aspuru-list April 2018

aspuru-list@lists.fas.harvard.edu

15 participants
40 discussions

[Aspuru-Guzik Group List] [qip] group meeting, colloquium

by Aram Harrow

Dear quanta, Tomorrow the physics colloquium (4pm, 10-250) is Jian-Wei Pan, who is probably the leading QI experimentalist in China. Friday we will have group meeting in the usual time and place, and Anand will talk about his recent arxiv update and the quantum games PCP theorem. -aram _______________________________________________ qip mailing list qip(a)mit.edu http://mailman.mit.edu/mailman/listinfo/qip

6 years

[Aspuru-Guzik Group List] Group meeting, Thu April 12, 3:30 PM, Div Room

by Ian Kivlichan

Hi all, Tomorrow Ben will speak at group meeting (with some of it going into his independent study proposal = feedback appreciated :) ). See below for his title and abstract. All the best, Ian ----------------- Title: GPMol - Gaussian Process on molecules Abstract: Gaussian processes (GPs), Neural networks, Quantifying uncertainty, Interpretable models, and real chemical applications. All this and more in this talk. I'll be showcasing GPMol a GP library I have been working on with other people in the group. part of the talk is a tutorial on building models with GP for many applications, part is how GPs fit in the grander scheme of prediction in machine learning (ML). The last part of my talk is about interpretable ML in chemistry, will be used for my independent study proposal, so feedback is welcomed.

6 years

[Aspuru-Guzik Group List] ITAMP lunch seminar - Dominik Wild, Harvard University (April 12th)

by Bekenstein, Rivka

*ITAMP Lunch Seminar* *Speaker:* Dominik Wild (Harvard University) *Date:* Thursday, April 12th *Time:* 12:00-1:00 pm Includes Pizza. *Title: Few- and Many-Body Quantum Mechanics in 2D Semiconductors* *Abstract: Atomically thin semiconductors such as monolayers of transition metal dichalcogenides (TMDs) are promising candidates for applications in quantum optics and the study of many-body dynamics. In the first part of the talk, I show that TMDs are excellent reflectors close to an exciton resonance. A TMD monolayer thus forms an optical resonator when placed in front of a conventional mirror, which can be exploited to enhance quantum nonlinearities of the TMD. In the second part, I discuss polarons formed by excitons interacting with a Fermi sea of electrons, highlighting key differences from polarons in ultracold gases. In particular, I demonstrate that the polaron dispersion in TMDs develops a roton-like minimum at finite momentum due to Pauli blocking, which may account for experimentally observed discrepancies between absorption and photoluminescence spectra.* *Location: *B-106 @ Center for Astrophysics (60 Garden Street) *Directions: *After entering the lobby of the CfA, turn right to enter the hallway of the B building. In the hallway, turn right again, B-106 will be at the end of the hallway on the left side.

6 years

[Aspuru-Guzik Group List] [qip] Fwd: [MIT Combin Sem] Fwd: [Theory-reading-group] CMSA workshop on coding and information theory next week

