Center for Quantum Technologies

Singapore, Singapore

Center for Quantum Technologies

Singapore, Singapore
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News Article | April 17, 2017

For the past five years the Quantum Shorts initiative from the Center for Quantum Technologies at the National University of Singapore has inspired artists and writers from around the world to try their hand at a unique kind of scientific storytelling. The contest alternates each year between calls for films or short stories that explore the ramifications of quantum mechanics. The key requirement? Each entry must take no more than five minutes to watch or read. For 2016 the contest focused on film and drew more than 200 entries, with 10 finalists selected. Now, in partnership with Scientific American and Nature (as well as with several scientific institutions), Quantum Shorts 2016 has revealed this year’s first- and second-place winners as selected by a six-member panel of expert judges as well as a “people’s choice” winner selected via public online polling. The overall winner is Novae, filmmaker Thomas Vanz’s breathtakingly beautiful visualization of a giant star’s explosive death by supernova and subsequent transformation into a black hole. Like a latter-day William Blake—the English poet and painter who famously mused about seeing “a world in a grain of sand” and “a heaven in a wild flower”—Vanz envisioned a supernova in drops of colored ink. Working for months in his garage in Paris, he filmed inks billowing through a water-filled fish tank, later using computer software to stitch and process the raw footage into his dramatic vision of stellar death. His behind-the-scenes shorts detailing the making of Novae are at least as entertaining as the final film itself. A supernova can form a black hole by compressing a star’s core to an infinitesimally minuscule size, creating a gravitational field so intense that it devours light itself. The compressed core of a black hole—a “singularity,” in the parlance of physics—is thus hidden behind a black, lightless “event horizon,” the boundary beyond which anything falling in cannot come back out. Black holes represent a mysterious union between gravity, which dictates the overall structure of the universe, and quantum mechanics, which describes the cosmos at subatomic scales. Probing the properties of these strange macroscale quantum objects is likely to be our best path forward to a deeper understanding of the nature of reality. The runner-up, The Guardian, is also the people’s choice winner. The film uses a love triangle between three people to explore the counterintuitive nature of the quantum world, in which an entity can exist either as a particle or as a wave—or, really, as both at the same time, in a hazy cloud of probability. It is the brainchild of Chetan  Kotabage, an assistant professor of physics at KLS Gogte Institute of Technology in Karnataka, India. “I love that it is looking at quantum physics through a cultural lens,” says Eliene Augenbraun, Scientific American’s video producer and multimedia managing editor for Nature Research Group, who also served as a contest judge and chose The Guardian as her favorite. Other entries that earned honorable mentions from the judges include Approaching Reality, Together—Parallel Universe and Bolero. Charlotte Stoddart, Nature’s chief multimedia editor and contest judge, says she was “really impressed by the quality of the filmmaking and the ideas.” You can watch all the finalists here. The next call for Quantum Shorts entries will occur later this year. Continuing its annual alternation between cinema and prose, 2017’s contest will be for short stories. Announcements will be available via the Quantum Shorts Twitter account and Facebook page.

Baden M.P.,Center for Quantum Technologies | Arnold K.J.,Center for Quantum Technologies | Grimsmo A.L.,Norwegian University of Science and Technology | Parkins S.,University of Auckland | And 2 more authors.
Physical Review Letters | Year: 2014

We realize an open version of the Dicke model by coupling two hyperfine ground states using two cavity-assisted Raman transitions. The interaction due to only one of the couplings is described by the Tavis-Cummings model and we observe a normal mode splitting in the transmission around the dispersively shifted cavity. With both couplings present the dynamics are described by the Dicke model and we measure the onset of superradiant scattering into the cavity above a critical coupling strength. © 2014 American Physical Society.

Barrett M.D.,Center for Quantum Technologies | Barrett M.D.,National University of Singapore
New Journal of Physics | Year: 2015

We show that by averaging over transitions to multiple hyperfine levels, quadrupole shifts and dominant Zeeman effects exactly cancel whenever the nuclear spin, I, is at least as large as the total electronic angular momentum, J. The average frequency thus defines a frequency reference which is inherently independent of external magnetic fields and electric field gradients. We use to illustrate the method although the approach could be readily adapted to other atomic species. This approach practically eliminates the quadrupole and Zeeman shift considerations for many potential clock transitions. © 2015 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.

