**William Mong Distinguished Lecture**

Time and location:Wednesday April 26th 2017, 5-6pm (Reception at 4:30pm), Lecture Theater A, Chow Yei Ching Building

Time and location:

**Speaker:**Gilles Brassard (Department of Computer Science and Operations Research, Université de Montréal)

**Title:**Cryptography in a Quantum World

**Abstract:**Cryptography, although practiced as an art and science for thousands of years, had to wait until the end of the 1940s before Claude Shannon gave it a strong mathematical foundation. However, Shannon's approach was rooted in his own information theory, itself inspired by the classical physics of Newton. But our world is ruled by the laws of quantum mechanics. When quantum-mechanical phenomena are considered, new vistas open up both for cryptographers (code makers) and cryptanalysts (code breakers). Some theorems (including by Shannon) remain mathematically correct, but become irrelevant in our quantum world. Most strikingly, it is possible for two people who do not share ahead of time a long secret key to communicate in perfect secrecy under the nose of an eavesdropper with unlimited computing power and whose technology is limited only by the known laws of physics. Conversely, quantum mechanics provides powerful tools to threaten the mechanisms that are currently used on the Internet to protect electronic transactions. Furthermore, it seems — but is not yet proven — that quantum mechanics provides more benefits to cryptanalysts than cryptographers if the latter are restricted to using only classical communication channels. So, in the end, is quantum mechanics a blessing or a curse to the protection of privacy? The jury is still out. No prior knowledge in quantum mechanics or cryptography will be expected from the audience.

**Bio:**

*Professor of computer science since 1979 and Canada Research Chair at the Université de Montréal, Gilles Brassard FRS, O.C., laid the foundations of quantum cryptography at a time when only a handful of people worldwide were interested in quantum information science. This thirty-year-old theoretical idea has lead to new enabling technologies for secure quantum communication on Earth and via satellite in which China has taken the international lead. Professor Brassard is also among the inventors of quantum teleportation, a universally recognized fundamental keystone of the entire quantum information discipline, for which Thomson Reuters has predicted that he will one day receive the Nobel Prize in Physics. Editor-in-Chief for Journal of Cryptology from 1991 until 1997, he is the author of three books that have been translated into eight languages (including Chinese). He is a Fellow of the Royal Society of London, the Royal Society of Canada, the Canadian Institute for Advanced Research and the International Association for Cryptologic Research. He was awarded honorary doctorates by the ETH in Zürich, the University of Ottawa and the Università della Svizzera italiana in Lugano, and made an Officer of the Order of Canada.*

Time and location: Thursday April 27th 2017, 1-3pm, RM 308, Chow Yei Ching BuildingSpeaker: Gilles Brassard (Department of Computer Science and Operations Research, Université de Montréal)Title: Nonsignalling theories are local-realisticAbstract: Most physicists take it for granted that the experimental violation of Bell’s inequality provides evidence that it is not possible to completely describe the state of a physical system in terms of purely local information when this system is entangled with some other system. We disagree. Provided we redefine appropriately what is the information-theoretic state of a quantum system, it becomes possible to recover the whole from the description of its parts. This is in sharp contrast with the standard formalism of quantum mechanics in which the density matrix provides all there is to say about the state of a system. According to our formalism, there is no need to invoke supernatural nonlocality in order to explain everything standard quantum mechanics tells us that we can observe. We show, however, that this is inconsistent with the usual belief held among Everettians that the universal wavefunction can be taken as the complete representation of reality. Inspired by Plato and Kant, we introduce and contrast the notions of noumenal and phenomenal states of physical systems: the former corresponds to the complete but unknowable state of the system and the latter to what can be perceived about it with the help of arbitrary technology. We exhibit an explicit epimorphism from the former to the latter, which explains the relationship between all that there is and all that can be apprehended. |

Time and location: Tuesday April 18th 2017, 5-6pm, RM 308, Chow Yei Ching BuildingSpeaker: Xing Rong (CAS Key Laboratory of Microscale Magnetic Resonance and Department of Modern Physics, University of Science and Technology of China, Hefei, China )Title: Quantum Control of Spins in SolidsAbstract:Quantum computation provides great speedup over its classical counterpart for certain tasks. Spin system is one of the most important candidates to realize quantum computations. The initialization, readout and quantum gate operations of spin qubits can be accomplished by advanced spin resonance techniques, which include nuclear magnetic resonance, electron paramagnetic resonance and optically detected magnetic resonance. My talk aims to summarize our recent experimental progresses in spin-based quantum computing. I will introduce how to suppress the noise from the environment to protect quantum states[1,2]. Then the realization of ultra-high fidelity quantum gates will be discussed[3,-6]. Finally, I will show how to realize time-optimal control of spins[7].1. Nature 461, 1265 (2009) 2. PRL 106, 040501 (2011) 3. PRL 109, 070502 (2012) 4. PRL 112, 010503 (2014) 5. PRL 112, 050503 (2014) 6. Nature Communications 6, 8748 (2015) 7. PRL 117, 170501 (2016) |