by Ramis Movassagh

Fyi... ---------- Forwarded message ---------- From: Yufei Zhao <yufeiz(a)mit.edu> Date: Sun, Apr 8, 2018 at 11:49 PM Subject: [MIT Combin Sem] Fwd: [Theory-reading-group] CMSA workshop on coding and information theory next week To: MIT Combinatorics Seminar announcements <mitmathcombin(a)mit.edu> Workshop at Harvard this week: http://cmsa.fas.harvard.edu/coding/ Workshop on Coding and Information Theory <http://cmsa.fas.harvard.edu/coding/> The workshop on coding and information theory will take place *April 9-13, 2018 *at the *Center of Mathematical Sciences and Applications*, located at 20 Garden Street, Cambridge, MA. This workshop will focus on new developments in coding and information theory that sit at the intersection of combinatorics and complexity, and will bring together researchers from several communities — coding theory, information theory, combinatorics, and complexity theory — to exchange ideas and form collaborations to attack these problems. Squarely in this intersection of combinatorics and complexity, locally testable/correctable codes and list-decodable codes both have deep connections to (and in some cases, direct motivation from) complexity theory and pseudorandomness, and recent progress in these areas has directly exploited and explored connections to combinatorics and graph theory. One goal of this workshop is to push ahead on these and other topics that are in the purview of the year-long program. Another goal is to highlight (a subset of) topics in coding and information theory which are especially ripe for collaboration between these communities. Examples of such topics include polar codes; new results on Reed-Muller codes and their thresholds; coding for distributed storage and for DNA memories; coding for deletions and synchronization errors; storage capacity of graphs; zero-error information theory; bounds on codes using semidefinite programming; tensorization in distributed source and channel coding; and applications of information-theoretic methods in probability and combinatorics. All these topics have attracted a great deal of recent interest in the coding and information theory communities, and have rich connections to combinatorics and complexity which could benefit from further exploration and collaboration. *Participation: *The workshop is open to participation by all interested researchers, subject to capacity. *Click here to register. <https://docs.google.com/a/g.harvard.edu/forms/d/1OTOkUPiZAjV6STdzeRqrGDEUAa…>* *Schedule:* *Monday, April 9* *Time.* *Speaker* *Title/Abstract* 9:00 – 9:30am *Breakfast * 9:30 – 10:00am *Noga Ron-Zewi* *Title:* Some recent results on list-decoding and local list-decoding 10:00 – 10:30am *Dean Doron* *Title: *Near-Optimal Erasure List-Decodable Codes *Abstract:*For every small ε, we explicitly construct binary erasure list-decodable codes that can be list-decoded from 1-ε fraction of erasures, with near-optimal list-size poly(log(1/ε)) and near-optimal rate O(ε^(1+γ)) where γ>0 is any constant. This is the first construction to break the ε^2 rate barrier, solving a long-standing open problem raised by Guruswami and Indyk, and it does so with a remarkably small list size (we remark that Guruswami showed such a result is impossible using linear codes, and indeed our code is not linear). Equivalently, in terms of dispersers, we construct ε-error one-bit strong dispersers with near-optimal seed-length and near-optimal entropy-loss.The main ingredient in the construction is a new (and nearly optimal) unbalanced two-source extractor. The extractor extracts one bit with constant error from two independent sources, where one source has length n and tiny min-entropy O(loglog n) and the other source has length O(log n) and arbitrarily small constant min-entropy rate. Joint work with Avraham Ben-Aroya and Amnon Ta-Shma. 10:30 – 11:10am *Break* 11:10 – 12:00pm *Young-Han Kim* *Title*: Elements of Index Coding *Abstract:* Originally introduced to minimize the number of transmissions in satellite communication, the index coding problem provides a simple yet rich model for several important engineering problems in network communication, such as content broadcasting, peer-to-peer communication, distributed caching, device-to-device relaying, and interference management. This talk aims to provide a broad overview of this fascinating problem — arguably one of the most important open problems in network information theory — in four parts. First, we give a mathematical description of the problem and introduce basic properties of optimal index codes. Second, we discuss several coding schemes based on algebraic, graph-theoretic, and information-theoretic tools. Third, we establish performance bounds and discuss their implications in network information theory. Fourth, we explore the relationship between index coding and other problems such as network coding, distribute storage, and guessing games. No prior exposure to network information theory is assumed. 12:00 – 1:30pm *Lunch* 1:30 – 2:00pm *Navin Kashyap* Indian Institute of Science *Title*: Probabilistic Forwarding Over Networks: To Code Or Not To Code? *Abstract:* Consider a network (i.e., a connected graph) with a large number of nodes, one of which is distinguished as a source node. The source node has $k$ data packets to be broadcast to all nodes in the network. The $k$ data packets are encoded into $n \ge k$ coded packets (using an MDS code over a large enough field), such that any $k$ of the $n$ coded packets are sufficient to recover the original $k$ data packets. The source transmits all $n$ packets to its one-hop neighbours. Thereafter, the nodes follow a probabilistic forwarding protocol: Each node, upon receiving a particular coded packet for the first time, forwards it to all its one-hop neighbours only with a certain probability $p$. This probabilistic forwarding happens independently across all nodes and packets. We declare such a probabilistic forwarding protocol to be useful if the expected fraction of network nodes that receive at least $k$ of the $n$ coded packets is at least $1-\delta$ for some small $\delta > 0$. We would of course like to operate the protocol at the smallest value of the probability $p$ that makes the protocol useful, as this would minimize the expected total number of transmissions across all network nodes. It turns out that the redundancy $\rho := (n-k)/k$ introduced by the coding scheme significantly influences this minimum expected number of transmissions. Simulation results indicate that over network topologies that are tree-like, the introduction of redundancy in the form of coding is not beneficial; but over topologies that are well-connected, the introduction of some (but not too much) redundancy can significantly reduce the expected total number of transmissions needed. We describe our preliminary analysis of this phenomenon using ideas from percolation theory. This talk is based on work with B.R. Vinay Kumar and Roshan Antony. 