Matthews W.,University of Waterloo | Matthews W.,University of Cambridge | Wehner S.,National University of Singapore | Wehner S.,Center for Quantum Technologies
IEEE Transactions on Information Theory | Year: 2014

We derive upper bounds on the rate of transmission of classical information over quantum channels by block codes with a given blocklength and error probability, for both entanglement-assisted and unassisted codes, in terms of a unifying framework of quantum hypothesis testing with restricted measurements. Our bounds do not depend on any special property of the channel (such as memorylessness) and generalize both a classical converse of Polyanskiy, Poor, and Verdú as well as a quantum converse of Renner and Wang, and have a number of desirable properties. In particular, our bound on entanglement-assisted codes is a semidefinite program and for memoryless channels, its large blocklength limit is the well-known formula for entanglement-assisted capacity due to Bennett, Shor, Smolin, and Thapliyal. © 2014 IEEE.

News Article | April 1, 2016

Scientific American partnered on a writing contest for science fiction short stories inspired by the realm of quantum physics Clara Moskowitz The bizarre quantum rules that govern the microscopic universe sometimes seem more like fiction than fact, even to physicists. To capitalize on the fantastic aspects of quantum mechanics, the 2015 Quantum Shorts competition solicited short stories inspired by quantum physics. Scientific American partnered with the Center for Quantum Technologies at the National University of Singapore, which sponsored the competition, and we’re proud to announce the 2015 winners. First prize in the Open Category goes to Ana by Liam Hogan. It is the story of a young girl who suspects that looking under her bed for monsters has deadly consequences for another version of her in a parallel universe. This tale was the favorite of an international panel of judges, and you can read it below in full. The runner-up in the category, Don’t Die before You’re Dead, Sally Wu by Andrew Neil Gray, imagines an e-mail list of people communicating with their other selves in the multiverse. The People’s Choice winner, decided by popular vote online, is The Qubits of College Acceptance by Lily Turaski, which equates an unopened envelope of letters from colleges to the box containing Schrödinger's cat. And in the Youth Category, for which I was a judge, the winning story is Unrequited Signals by Tara Abrishami, about unrequited love between a pair of scientists trying to make contact with an alternate universe. We’re thrilled to share these creative and scientifically stimulating stories with you. To read all the entries, visit Quantum Shorts 2015. It’s weird, the things that can mess up a kid’s head. Take Ana, for example. She was convinced that every time she looked under her bed, the Universe split in two. In a parallel world in which a mirror Ana also looked under her bed before going to sleep and after saying her prayers and where, up until then, she’d never found anything bad, this time there would be a ghastly demon with wicked teeth and blood-stained claws, whose only desire was to catch and tear apart Ana, aged six and three quarter years. Little wonder she said her prayers before she looked. Little wonder she had nightmares. I told her that wasn’t the way the multiverse theory worked. That for every Ana that found a slavering beast, there was one that found a toy she’d lost, or one that forgot to look under the bed. She skewered me with her most outraged look. This Ana never forgot. But it’s hard arguing theoretical physics with a child yet to turn seven and, as I wasn’t prepared to deny the theory outright, it was clear this notion was not going to be an easy one to shift. It wasn’t simply that she had a binary, yes versus no, either-or view of the coin toss that happened in her imagination every time she lifted the skirt that kept under-the-bed out-of-sight. It was because what terrified her, wasn’t the finding a monster under her bed, it was the not finding a monster under