CQT Colloquium by Rubem Mondaini, Beijing Computational Sciences Research Center
Title: Many-body localization: When thermalization fails and how to observe it experimentally Date/Time: 21-Nov, 04:00PM Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: The observation of many-body localization is a paradigmatic example of the amount of time an idea takes to get mature enough, and the numerical and experimental methods to develop sufficiently, in order to settle its existence. After the original study of Philip Anderson in 1958, demonstrating localization of non-interacting quantum particles in disordered settings, a natural question is on the resulting effects of the inter-particle interactions on this phenomenon. Only after 50 years, substantial theoretical progress was made in solving this puzzle and, in 2016 the first experimental observation was realized. The advent of platforms involving ultracold atoms allowed the inspection of an inherently dynamical quantum phase transition, that goes beyond the standard ground-state classification of the quantum matter, and its associated low-lying excitations. In this talk, after introducing the general conditions where it occurs, and review the experiments tackling it so far, I will show numerical and experimental results using quantum circuits of superconducting qubits that shed light on yet another highly debated aspect: the possible existence of many-body mobility edges.
Abstract: We live in an era where everyone is constantly connected via the internet. The way we work, play, socialise and perform transactions can no longer be dissociated with our smartphones. This is possible only because we could communicate securely over the virtual world, keeping our sensitive information away from prying eyes. However, quantum computers could break our current encryption scheme, completely disrupting our way of life in the current digital age. Fortunately, quantum theory also provides a means of encrypting unconditionally secure messages. In this talk, the art and science of quantum cryptography will be introduced. Our vision of connecting people securely via quantum links in the upcoming quantum internet will also be shared.
Speaker: Dr Goh Koon Tong, Research Fellow, Department of Electrical & Computer Engineering, National University of Singapore (NUS)
Speaker’s Profile: Dr Goh Koon Tong, Research Fellow, Department of Electrical & Computer Engineering, National University of Singapore (NUS) Dr Goh Koon Tong is a Research Fellow at the Department of Electrical & Computer Engineering, NUS and leads a theoretical team of scientists in the Quantum Communications Laboratory. His research interest is centred on quantum cryptography, focusing on the development of practical quantum protocols for the future Quantum Internet.
CQT Talk by Shayan Mookherjea, University of California, San Diego
Title: Integrated photonics for high-performance quantum sources and detectors Date/Time: 14-Nov, 04:00PM Venue: CQT Level 3 Seminar Room, S15-03-15
Integrated photonics can greatly reduce the size, weight and power (SWaP) of optical assemblies, and also improve functionality and utility of quantum photonic sources and detectors, which may, in turn, benefit large-scale deployment and adoption of quantum technology.
We report on good performance in entangled photon-pair and heralded single-photon generation using spontaneous four-wave mixing (SFWM) in silicon photonic micro-resonators, and using spontaneous parametric down-conversion (SPDC) in periodically-poled thin-film lithium niobate waveguides. Using superconducting nanowire single-photon detectors, we demonstrated capture of ultra-high bandwidth (>100 GHz) optical modulated signals at ultra-low received average power (below -100 dBm), a new milestone in optical oscilloscopy.
Shayan Mookherjea received the BS degree with honors from Caltech, the SM degree from MIT, and the PhD from Caltech in Electrical Engineering with a minor in Physics. He is a Professor of Electrical and Computer Engineering at the University of California, San Diego. His awards include: Wilts Prize, Hellman Faculty Fellow, NSF CAREER grant, IEEE Senior Member, and OSA Fellow. He leads the Micro/Nano-Photonics Group at UCSD (http://mnp.ucsd.edu).