2:00 – 2:30pm *Chandra Nair* *TItle: *Observations regarding extremizers of some non-convex optimization problems in information theory and some open questions motivated by them *Abstract: *Rate regions in network information theory have long been characterized using auxiliary random variables. In the last few years, optimality or sub-optimality of some fundamental rate regions have been established by computing the optimal extremal auxiliaries, i.e. by computing the optimizers that belong to a family of non-convex optimization problems. These results and computations seem to reveal an intriguing relationship between “local tensorization” and “global tensorization”. Indeed several of the counterexamples to various optimality questions have been deduced based on the above intuitive connection. In this talk, I will try to outline these observations, and formally state various observations as open problems. I will also talk about a potential pathway (verified on small examples by simulations) that supports this connection as well as potential consequences, such as (potentially efficient) computation of hypercontractivity parameters. 2:30 – 3:30pm *Break* 3:30 – 4:30pm *Amnon Ta-Shma* *Title: *Explicit, Epsilon-Balanced Codes Close to the GV Bound I will show an explicit construction of a binary error correcting code with relative distance 1/2-epsilon and relative rate epsilon^{2+o(1)}. This comes close to the Gilbert-Varshamov and the LP lower bound. Previous explicit constructions had rate about epsilon^3, and this is the first explicit construction to get that close to the Gilbert-Varshamov bound. Technically, we define Parity Samplers. A parity sampler is a collection of sets {S_i} with the property that for every “test” set A of a given constant density epsilon_0, the probability a set S_i from the collection falls into the test set A an even number of times is about half. A sparse parity sampler immediately implies a good code with distance close to half. The complete t-complex of all sequences of cardinality t is a good parity sampler, but with too many sets in the collection. Rozenman and Wigderson, and independently Alon, used random walks over expanders to explicitly construct sparse parity samplers, and their construction implies explicit codes with relative rate epsilon^4. I will show how one can get better explicit parity samplers (and therefore also better explicit codes) using a variant of the zig-zag product. In the random walk sampler, there exist many sets with substantial overlap. One way to look at the zig-zag product is that it takes a sub-collection of the random walk sampler, and this sub-collection has a smaller overlap between sets in the collection. The zig-zag product achieves that by keeping a small internal state. I will show that by enlarging the internal state one can further reduce the overlap, and as a result improve the quality of the parity sampler. 4:30 – 6:00pm *Welcome Reception* *Tuesday, April 10 * *Time* *Speaker* *Title/Abstract* 9:00 – 9:30am *Breakfast * 9:30 – 10:00am *Himanshu Tyagi* *Title: *Distributed independence testing and a new question related to hypercontractivity *Abstract: *Two parties observing sequences of unbiased, random bits seek to determine if the bits are independent or have a specified correlation. A trivial scheme will entail one party sending its bits to the other who in turn will decide if the bits are independent or correlated. Can any other scheme accomplish the independence test by using less communication than the trivial scheme? We exhibit a one-way communication scheme building on linear correlation that indeed outperforms the trivial scheme. In fact, our scheme extends to an arbitrary distribution. Furthermore, we establish a lower bound for one-way communication required to enable independence testing, which is tight for the cases of binary symmetric distribution and Gaussian symmetric distribution. Our lower bound leverages hypercontractivity to obtain a measure change bound, but works only for one-way communication protocols. Its extension to multiround protocols opens up new interesting questions related to hypercontractivity. 10:00 – 10:30am *Alex Samorodnitsky* *Title:* On the entropy of a noisy function. *Abstract:*Let X be a uniformly distributed binary sequence of length n. Let Y be a noisy version of X, obtained by flipping each coordinate of X independently with probability epsilon. We want to come up with a one-bit function of Y which provides as much information as possible about X. Courtade and Kumar conjectured that the best one can do is to choose a coordinate function f(Y) = Y_i, for some i between 1 and n. We prove the conjecture for large values of epsilon (epsilon > 1/2 – delta, for some absolute constant delta). The main new technical ingredient in the proof is the claim that if F is a real-valued function on the boolean cube, and G is a noisy version of F, then the entropy Ent(G) is upper-bounded by the expected entropy of a projection of F on a random coordinate subset of a certain size. 10:30 – 11:10am *Break* 11:10 – 12:00pm *Maxim Raginsky * Algorithmic stability and generalization in machine learning: an information-theoretic analysis 12:00 – 1:30pm *Lunch* 1:30 – 2:00pm *Tom Gur* *Title:* Quantum zero knowledge via local codes *Abstract:* The seminal work of Ben-Or et al. (STOC 1988) shows that zero knowledge can be achieved unconditionally by making the physical assumption that spatially-isolated provers are playing independent strategies. In this talk we will discuss new techniques involving locally testable/correctable codes that allow us to strengthen the foregoing result to hold even in light of quantum mechanics, which tells us that spatially-isolated provers can realize non-local correlated strategies by sharing a quantum entangled state. These techniques include structural results regarding subcube sums of the Reed-Muller code. We provide the first construction of a zero-knowledge proof system that is sound against quantum entangled provers. Along the way, we introduce a framework that leverages local testability to “lift’’ classical protocols to quantum protocols in a blackbox manner. The talk is self-contained and does not assume any quantum or complexity preliminaries. Joint work with Alessandro Chiesa, Michael Forbes, and Nicholas Spooner. 