Time and location: Wednesday April 19th 2017, 5-6pm, RM 328, Chow Yei Ching BuildingSpeaker: Zhaokai Li (CAS Key Laboratory of Microscale Magnetic Resonance and Department of Modern Physics, University of Science and Technology of China, Hefei, China )Title: Compressed Quantum SimulationAbstract: Certain n-qubit quantum systems can be faithfully simulated by quantum circuits with only O(log(n)) qubits [1]. I will report an experimental realization of the compressed quantum simulation on a one-dimensional Ising chain [2]. By utilizing an nuclear magnetic resonance quantum simulator with only five qubits, the property of ground-state magnetization of an open-boundary 32-spin Ising model is experimentally simulated, prefacing the expected quantum phase transition in the thermodynamic limit. This experimental protocol can be straightforwardly extended to systems with hundreds of spins by compressing them into up to merely 10-qubit systems. Besides, I will also talk about the experimental platforms we have in USTC and some recent experimental studies in our lab, including experimental quantum machine learning [3] and quantum factorization [4]. [1] Phys. Rev. Lett. 107, 250503 (2011) [2] Phys. Rev. Lett. 112, 220501 (2014) [3] Phys. Rev. Lett. 114, 140504 (2015) [4] ArXiv.org/1611.03293 (2016) |

Time and location: Tuesday, December 13,2016, 3-4pm, rm 308, Chow Yei Ching BuildingSpeaker: Adan Cabello (University of Seville)Activity: FQXi Mini-Workshop on Quantum Information and FoundationsTitle: The message of the Bell inequalityAbstract: What can we learnt about the universe from the recent experiments confirming the violation of the Bell inequality? [1-5] A first, obvious, lesson is that the universe cannot be explained as Einstein would like. More importantly, we learn that in the universe there are "measurements" for which there is not joint probability distribution, and that any good theory of the universe should use "states" which give maximum information about a system without giving any information about its parts. But this is just the beginning. The specific way quantum theory violates the Bell inequality tell us much more than that. The maximum value predicted by quantum theory and confirmed in a recent experiment [6] tells us that there are no physical laws limiting the value of the Bell parameter: what we observe is already the largest value possible in any "nice" theory of the universe.[1] B. Hensen et al., Nature 526, 682 (2015). [2] M. Giustina et al., et al. Phys. Rev. Lett. 115, 250401 (2015). [3] L. K. Shalm et al., Phys. Rev. Lett. 115, 250402 (2015). [4] B. Hensen et al., arXiv:1603.05705. [5] H. Weinfurter, https://play.lnu.se/media/t/0_i7ibwbp6 [6] H. S. Poh et al. Phys. Rev. Lett. 115, 180408 (2015). |

Time and location: Monday, December 12, 2016, 2-3pm, rm 313, Chow Yei Ching BuildingSpeaker: Masahito Hayashi (Nagoya University and CQT, Singapore)FQXi Mini-Workshop on Quantum Information and FoundationsActivity: Title: Memory Size Reduction of Approximate Sufficient StatisticsAbstract: Given a sufficient statistic for a parametric family of distributions, one can estimate the parameter of the distribution generating the data without access to the data itself. However, the memory or code size for storing the sufficient statistic may nonetheless still be prohibitive. Indeed, by the method of types, it is known that for $n$ independent data samples drawn from a $k$-nomial distribution with $d=k-1$ degrees of freedom, the length of the code scales as $d\log n+O(1)$ as $n\to\infty$. In many applications though, we may not need to reconstruct the unknown parameter or generating distribution exactly. By adopting a Shannon-theoretic approach in which we allow a small (but non-vanishing error) in estimating the parameter, we consider various constructions of {\em approximate sufficient statistics} and show that under regularity assumptions on the parametric family, the code length can be reduced to $\frac{d}{2}\log n+O(1)$ as $n\to\infty$. We consider errors measured according to both the relative entropy and variational distance criteria. For the code construction parts, we leverage Rissanen's minimum description length principle---namely, a two-step encoding procedure that yields a universal variable-length source code. For the converse parts, we use Clarke and Barron's asymptotic expansion for the relative entropy of a product parametrized distribution and a mixture distribution. In addition to weak converses, we also prove strong converses for our statements. This means that even if the code is allowed to have a non-vanishing error, its length must still be at least $\frac{d}{2}\log n+O(1)$.This work is a joint work with Vincent Y. F. Tan. |