Abstract: How to characterize fundamental states of quantum matter is an important theme in condensed matter physics. Characterizations of the celebrated Landau symmetry-breaking and topological quantum phases are mainly developed based on the equilibrium theories. In this talk, I will introduce how to characterize equilibrium symmetry-breaking and topological quantum phases by far-from-equilibrium quantum quench dynamics. For topological phases, a generic theory is established by showing a dynamical bulk-surface correspondence, which connects the dD bulk topology of equilibrium phases to topological pattern of quench dynamics emerging in the (d-1)D momentum subspace, dubbed band-inversion surfaces (BISs), similar to the well-known bulk-boundary correspondence for equilibrium topological phases in the real space. Further, we consider the Haldane-Hubbard model, which hosts both the symmetry-breaking orders and topological phases. We show that the correlated pseudospin quench dynamics exhibits robust universal behaviors on the BISs, from which both the topology and symmetry-breaking orders are extracted. In particular, the topology of the post-quench regime is characterized by the emergent topological pattern of quench dynamics on BISs, which is robust against dephasing and heating induced by interactions; the pre-quench symmetry-breaking orders are read out from a universal scaling of the quench dynamics emerging on the BIS, which is valid beyond the mean-field theory. These results may show insights into the exploration of the characterization of both symmetry-breaking and topological phases by quench dynamics.
References:  L. Zhang, L. Zhang, etal., Science Bull. 63, 1385 (2018).  W. Sun et al., Phys. Rev. Lett. 121, 250403 (2018).  B. Song, C. He, S. Niu et al. Nature Physics, 15, 911 (2019).  L. Zhang et al., Phys. Rev. A 99, 053606 (2019).  L. Zhang et al., arXiv:1903.09144v2.  C. R. Yi, L. Zhang et al., Phys. Rev. Lett. 123, 190603 (2019).
About the speaker: Xiong-Jun Liu received Ph.D in Texas A&M University in 2011, and was a postdoctoral fellow in University of Maryland, IAS HKUST and MIT (2011-2014). He joined the faculty of International Center for Quantum Materials at Peking University (09/2014), became tenured (07/2018), and now a full professor (from 01/2019). He works in condensed matter theory and ultracold atoms, focusing on quantum simulation and topological matter: topological superconductors, synthetic gauge fields, non-equilibrium topological quantum systems, and strongly correlated topological states. He has been awarded The National Science Fund for Distinguished Young Scholars (2018), and AAPPS-APCTP CN Yang Award (2019).
CQT Talk by Tom Stace, The University of Queensland
Title: Buildinga bigger Hilbert space for superconducting devices, one Bloch state at a time Date/Time: 13-Nov, 02:00PM Venue: CQT Level 3 Seminar Room, S15-03-15
Abstract: Noise and errors have been the bottlenecks for building robust quantum machines. I will describe a new class of superconducting devices that has built-in error rejection. Fundamentally, the encoding that facilitates this intrinsic robustness comes from the fact that these systems have much bigger Hilbert spaces than has been traditionally considered. The extra space affords new, robust qubit encodings, which I describe in two different instantiations.
CQT Talk by Christopher Williamson, The Chinese University of Hong Kong
Abstract: The epsilon-approximate degree of a Boolean function f isthe least degree of a real-valued polynomial that approximates f pointwise toerror epsilon. The approximate degree of f is at least k iff there exists apair of probability distributions, also known as a dual polynomial, that areperfectly k-wise indistinguishable, but are distinguishable by f with advantage1-epsilon.
Inspired by the study of secret sharing complexity and anatural generalization of bounded independence, we discuss the followingcontributions:
A simple new construction of a dual polynomial for theAND function, certifying that the approximate degree of this function is Omega(sqrt(n)). This construction is the first to extend to the notion of weighteddegree, and yields the first explicit certificate that the 1/3-approximatedegree of any read-once DNF is at least Omega (sqrt(n)).
A proof that any pair of symmetric distributions onn-bit strings that are perfectly k-wise indistinguishable are alsostatistically K-wise indistinguishable with error at most essentially exp(-k^2/K), for all k < K < n/64. This implies that any symmetric functionf is a reconstruction function with constant advantage for a ramp secretsharing scheme that is secure against size K coalitions with error exp(approx-deg(f)^2/K) for all values of K up to n/64 simultaneously. Previoussecret sharing schemes required that K be determined in advance and only workedfor f=AND.
Our analyses draw new connections between approximate degreeand concentration phenomena.