2:00 – 2:30pm *Nicolas Resch* 2:30 – 3:30pm *Break* 3:30 – 4:30pm *Open problem session* *Wednesday, April 11* *Time* *Speaker* *Title/Abstract* 9:00 – 9:30am *Breakfast * 9:30 – 10:00am *Amirbehshad Shahrasbi * 10:00 – 10:30am *Mahdi Cheraghchi* *Title.* Capacity Upper Bounds for Deletion-type Channels *Abstract.* We develop a systematic approach, based on convex programming and real analysis, for obtaining upper bounds on the capacity of the binary deletion channel and, more generally, channels with i.i.d. insertions and deletions. Other than the classical deletion channel, we give a special attention to the Poisson-repeat channel introduced by Mitzenmacher and Drinea (IEEE Transactions on Information Theory, 2006).Our framework can be applied to obtain capacity upper bounds for any repetition distribution (the deletion and Poisson-repeat channels corresponding to the special cases of Bernoulli and Poisson distributions). Our techniques essentially reduce the task of proving capacity upper bounds to maximizing a univariate, real-valued, and often concave function over a bounded interval. The corresponding univariate function is carefully designed according to the underlying distribution of repetitions and the choices vary depending on the desired strength of the upper bounds as well as the desired simplicity of the function (e.g., being only efficiently computable versus having an explicit closed-form expression in terms of elementary, or common special, functions). Among our results, we show the following: 1. The capacity of the binary deletion channel with deletion probability $d$ is at most $(1-d) \log \varphi$ for $d \geq 1/2$, and, assuming the capacity function is convex, is at most $1-d \log(4/\varphi)$ for $d<1/2$, where $\varphi=(1+\sqrt{5})/2$ is the golden ratio. This is the first nontrivial capacity upper bound for any value of $d$ outside the limiting case $d \to 0$ that is fully explicit and proved without computer assistance. 2. We derive the first set of capacity upper bounds for the Poisson-repeat channel. Our results uncover further striking connections between this channel and the deletion channel, and suggest, somewhat counter-intuitively, that the Poisson-repeat channel is actually analytically simpler than the deletion channel and may be of key importance to a complete understanding of the deletion channel. 3. We derive several novel upper bounds on the capacity of the deletion channel. All upper bounds are maximums of efficiently computable, and concave, univariate real functions over a bounded domain. In turn, we upper bound these functions in terms of explicit elementary and standard special functions, whose maximums can be found even more efficiently (and sometimes, analytically, for example for $d=1/2$). Along the way, we develop several new techniques of potentially independent interest. (Based on https://arxiv.org/abs/1711.01630) 10:30 – 11:10am *Break* 11:10 – 12:00pm *Yuval Peres * *Title: *Iterated von-Neumann unbiasing and Trace reconstruction for the deletion channel *Abstract: *In the first part of the talk, I will describe the connections between an efficient unbiasing method from 1992 (Iterated von-Neumann unbiasing), and polar codes. In the second part, I will discuss the trace reconstruction problem: Suppose that an unknown string $x$ of $n$ bits is observed through the deletion channel; how many independent outputs (traces) of this channel are needed to reconstruct $x$ with high probability? The best lower bound known is linear in $n$. Currently, the best upper bound is exponential in the cube root of $n$, proved with with Fedor Nazarov, STOC 2017 (Similar results were obtained independently by De, O’Donnell and Servedio). This upper bound is sharp for tests that only use linear combinations of the output. If the string $x$ is random, we show that a subpolynomial number of traces suffices, by comparison to a random walk. Joint works with Alex Zhai (FOCS 2017) and with Nina Holden & Robin Pemantle, preprint (2017). 12:00 – 1:30pm *Lunch* 1:30 – 2:00pm *Itzhak Tamo* *Title: *Detecting and Correcting Errors in Codes over Graphs *Abstract: *Let G be a graph on n vertices and C be an (n, k, d) code over an alphabet X. Assume that vertex i stores a symbol c_i, where the vector of stored symbols (c_1,…,c_n) forms a codeword of the code C. We assume that two vertices can communicate if there is a communication link (edge) connecting them. In this talk we consider the communication complexity of error detection and correction of the information stored over the vertices of the graph. For error detection, we obtain general lower bounds for the communication complexity as functions of n, k, d, |X|, which are tight for several graphs and codes. For error correction, building on the work of Alon, Efremenko and Sudakov, we design a protocol which can efficiently correct a single input error for repetition codes. We conclude with some interesting problems for further research. Joint work with Chong Shangguan 2:00 – 2:30pm *Arya Mazumdar* *Title:* Open problems in locally recoverable codes *Abstract:* Locally repairable codes (LRCs) have recently been a subject of much research interest due to their theoretical appeal and application in distributed storage systems. In an LRC, any coordinate of a codeword can be recovered by accessing only few other coordinates. In this talk we describe three aspects of LRCs and some open problems. Namely, we look into the rate-distance trade-off of LRCs, the problem of disjoint repair groups, and the capacity of LRCs. 2:30 – 3:15pm *break* 3:15 – 3:45pm *Sankeerth Rao* 3:45 – 4:15pm *Sivakanth Gopi* *Title*: Maximally Recoverable Local Reconstruction Codes *Abstract:* Protecting huge amounts of data stored in the cloud from server crashes resulted in distributed storage systems transitioning to erasure coding based schemes. Local Reconstruction Codes (LRCs) have emerged as the codes of choice for these applications. They allow super fast recovery in the typical case of a small number of crashes by reading only a few number of healthy servers (called ‘locality’), while still protecting the data from the unlikely event