**Bio:**

*Masahito Hayashi was born in Japan in 1971. He received the B.S. degree from the Faculty of Sciences in Kyoto University, Japan, in 1994 and the M.S. and Ph.D. degrees in Mathematics from Kyoto University, Japan, in 1996 and 1999, respectively. He worked in Kyoto University as a Research Fellow of the Japan Society of the Promotion of Science (JSPS) from 1998 to 2000, and worked in the Laboratory for Mathematical Neuroscience, Brain Science Institute, RIKEN from 2000 to 2003, and worked in ERATO Quantum Computation and Information Project, Japan Science and Technology Agency (JST) as the Research Head from 2000 to 2006. He also worked in the Superrobust Computation Project Information Science and Technology Strategic Core (21st Century COE by MEXT) Graduate School of Information Science and Technology, The University of Tokyo as Adjunct Associate Professor from 2004 to 2007. In 2006, he published the book "Quantum Information: An Introduction'' from Springer. In 2007, he joined the Graduate School of Information Sciences, Tohoku University as Associate Professor. He also worked in Centre for Quantum Technologies, National University of Singapore as Visiting Research Associate Professor from 2009. He is on the Editorial Board of International Journal of Quantum Information and International Journal On Advances in Security. His research interests include quantum information theory, quantum statistical inference, and Shannon Theory.*

Time and location: Monday, December 12, 2016, 2-3pm, rm 313, Chow Yei Ching BuildingSpeaker: Debbie Leung (University of Waterloo) FQXi Mini-Workshop on Quantum Information and FoundationsActivity: Title: Embezzlement of entanglement, conservation laws, and nonlocal gamesAbstract: Consider two remote parties Alice and Bob, who share quantum correlations in the form of a pure entangled state. Without further interaction, the "Schmidt coefficients" of the entangled state are invariant; in particular, the amount of entanglement is conserved. van Dam and Hayden found that reordering these coefficients (corresponding to allowed local operations) can effect an apparent violation of the conservation law nearly perfectly, a phenomenon called "embezzlement". We discuss how the same mathematics can explain coherent manipulation of spins in NMR and other approximate violation of conservation laws. We show how this phenomenon gives rise to a quantum generalization of nonlocal games that cannot be won with finite amount of entanglement. (Joint work with Ben Toner, John Watrous and Jesse Wang.) |

**Time and location:**Friday, September 23 2016, 2-3pm, rm 308, Chow Yei Ching Building

**Speaker:**Simon Benjamin (University of Oxford)

**Title:**Harnessing the quantum world: can lab experiments become practical technologies?

**Abstract:**The effort to develop practical quantum technology is ramping up worldwide,

with the UK and other governments recently announcing hundreds of millions of dollars

worth of investment. Some say this is premature! Are we now sure that complex quantum

systems can be adequately controlled, and sufficiently scaled, that they will be useful

outside the lab? I will ague that the answer is "yes". After reviewing recent achievements

in experiments, theory and applications, I will describe the Q20:20 machine that the

Oxford-led National Quantum Hub is building in the UK. More importantly, I'll try to offer

an answer to the question "What would the first generation of such machines be useful

for?"

**Bio:**

*Simon Benjamin is Professor of Quantum Technologies at Materials Department in the University of Oxford, and an Associate Director of the Oxford-led UK Hub on Networked Quantum Information Technologies (NQIT). Simon leads a team of ten staff and students called the Quantum and Nanotechnology Theory group (QuNaT). They are applied theorists, who study various questions relating to how build and use a new generation of technologies based on harnessing quantum effects. The group has interests ranging from energy harvesting to sensors to secure communications, and even the question of whether quantum effects are exploited by biology; but their primary interest is in information processing systems.*

Simon has been a Visiting Professor at the Centre for Quantum Technologies in Singapore, and is currently a visiting researcher at the Singapore University of Technology and Design.

Simon has been a Visiting Professor at the Centre for Quantum Technologies in Singapore, and is currently a visiting researcher at the Singapore University of Technology and Design.