her bed. In her head, every time she survived, she doomed the parallel Universe Ana to a grisly death. It was the guilt that was crushing her. “I have to,” she replied with an air of ancient sorrow. “There might be a monster under the bed. I have to check. And even if I don’t, the other Ana will.” This had me scratching my head, figuratively speaking. I’m a psychologist by trade, not a physicist. Wouldn’t that require the Universe to have already split? And, once the other Ana looked, it would be her Universe that split again, not this Ana’s. Maybe this was something I could use. I thought of her parents. Reading between the lines, not a tricky task with those two, they wanted me to crush Ana’s creativity. To make her as easy to handle as she had been twelve months earlier. To make her ‘normal’. But normal wasn’t an option; it was clear this precocious child had the potential to far exceed the pretensions of her middle class parents. “Ana,” I said, “Who looks first? You, or the other Ana?” She suspected a trick and trod carefully. “We both...” then she corrected herself. “There is no other Ana, not until I look. Or there is, but it’s me and we haven’t split yet.” “If she is you, will she react to finding the monster the same way you would?” She sucked air through the gap in her front teeth. “I guess.” “And how would you react, if, when you looked under the bed, you found a monster there? What would you do?” I waited. The silence stretched between us. This was somewhere she hadn’t been before. “I don’t know,” she said quietly. “But you’d do something? You wouldn’t just sit there?” “I’m sure you would. And what would your parents do, if they heard you scream?” “They’d come running,” she said, and they would. Any parent would. I let her think about this for a moment. “Ana, you’re intelligent, resourceful, and brave. And the other Ana, she is exactly the same. She is, after all, you. She - you - would not take it lying down. You’d fight, you’d run. Your parents would help. The one thing you would never be, is a victim. Don’t think I haven’t noticed the hobby horse propped up against the toy chest, ready for action.” “And the roller skates on the landing,” she said. I wasn’t sure how the roller skates would help. Perhaps she hoped the monster would trip on them. She’d be upset if I told her that her mother wordlessly tidied them up each night. “And the skates,” I diplomatically agreed. “It’s not much, perhaps, but you’re doing the best you can. And so would the other Ana. No monster is going to get a free lunch in this house.” She laughed, a lovely little laugh, made all the more charming by its rarity of use. I pushed on. “So it’s not a foregone conclusion that the monster always wins. And if it does not-” “-Then there are two Anas!” she interrupted. This wasn’t quite where I’d been going. I wanted her to acknowledge that she wasn’t responsible for what happened in the other Universes. How could she be? But sometimes, usually in fact, you had to let your patient find their own path. “And then four, and then eight, and then...” she babbled on. A small chime rang out on my wristwatch. “Okay Ana. I think we’ve made good progress. We’ll leave it there for today.” A muffled voice came through the door. “Ana? Honey? Who are you talking to in there?” Which was an illuminating denial. I jotted it down for future discussion, curious to see if Ana’s mother would come into the bedroom. “Okay sweetie,” she caved in, as I suspected she would. “But go to sleep now, you hear?” Ana waited until the footsteps faded away down the hall. “Goodnight, Doctor.” And then I slid myself back under her bed, listening to her breathing softly slow and waiting for tomorrow night, when she would once again lift the covers, and - all being well - discover me lying there, ready for our next session. Liam Hogan is a London-based writer and host of the award-winning monthly literary event, Liars' League. He was a finalist in Sci-Fest LA's Roswell Award 2015 and has had work published in Leap Books' Beware the Little White Rabbit, #Alice150 anthology and in Sci Phi Journal.