of a large number of crashes. They have many good properties of simply replicating data while being much more storage efficient and crash resilient. Maximally Recoverable LRCs (MR LRCs) are information theoretically the most optimal LRCs; they can recover from every feasible crash scenario for a given storage efficiency and locality. MR LRCs have already been deployed in Microsoft storage systems, outperforming traditional erasure coding systems. Designing such codes over small finite fields is crucial for applications. Unfortunately, we are far from understanding the minimal field size required for these codes. In this talk, we prove the first polynomially growing lower bounds on field size for MR LRCs using an interesting connection to incidence geometry, prior to our work no super linear lower bounds were known. We also present some linear field size MR LRC constructions in some parameter ranges which are very relevant in practice. *Thursday, April 12* *Time* *Speaker* *Title/Abstract* 9:00 – 9:30am *Breakfast * 9:30 – 10:00am *Boris Bukh* *Title*: Small antipodal spherical codes *Abstract*: We prove sharp bounds on the distance of antipodal spherical codes in R^d with 2d+2k points for k=1,2,3,7,23. In addition, we obtain asymptotics for general k, and solve the analogous problem for spherical codes in C^d. Instead of linear programming, we use a new approach that relies on bounding the first moment of isotropic measures. Joint work with Chris Cox. 10:00 – 10:30am *William Martin* *Title: *Problems on spherical codes motivated by quantum information theory *Abstract:* A pair B, B’ of orthonormal bases in R^d or C^d are said to be unbiased if |<b,b’>| is constant for b in B and b’ in B’. In most cases, finding the maximum number of pairwise unbiased bases (or mutually unbiased bases, “MUBs”) appears to be a hard problem. The real and complex cases are quite different. The same dichotomy arises in finding the maximum possible number of equiangular lines in R^d or C^d. One may also ask for the maximum number of linked simplices in R^d: full-dimensional simplices with vertices on the unit sphere such that only two possible angles occur between vectors in distinct simplices. In this talk, I will briefly introduce the problems in quantum information theory which gave rise to the complex versions of these questions. I will discuss connections to association schemes, coding theory and finite geometry and I will summarize recent work of my student, Brian Kodalen. 10:30 – 11:10am *Break* 11:10 – 12:00pm *Emmanuel Abbe* *Title: *The stochastic block model: where we stand *Abstract:* The talk overviews some of the recent developments on the stochastic block model, staring with the two community case, and extending to multiple communities. We will cover various recovery requirements, phase transitions, algorithms and the information-computation gap. Time permitting, geometric block models and open problems will be discussed. 12:00 – 1:30pm *Lunch* 1:30 – 2:00pm *Olgica Milenkovic* *Title:* Leveraging data popularity in distributed storage systems via MinMax Steiner systems This is a joint work with Charles Colbourn and Hoang Dau. 2:00 – 2:30pm *Alexander Barg* *Title:* Lattices with exponentially large kissing numbers (after a paper by Serge Vladuts) *Abstract:* Serge Vladuts recently constructed lattices with the property described in the title (see arXiv:1802.00886). Vladuts’s construction is based on an earlier construction of binary codes with exponentially many codewords of the smallest weight, which in its turn relied on results on the weight spectra of algebraic geometric codes. We explain his approach and describe families of AG codes used to achieve several versions of his result. 2:30 – 3:30pm *Break* 3:30 – 4:30pm *Rump Session* *Friday, April 13* *Time* *Speaker* *Title/Abstract* 9:00 – 9:30am *Breakfast * 9:30 – 10:00am *Min Ye * 10:00 – 10:30am *Ankit Singh Rawat* *Title:* MDS Codes with Small Sub-packetization and Near-optimal Repair Bandwidth *Abstract:* Minimum storage regenerating (MSR) codes form a special sub-class of maximum distance separable (MDS) codes by providing mechanisms for exact regeneration of a single code-block by downloading the minimum amount of information from the remaining code-blocks. As a result, the MSR codes find application to distributed storage systems to ensure node repairs with optimal repair-bandwidth. However, constructions of MSR codes require large sub-packetization levels, which restricts their applicability in practice. In this talk, I’ll present a general approach to construct MDS codes that significantly reduce the required sub-packetization level by incurring slightly higher repair-bandwidth as compared to the MSR codes. 10:30 – 11:10am *Break* 11:10 – 12:00pm *Hamed Hassani * *Title:* Non-asymptotic analysis of channel codes and its practical significance *Abstract:* Since Shannon’s 1948 landmark paper, coding theory has focused on achieving capacity in the asymptotic sense: design low-complexity codes that become reliable at rates approaching capacity in the limit of large blocklengths. Throughout the seven decades of the development of coding theory, we have witnessed many remarkable code designs. Today, we have two families of low-complexity codes that can asymptotically achieve capacity for a wide range of channels: polar codes and spatially coupled codes. In this talk, I will consider a practically significant question that has emerged in recent years as the new grand challenge in coding theory: Design codes that are optimal in the non-asymptotic sense (at short lengths). I will explain why the performance of the state-of-the-art code designs are unsatisfactory with respect to this challenge. I will then discuss new promising ways for improving the current codes as well as new coding paradigms to address this challenge. 12:00 – 1:30pm *Lunch* 1:30 – 2:00pm *Nati Linial* *Title:* On the weight distribution of random linear codes *Abstract: *Random linear codes play an important role in several parts of computer science. Yet, not much seems to be known about their properties. In this work we investigate their weight distribution – One of the most informative set of parameters of a code. This is joint work with my PhD student Jonathan Mosheiff. 2:00 – 2:30pm *Bernhard Haeupler* 2:30 – 3:30pm *Simeon Ball * *Title:* On Maximum Distance Separable Codes *Abstract:* A block code C of length n, minimum distance d over an alphabet with q symbols, satises, jCj 6 qn

