-442

Sergey Bravyi (IBM)


:: Mon 5/18/15, 1:30 in 6C-442

Lidia del Rio (University of Bristol)


:: Tue 6/23/15, 1:30 in 6C-442

Rotem Arnon Friedman (ETH Zurich)


:: Fri 6/26/15, 1:30 in 6C-442

Marco Tomamichel (The University of Sydney)



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