Lee T.,Center for Quantum Technologies | Mittal R.,University of Waterloo | Reichardt B.W.,University of Waterloo | Spalek R.,Google | Szegedy M.,Rutgers University
Proceedings - Annual IEEE Symposium on Foundations of Computer Science, FOCS | Year: 2011

State conversion generalizes query complexity to the problem of converting between two input-dependent quantum states by making queries to the input. We characterize the complexity of this problem by introducing a natural information-theoretic norm that extends the Schur product operator norm. The complexity of converting between two systems of states is given by the distance between them, as measured by this norm. In the special case of function evaluation, the norm is closely related to the general adversary bound, a semi-definite program that lower-bounds the number of input queries needed by a quantum algorithm to evaluate a function. We thus obtain that the general adversary bound characterizes the quantum query complexity of any function whatsoever. This generalizes and simplifies the proof of the same result in the case of boolean input and output. Also in the case of function evaluation, we show that our norm satisfies a remarkable composition property, implying that the quantum query complexity of the composition of two functions is at most the product of the query complexities of the functions, up to a constant. Finally, our result implies that discrete and continuous-time query models are equivalent in the bounded-error setting, even for the general state-conversion problem. © 2011 IEEE.

Lee T.,Center for Quantum Technologies | Roland J.,University of America
Proceedings of the Annual IEEE Conference on Computational Complexity | Year: 2012

We show that quantum query complexity satisfies a strong direct product theorem. This means that computing k copies of a function with less than k times the quantum queries needed to compute one copy of the function implies that the overall success probability will be exponentially small in k. For a boolean function f we also show an XOR lemma - computing the parity of k copies of f with less than k times the queries needed for one copy implies that the advantage over random guessing will be exponentially small. We do this by showing that the multiplicative adversary method, which inherently satisfies a strong direct product theorem, characterizes bounded-error quantum query complexity. In particular, we show that the multiplicative adversary bound is always at least as large as the additive adversary bound, which is known to characterize bounded-error quantum query complexity. © 2012 IEEE.

Kulkarni R.,Center for Quantum Technologies
ITCS 2013 - Proceedings of the 2013 ACM Conference on Innovations in Theoretical Computer Science | Year: 2013

A function f : {0, 1}n → {0, 1} is called evasive if its decision tree complexity is maximal, i.e., D(f) = n. The long-standing Anderaa-Rosenberg-Karp (ARK) Conjecture asserts that every non-trivial monotone graph property is evasive. The Evasiveness Conjecture (EC) is a generalization of ARK Conjecture from monotone graph properties to arbitrary monotone transitive Boolean functions. In this paper we study a weakening of the Evasiveness Conjecture called Weak Evasivenss Conjecture (weak-EC). The weak-EC asserts that every non-trivial monotone transitive Boolean function must have D(f) ≥ n1-∈, for every ∈ > 0. The purpose of this note is to make some remarks on weak-EC that hint towards a plausible attack on EC. First we observe that weak-EC is equivalent to EC. Further we observe that ruling out only certain simple (monotone-NC1) counter-examples to weak-EC suffices to confirm EC in its whole generality. Finally we rule out some simple counter-examples to weak-EC (AC0 : unconditionally; and monotone-TC0 : under a conjecture of Benjamini, Kalai, and Schramm on their noise stability). We also investigate an analogue of weak-EC for the stronger model of parity decision trees and provide a counter-example to this seemingly stronger version under a conjecture of Montanaro and Osborne. © 2013 ACM.

Datta S.,Chennai Mathematical Institute | Hesse W.,Google | Kulkarni R.,Center for Quantum Technologies
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2014

We report progress on dynamic complexity of well-known graph problems such as reachability and matching. In this model, edges are dynamically added and deleted and we measure the complexity of each update/query. We propose polynomial-size data structures for such updates for several related problems. The updates are in very low level complexity classes such as quantifier-free first order formulas, AC0[2], TC0. In particular, we show the following problems are in the indicated classes: (a) maximum matching in non-uniform DynTC0; (b) digraph reachability in non-uniform DynAC0[2]; (c) embedded planar digraph reachability in DynFO(= uniform DynAC0). Notably, the part (c) of our results yields the first non-trivial class of graphs where reachability can be maintained by first-order updates; it is a long-standing open question whether the same holds for general graphs. For (a) we show that the technique in [7] can in fact be generalized using [8] and [9] to maintain the determinant of a matrix in DynTC0. For (b) we extend this technique with the help of two more ingredients namely isolation [1,13] and truncated approximation using rational polynomials. In fact, our proof yields DynAC0[p] bound for any prime p > 1. For (c) we exploit the duality between cuts and cycles in planar graphs to maintain the number of crossings between carefully chosen primal and dual paths, using several new structural lemmas. © 2014 Springer-Verlag.

Alon N.,Tel Aviv University | Lee T.,Center for Quantum Technologies | Shraibman A.,The Academic College of Tel-Aviv-Yaffo | Vempala S.,Georgia Institute of Technology
Proceedings of the Annual ACM Symposium on Theory of Computing | Year: 2013

We study the ε-rank of a real matrix A, defined for any ε > 0 as the minimum rank over matrices that approximate every entry of A to within an additive ε. This parameter is connected to other notions of approximate rank and is motivated by problems from various topics including communication complexity, combinatorial optimization, game theory, computational geometry and learning theory. Here we give bounds on the ε-rank and use them for algorithmic applications. Our main algorithmic results are (a) polynomial-time additive approximation schemes for Nash equilibria for 2-player games when the payoff matrices are positive semidefinite or have logarithmic rank and (b) an additive PTAS for the densest subgraph problem for similar classes of weighted graphs. We use combinatorial, geometric and spectral techniques; our main new tool is an algorithm for efficiently covering a convex body with translates of another convex body. Copyright 2013 ACM.

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