6 years

[Aspuru-Guzik Group List] [qip] Fwd: The Robert H. Meservey Memorial Lecture on 4/6

by Aram Harrow

Dear quanta, This talk is at our usual group meeting time. I suggest we go to this and skip the group meeting that week. We also won't have a group meeting this week because of spring break. We are looking for speakers for future weeks. Send me an email if you're intersted in speaking. aram ---------- Forwarded message ---------- From: Nina Wu <ninawu(a)mit.edu> Date: Tue, Mar 27, 2018 at 2:22 PM Subject: The Robert H. Meservey Memorial Lecture on 4/6 To: Nina Wu <ninawu(a)mit.edu> *Robert H. Meservey Memorial Lecture* *Friday, April 6, 2018, 11:00am - 12:00pm in the Pappalardo Community Room, 4-349* *Reception to precede talk at 10:30am in the lobby of 4-349* Charles Marcus Niels Bohr Institute, University of Copenhagen and Director of the Center for Quantum Devices “Using Topology to Build a Better Qubit” This talk will describe an adventure currently underway to coax into existence excitations (particles) that have non-Abelian braiding statistics—something yet unseen in the physical world—and to not stop there, but to try to employ these new excitations, Majorana zero modes, for a topological quantum computing. Which is more challenging: the mathematics of computing by braiding particles? The material science of creating hybrid materials that support Majorana modes? The nanotechnology of fabricating the devices? The condensed matter physics of producing them in the lab? The electrical engineering of controlling and reading out their state? The software to control the electronics on submicrosecond timescales? This talk will try to cover a small amount of each of these aspects, to convey the sense of complexity of quantum computing generally. *Robert H. Meservey Memorial Lectureship* A lectureship established by family and friends in memory of Robert H. Meservey. The lectureship brings in speakers in the field of Experimental and Theoretical Condensed Matter Physics for the Physics Department, as well as providing support for research and educational activities in the fields of Condensed Matter and Atomic Physics. _______________________________________________ qip mailing list qip(a)mit.edu http://mailman.mit.edu/mailman/listinfo/qip

6 years

[Aspuru-Guzik Group List] Group meeting, Thu April 05, 3:30 PM, Div Room

by Ian Kivlichan

Hi all, Semion will talk at group meeting tomorrow - see below for his title and abstract! All the best, Ian ----------------- Title: Excitonic bands in 2D aggregates of heterotriangulenes Abstract: I will present my ongoing work on two-dimensional supramolecular aggregates of organic dyes where the exciton transport can be not as trivial as we used to think.

6 years

[Aspuru-Guzik Group List] TODAY: ITAMP/HQOC Joint Quantum Sciences Seminar - Wednesday, April 4 2018 @ 4:30

by Dakoulas, Samantha R

ITAMP/HQOC Joint Quantum Sciences Seminar Wednesday, April 4th, 2018 4:30 PM, Jefferson 250 Falko Pientka “Polaron drag of excitons in semiconductors” Mobile quantum impurities interacting with a fermionic bath form quasiparticles known as Fermi polarons. We demonstrate that a force applied to the bath particles can generate a drag force of similar magnitude acting on the impurities, realizing a novel, nonperturbative Coulomb drag effect. We apply our theory to excitons interacting with a bath of charge carriers in a doped semiconductor. Our findings establish transport measurements as a novel, powerful tool for probing the many-body physics of mobile quantum impurities. Javier Sanchez-Yamagishi “Current instability in a driven 2d electron liquid probed by nanoscale magnetometry” A moving fluid can become unstable in the presence of an obstruction, leading to a flow pattern that fluctuates in time due to nonlinear dynamics. It has been recently found that some materials host electrons that behave like a collective fluid. In particular, experiments with graphene electrons have demonstrated linear hydrodynamic phenomena, such as viscous drag at sample boundaries, but nonlinear effects have yet to be reported. We observe a AC current instability that develops when driving a DC current through a graphene device in the electron hydrodynamic regime. The current fluctuations are substantially larger than typical electronic noise and broadly peaked in the GHz frequency range. To probe the local structure of the instability, we use diamond NV magnetometry to measure the current fluctuations at the nanoscale. We find that the current fluctuations vary across the sample. Remarkably, some regions exhibit fluctuations that are strongly dependent on the direction of the current, breaking the directional symmetry of the device as would be expected for a fluid instability seeded by disorder. The combined global and local measurements indicate non-linear effects which arise when driving the graphene electron liquid. In addition, this work demonstrates the power of using local magnetometry probes in combination with traditional global measurements to gain deeper insight into electronic phenomena. Guest Presentation will begin at 4:30 PM (No 10-minute speaker) Refreshments will be provided. Samantha Dakoulas Faculty Assistant to Professors Lukin & Greiner & their groups Department of Physics 17 Oxford St., Lyman 324A Cambridge, MA 02138 P. (617) 496-2544

6 years

[Aspuru-Guzik Group List] ITAMP/HQOC Joint Quantum Sciences Seminar - Wednesday, April 4 2018 @ 4:30

by Dakoulas, Samantha R

ITAMP/HQOC Joint Quantum Sciences Seminar Wednesday, March 21, 2018 4:30 PM, Jefferson 250 Falko Pientka “Polaron drag of excitons in semiconductors” Mobile quantum impurities interacting with a fermionic bath form quasiparticles known as Fermi polarons. We demonstrate that a force applied to the bath particles can generate a drag force of similar magnitude acting on the impurities, realizing a novel, nonperturbative Coulomb drag effect. We apply our theory to excitons interacting with a bath of charge carriers in a doped semiconductor. Our findings establish transport measurements as a novel, powerful tool for probing the many-body physics of mobile quantum impurities. Javier Sanchez-Yamagishi “Current instability in a driven 2d electron liquid probed by nanoscale magnetometry” A moving fluid can become unstable in the presence of an obstruction, leading to a flow pattern that fluctuates in time due to nonlinear dynamics. It has been recently found that some materials host electrons that behave like a collective fluid. In particular, experiments with graphene electrons have demonstrated linear hydrodynamic phenomena, such as viscous drag at sample boundaries, but nonlinear effects have yet to be reported. We observe a AC current instability that develops when driving a DC current through a graphene device in the electron hydrodynamic regime. The current fluctuations are substantially larger than typical electronic noise and broadly peaked in the GHz frequency range. To probe the local structure of the instability, we use diamond NV magnetometry to measure the current fluctuations at the nanoscale. We find that the current fluctuations vary across the sample. Remarkably, some regions exhibit fluctuations that are strongly dependent on the direction of the current, breaking the directional symmetry of the device as would be expected for a fluid instability seeded by disorder. The combined global and local measurements indicate non-linear effects which arise when driving the graphene electron liquid. In addition, this work demonstrates the power of using local magnetometry probes in combination with traditional global measurements to gain deeper insight into electronic phenomena. Guest Presentation will begin at 4:30 PM (No 10-minute speaker) Refreshments will be provided. Samantha Dakoulas Faculty Assistant to Professors Lukin & Greiner & their groups Department of Physics 17 Oxford St., Lyman 324A Cambridge, MA 02138 P. (617) 496-2544

6 years

[Aspuru-Guzik Group List] ITAMP lunch seminar - Nicholas Rivera, MIT (April 5th)

by Bekenstein, Rivka

*ITAMP Lunch Seminar* *Speaker: *Nicholas Rivera (MIT) *Date:* Thursday, April 5th *Time:* 12:00-1:00 pm Includes Pizza. *Title: Quantum electrodynamics at the surface of a polaritonic medium* *Abstract: Quantum electrodynamics is one of the most successful theories in the history of science, d*escribing accurately nearly all known phenomena involving electromagnetic fields. Central in QED is the concept of the photon, a quantum of electromagnetic energy with properties such as energy, momentum, and angular momentum. For photons in complex arrangements of optical media, these properties are in general very different than in free space, leading to vastly altered interactions between matter and electromagnetic fields. As a result, "QED in a medium" features a vast set of phenomena that either do not exist or are very hard to realize in "free space QED". For example, QED effects in photonic crystals and optical cavities have been leveraged to achieve ultrafast spontaneous emission of light by molecules, vacuum Rabi oscillations, atom-photon bound states in photonic crystals, and exciton-polariton condensation. *This talk presents new developments in this field which result from extreme (nanoscale) confinement of light by optical media supporting surface polaritons. These polaritons, which include the family of plasmon-, phonon-, exciton-, or magnon- polaritons, enable new quantum regimes of interactions with different kinds of matter such as atom-like emitters, electrons in quantum wells and in electron microscopes, and ultrarelativistic Dirac fermions. From this theory, we find that nanoscale confinement of electromagnetic energy enables high-order angular momentum transfer with light [1], indirect optical transitions [2], efficient high-order QED processes such as multi-photon spontaneous emission [3], as well as conversion of plasmons into x-rays and gamma rays by relativistic electrons, and high-harmonic emission of plasmons by electrons modulated by mid-to-far-IR driving fields.* *[1] N. Rivera*, I. Kaminer*, B. Zhen, J.D. Joannopoulos, and M. Soljacic. Science 353.6296 (2016): 263-269.* *[2] Y. Kurman, N. Rivera, T. Christensen, S. Tsesses, J.D. Joannopoulos, M. Soljacic, M. Orenstein, and I. Kaminer. In press.* *[3] N. Rivera, G. Rosolen, J.D. Joannopoulos, I. Kaminer, and M. Soljacic. PNAS (2017): 201713538.* *Location: *B-106 @ Center for Astrophysics (60 Garden Street) *Directions: *After entering the lobby of the CfA, turn right to enter the hallway of the B building. In the hallway, turn right again, B-106 will be at the end of the hallway on the left side.

6 years

[Aspuru-Guzik Group List] [ IMPORTANT ] Regarding Administrative Support May 1-16th

by Siria Serrano

Dear Group, This is a quick note to alert everyone that* Monday, April 30th will be Felix's last day in the lab and I will be on vacation May 1st to the 16th.* We ask all Group members to think carefully and try to anticipate any administrative needs they may have during this two week period and alert us as soon as possible so we may resolve the matter *before April 30th* We will not be hiring a new Lab Admin, instead, I will take over the administration of the group upon my return on May 17th-in addition to my current responsibilities. Understand that this will likely mean a slower turn around on requests, and please plan accordingly. Our very best, Siria and Felix -- *Siria Serrano* *Faculty Assistant* *Aspuru-Guzik Group* *Harvard University **Department of Chemistry and Chemical Biology* *12 Oxford St. M 136* *Cambridge, MA 02138* *P:** (617) 496-1716 <%28617%29%20496-1716>** F: **617-496-9411 <617-496-9411>*

6 years

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aspuru-list April 2018